Advertisement

Reduced Mortality After Oral Polio Vaccination and Increased Mortality After Diphtheria-tetanus-pertussis Vaccination in Children in a Low-income Setting

      Highlights

      • Up to 3 years of age, children who had DTP as most recent vaccination had higher mortality than both unvaccinated and MV-vaccinated children.
      • Children who received oral polio vaccine (OPV) without DTP had lower mortality than children who received DTP with or without OPV.
      • Children who received DTP and OPV had lower mortality than children who received DTP without OPV.

      Abstract

      Purpose

      The diphtheria-tetanus-pertussis vaccine (DTP) and oral polio vaccine (OPV) were introduced in children 3 of 5 months of age in 1981–1983 in Bandim, in the capital of Guinea-Bissau. Because DTP has been linked to deleterious nonspecific effects (NSEs) and OPV to beneficial NSEs, we followed up this cohort to 3 years of age and examined the effects of DTP with OPV on all-cause mortality and the interactions of DTP and OPV with the measles vaccine (MV).

      Methods

      DTP and OPV were offered at 3 monthly community weighing sessions. Vaccination groups were defined by the last vaccine received. We compared overall mortality for different groups in Cox proportional hazards regression models, reporting hazards ratios (HRs) with 95% CIs.

      Findings

      The study cohort included 1491 children born in Bandim from December 1980 to December 1983. From 3 to 35 months of age, with censoring for MV, children vaccinated with DTP and/or OPV had higher mortality than both unvaccinated children (HR = l.66; 95% CI, 1.03–2.69) and OPV-only vaccinated children (HR = 2.81; 95% CI, 1.02–7.69); DTP-only vaccinated children had higher mortality than OPV-only vaccinated children (HR = 3.38; 95% CI, 1.15-–9.93). In the age group of 3–8 months, before MV is administered, DTP-only vaccination was associated with a higher mortality than DTP with OPV (HR = 3.38; 95% CI, 1.59–7.20). Between 9 and 35 months of age, when MV is given, DTP-vaccinated and MV-unvaccinated children had higher mortality (HR = 2.76; 95% CI, 1.36–5.59) than children who had received MV after DTP, and among children who received DTP with MV or after MV, DTP-only vaccination was associated with a higher mortality than DTP with OPV (HR = 6.25; 95% CI, 2.55–15.37).

      Implications

      Because the 2 vaccines had differential effects and the healthiest children were vaccinated first, selection biases are unlikely to explain the estimated impact on child survival. OPV had beneficial NSEs, and administration of OPV with DTP may have reduced the negative effects of DTP.

      Key words

      Introduction

      The effect of vaccines on overall survival had not been assessed in randomized clinical trials (RCTs) when the Expanded Program on Immunization was initiated in 1974. The disease-protective effects were known; effects on survival were assumed to be proportional to the burden of infection. Hence, the main interest was at which age to vaccinate.
      Expanded Programme on Immunization
      The optimal age for measles immunization.
      However, subsequent studies of the introduction of measles vaccine (MV)
      • Aaby P.
      • Bukh J.
      • Lisse I.M.
      • Smits A.J.
      Measles vaccination and child mortality.
      • Aaby P.
      • Bukh J.
      • Lisse I.M.
      • Smits A.J.
      Measles vaccination and reduction in child mortality: a community study from Guinea-Bissau.
      • Aaby P.
      • Bhuyia A.
      • Nahar L.
      • Knudsen K.
      • de Francisco A.
      • Strong M.
      The survival benefit of measles immunization may not be explained entirely by the prevention of measles disease: a community study from rural Bangladesh.
      • Holt E.A.
      • Boulos R.
      • Halsey N.A.
      • et al.
      Childhood survival in Haiti: protective effect of measles vaccination.
      The Kasongo Project Team
      Influence of measles vaccination on survival pattern of 7-35-month-old children in Kasongo, Zaire.
      suggested that the MV had beneficial nonspecific effects (NSEs) on child survival (ie, effects on survival not explained by prevention of the vaccine-targeted disease).
      • Aaby P.
      • Samb B.
      • Simondon F.
      • Coll Seck A.M.
      • Knudsen K.
      • Whittle H.
      Non-specific beneficial effect of measles immunisation: analysis of mortality studies from developing countries.
      The World Health Organization (WHO) recently sponsored a review of the potential NSEs of the BCG vaccine, diphtheria-tetanus-pertussis vaccine (DTP), and MV
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      ,
      Strategic advisory group of experts on immunization.
      for child mortality in low-income countries. BCG and MV had beneficial NSEs. The point estimate for DTP was in the opposite direction.
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      Although protective against the 3 target diseases, DTP apparently increased susceptibility to unrelated infections.
      • Aaby P.
      • Jensen H.
      • Samb B.
      • et al.
      Differences in female-male mortality after high-titre measles vaccine and association with subsequent vaccination with diphtheria-tetanus-pertussis and inactivated poliovirus: reanalysis of West African studies.
      • Aaby P.
      • Jensen H.
      • Gomes J.
      • Fernandes M.
      • Lisse I.M.
      The introduction of diphtheria-tetanus-pertussis vaccine and child mortality in rural Guinea-Bissau: an observational study.
      • Aaby P.
      • Benn C.S.
      • Nielsen J.
      • Lisse I.M.
      • Rodrigues A.
      • Ravn H.
      Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
      • Aaby P.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • Benn C.S.
      Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.
      Other nonlive vaccines may also be associated with increased mortality.
      • Benn C.S.
      • Fisker A.B.
      • Rieckmann A.
      • Sørup S.
      • Aaby P.
      Vaccinology: time to change paradigm?.
      ,
      • Aaby P.
      • Fisker A.B.
      • Björkman
      • Benn C.S.
      Rolling out the RTS,S malaria vaccine: to test or not to test the effect on mortality?.
      OPV was not examined.
      There are surprisingly few studies of OPV and DTP and child survival in the medical literature. We have data from 40 years ago when OPV and DTP were introduced in Guinea-Bissau in the 1980s. Few sites have similar data, so we have gone back to examine the natural experiment of introducing DTP and OPV. In an urban area, weighing sessions were organized every 3 months to identify malnourished children. When vaccines became available in June 1981, OPV and DTP were offered from 3 months of age in connection with the weighing sessions.
      Inadvertently, this created a natural experiment among 3- to 5-month-old children; some received vaccines when just 3 months old, whereas others were nearly 6 months old before they were vaccinated. Thus, allocation was determined by birthdays and the dates of weighing sessions and not by selection biases. In this natural experiment, DTP-vaccinated children had higher mortality than children not yet vaccinated with DTP from 3 to 5 months of age.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      We examine the effects of OPV and DTP up to 3 years of age in the same cohort.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      After 6 months of age, the unvaccinated group was increasingly composed of children who had not been vaccinated because they were frail or malnourished or had traveled to rural areas. Because most children were subsequently vaccinated with MV, we also examined possible interactions between DTP and OPV with MV. Because there are plans to stop OPV globally in 2024, we have particularly tried to assess the NSEs of OPV on child survival.

      Methods

       Demographic Surveillance

      Bandim Health Project (BHP) was started in 1978 in an urban district. In 1978–1979, mortality among children younger than 5 years was nearly 500 per 1000.
      • Smedman L.
      • Aaby P.
      • Lindeberg A.
      • Zetterstrom R.
      Survival 0-6 years of age in a periurban community in Guinea-Bissau: a longitudinal assessment.
      Malnutrition was assumed to be the main cause; BHP was initiated to determine why children were malnourished.
      • Aaby P.
      • Bukh J.
      • Lisse I.M.
      • Smits A.J.
      Measles mortality, state of nutrition, and family structure: a community study from Guinea-Bissau.
      However, severe malnutrition was not evident, and to understand the high mortality, we started population follow-up. Four health workers identified pregnant women, encouraged women to attend antenatal clinics, and followed up children younger than 3 years with anthropometric measurements. Each health care worker supervised 2 subdistricts; they kept lists of pregnant women and children younger than 3 years. BHP had no computerized registration system until 1990 but kept an A5 BHP card with weights and vaccination dates for each child. Growth cards were kept by the mother. The study of nutritional status was planned by the Swedish Agency for Research Collaboration and the Ministry of Health in Guinea-Bissau.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.

       Anthropometry

      The health care workers arranged 3 monthly weighing sessions in each subdistrict. Mothers were notified before a community weighing. The following morning, the child's weight was measured and noted on the BHP card.

       Vaccinations

      BHP organized MV campaigns in December 1979 and December 1980.
      • Aaby P.
      • Bukh J.
      • Lisse I.M.
      • Smits A.J.
      Measles vaccination and reduction in child mortality: a community study from Guinea-Bissau.
      ,
      • Mogensen S.W.
      • Aaby P.
      • Smedman L.
      • et al.
      Introduction of standard measles vaccination in an urban African community in 1979 and overall child survival: a reanalysis of data from a cohort study.
      In June 1981, BHP started to provide vaccines at the weighing sessions. A nurse from the health center followed the weighing team and vaccinated eligible children. DTP and OPV were provided to children from 3 months of age and MV to children from 9 months of age. OPV was not given at birth. Doses of DTP and OPV could be given with 1-month intervals, but because we only arranged quarterly weighing sessions, most children had longer intervals. In several periods, either OPV or DTP was missing.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      BCG was rarely provided because nurses were not trained to administer intradermal vaccinations.
      An expatriate nurse of the supervising field staff sometimes organized catch-up vaccination sessions without weighing, but vaccinations were noted on BHP cards. Both nurses and mothers thought that sick children should not be vaccinated; BHP cards often indicated that the child was sick, malnourished, or orphan to explain why an eligible child had not been vaccinated at a weighing session.

       Data Control

      A computerized system became available in 1990–1991; weights and vaccination dates from the BHP cards were entered. For the present analysis, dates of visits, weights, and vaccination dates were checked against the original cards (Figure 1). As described previously a few cards could not be found.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      Figure 1
      Figure 1Flowchart of study population and children included in the analyses. Numbers in parentheses indicate number of death. In the previous analysis,
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      1452 children were registered before date of birth plus 183 days.

       Study Cohort

      We included children born December 3, 1980, to December 31, 1983. The vaccination program started June 2, 1981.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      Children who never attended any weighing session were not included as unvaccinated. We excluded orphans because they were not breastfed and likely to have different care. Children were included from 91 days of age if examined before 3 months of age. If first seen after 3 months, they were included from the date seen (Figure 2). DTP and OPV were not administered at other health centers; follow-up time therefore counted as unvaccinated until BHP administered DTP or OPV (Figure 2). Time as unvaccinated also came from children not seen at 3–5 months but seen before 3 months of age (Figure 2). Hence, DTP- and OPV-vaccinated and unvaccinated children were from the same cohort born in Bandim; their randomization to the DTP/OPV vaccination group or DTP/OPV not yet vaccinated group depended on birthdate, timing of weighing sessions, and traveling. The death of a traveling child could usually be discovered from other members of the family who remained in the study area.
      Figure 2
      Figure 2Natural variation in the timing of vaccination.
      The 3- to 5-month age group corresponds to the natural experiment with unbiased allocation to vaccination.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      After 6 months, most unvaccinated children were frail children who had been weighed but not vaccinated or who had traveled.
      Because of the lack of vaccines, some children received DTP-only or OPV-only vaccination.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      Traveling patterns did not differ between children who had received DTP1 and OPV1 or DTP1 only, and these groups were equally likely to receive subsequent vaccinations (data not shown).
      This cohort born between December 1980 and December 1983 is entirely different from the cohort of children aged 6–35 months born between June 1978 and December 1980, which we have described elsewhere.
      • Aaby P.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • Benn C.S.
      Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.

       Statistical Analysis

      DTP and OPV vaccinations started on June 2, 1981. Groups defined by the most recent vaccination(s) were compared using Cox proportional hazards regression models with age as the underlying time; proportional hazard assumptions were tested using the Schoenfeld residuals test and visual inspection of the cumulative hazard ratios (HRs). A few children had received BCG vaccine without documentation at the maternity ward because they had a BCG scar but no vaccination card, and BCG vaccinations were therefore ignored in the analyses. In a sensitivity analysis, we censored the children who had documented BCG vaccination. Although this reduced the power of the study, it did not change the estimates (data not shown).
      To avoid survival bias, we used a landmark approach
      • Jensen H.
      • Benn C.S.
      • Lisse I.M.
      • Rodrigues A.
      • Andersen P.K.
      • Aaby P.
      Survival bias in observational studies of the impact of routine vaccinations on childhood survival.
      ; hence, vaccination status was only updated from the day the information was collected.
      We conducted 3 main analyses. First, we compared DTP and/or OPV-vaccinated with unvaccinated children in the 3- to 35-month period; children were censored when known to have received MV. Second, we conducted an analysis between 3 and 8 months of age before children receive MV. Third, we examined whether DTP and OPV interacted with MV; this analysis included only the 9- to 35-month age group. Because vaccine effects often differ by sex, we present main analyses by sex.
      Control for confounders was conducted in the 3- to 35-month age group. Subdistrict, ethnic group, and twinning did not change results. Control for year of birth slightly increased the HR of DTP-vaccinated compared with unvaccinated children. There was no clustering of deaths, and control for season did not change the estimates (data not shown). The WHO z score for weight for age (WAZ) was used to assess nutritional status. However, we did not adjust for WAZ in comparisons that involved unvaccinated children because most unvaccinated children had traveled and had therefore not been weighed at a similar age as the vaccinated children. In the comparison of vaccinated groups, the last observation of WAZ was carried forward if an observation was missing.

      Results

       DTP and OPV

      A total of 1184 children were included in the analysis of the 6- to 35-month age group (Figure 1). The vaccination coverage is indicated in Supplemental Table I; 95% received DTP1 and OPV1 before 3 years of age, but coverage for the third dose was approximately 80%. The groups did not differ with respect to background factors, such as birthweight and weight before 6 months of age (Table I). At 6–8 months, WAZ was better for children who had received DTP and OPV, DTP only, or OPV only than for children who remained unvaccinated after participating in a weighing session. After 9 months of age, there was no clear difference in nutritional status measured by WAZ. Background factors after 12 months of age are listed in Supplemental Table II; unvaccinated children participated in fewer weighing sessions, reflecting that they traveled more.
      Table IBackground factors for the different most recent vaccination groups observed up to 1 year of age.
      Sample sizes are the number of children in the group, using only 1 observation per child but allowing a child to be included in all groups at once.
      FactorUnvaccinated (n = 731)DTP Only (No MV) (n = 225)DTP and OPV (No MV) (n = 633)OPV Only (No MV) (n = 155)MV
      MV includes all children who have received MV and is therefore not necessarily the most recent vaccination.
      (n = 400)
      Birth WAZ, mean (SD)
      WAZ is defined as the WAZ measured at birth or before 14 days of age.
      [n]
      −0.18 (0.98) [313]−0.17 (1.02) [105]−0.26 (0.92) [281]−0.12 (1.25) [68]−0.18 (0.93) [156]
      WAZ at <6 months of age, mean (SD) [n]−0.37 (1.21) [685]−0.52 (1.36) [119]−0.38 (1.16) [394]−0.50 (1.35) [66]−0.86 (1.21) [8]
      WAZ at 6–8 months of age, mean (SD) [n]−1.07 (1.49) [77]−0.79 (1.27) [104]−0.65 (1.25) [344]−0.79 (1.22) [81]−0.78 (1.11) [75]
      WAZ at 9–11 months of age, mean (SD) [n]−0.98 (1.39) [42]−1.22 (1.15) [49]−1.11 (1.25) [117]−1.24 (1.36) [29]−0.87 (1.17) [359]
      Male sex, %
      The proportions of sex, twins, and ethnic group calculated before 9 months of age (where MV is scheduled).
      52.353.849.352.954.7
      Twin, %
      The proportions of sex, twins, and ethnic group calculated before 9 months of age (where MV is scheduled).
      2.54.61.94.41.3
      Ethnic group, %
      The proportions of sex, twins, and ethnic group calculated before 9 months of age (where MV is scheduled).
       Pepel48.745.150.048.550.7
       Balanta13.211.813.215.410.7
       Other38.143.136.836.038.7
      Examination rate,
      Examination rate calculated as the number of all observations starting with the child being present divided by person-years.
      (events/person-years)
      3.78 (728/192.8)3.44 (74/21.5)3.14 (273/87.0)3.06 (41/13.4)6.47 (48/7.4)
      DTP = diphtheria-tetanus-pertussis vaccine; MV = measles vaccine; OPV = oral polio vaccine; WAZ = weight for age z score.
      Sample sizes are the number of children in the group, using only 1 observation per child but allowing a child to be included in all groups at once.
      MV includes all children who have received MV and is therefore not necessarily the most recent vaccination.
      WAZ is defined as the WAZ measured at birth or before 14 days of age.
      § The proportions of sex, twins, and ethnic group calculated before 9 months of age (where MV is scheduled).
      || Examination rate calculated as the number of all observations starting with the child being present divided by person-years.
      In the first analysis, with censoring for MV, the DTP- and/or OPV-vaccinated children had a HR of 1.22 (95% CI, 0.73–2.04) between 6 and 35 months of age compared with unvaccinated children. Between 3 and 35 months, the HR was 1.66 (95% CI, 1.03–2.69) (Table II): 2.13 (95% CI, 1.00–4.54) for girls and 1.43 (95% CI, 0.78-2–59) for boys (Supplemental Table II). The results for different age groups have been visually presented in Figure 3 (Supplemental Figure 1). DTP- and/or OPV-vaccinated children had also higher mortality than OPV-only vaccinated children (HR = 2.81; 95% CI, 1.02–7.69), and DTP-only vaccination was associated with a higher mortality than OPV-only vaccination (HR = 3.38; 95% CI, 1.15–9.93) (Table II). Adjusting for WAZ, these HRs were 2.89 (95% CI, 1.06–7.94) and 3.33 (95% CI, 1.13–9.79), respectively.
      Table IIMortality rates and HRs by most recent vaccination and age (MV has been censored).
      All
      Children with no registered sex included.
      Mortality Rate (Deaths/Person-Years)
      Mortality rate is presented as (event/person-years) × 100.
      HR (95% CI)
      Age Group, moUnvaccinatedDTP and OPVDTP OnlyOPV OnlyDTP and/or OPVDTP and/or OPV vs UnvaccinatedDTP Only vs UnvaccinatedDTP and/or OPV vs OPV OnlyDTP Only vs OPV Only
      3–54.8 (6/125.3)13.6 (7/51.3)37.5 (6/16.0)0 (0/8.2)19.3 (13/67.3)4.98 (1.73–14.30)9.54 (2.86–31.87)P = 0.21
      Log-rank test for equality.
      P = 0.10
      Log-rank test for equality.
      6–119.1 (12/131.2)11.9 (25/211.0)14.2 (8/56.3)2.4 (1/41.2)12.3 (33/267.3)1.27 (0.66–2.46)1.46 (0.60–3.57)5.22 (0.71–38.18)6.00 (0.75–47.96)
      12–236.5 (7/107.2)7.1 (13/183.6)4.8 (3/62.0)2.2 (1/45.3)6.5 (16/245.6)0.99 (0.41–2.40)0.74 (0.19–2.86)2.91 (0.39–21.93)2.18 (0.23–20.93)
      24–352.6 (1/39.1)5.8 (5/86.9)6.9 (2/29.0)9.0 (2/22.1)6.0 (7/115.9)2.38 (0.29–19.34)2.75 (0.25–30.37)0.64 (0.13–3.11)0.75 (0.10–5.29)
      3–356.5 (26/403.0)9.4 (50/532.8)11.6 (19/163.3)3.4 (4/116.8)9.9 (69/696.1)1.66 (1.03–2.69)2.00 (1.08–3.70)2.81 (1.02–7.69)
      HR fails the Schoenfeld residual test of the proportional hazards assumption (P < 0.05).
      3.38 (1.15–9.93)
      HR fails the Schoenfeld residual test of the proportional hazards assumption (P < 0.05).
      DTP = diphtheria-tetanus-pertussis vaccine; HR = hazard ratio; MV = measles vaccine; OPV = oral polio vaccine.
      Children with no registered sex included.
      Mortality rate is presented as (event/person-years) × 100.
      Log-rank test for equality.
      § HR fails the Schoenfeld residual test of the proportional hazards assumption (P < 0.05).
      Figure 3
      Figure 3Summary of comparisons of different vaccines at different ages. DTP = diphtheria-tetanus-pertussis vaccine; HR = hazard ratio; MV = measles vaccine; OPV = oral polio vaccine.
      Both analyses with OPV only as the comparator (Table II) failed the Schoenfeld test of the proportional hazards assumption. Visual inspection of the cumulative HRs and the partitions presented in Table II indicates that this could be explained by the estimated HR for DTP and OPV versus OPV being larger at ages 3–8 years than at ages 9–35 months although >1 in both intervals (HR not defined at 3–8 months because of no OPV deaths; HR = 2.08; 95% CI, 0.74–5.85 at 9–35 months). Hence, the cumulative HRs of the groups having received DTP and OPV and OPV only did not cross and stayed apart (Figure 4).
      Figure 4
      Figure 4Cumulative hazard ratios of different vaccination groups. DTP = diphtheria-tetanus-pertussis vaccine; OPV = oral polio vaccine.
      In the second analysis of children aged 3–8 months, before MV, children vaccinated with DTP only (13 deaths) had significantly higher mortality than children vaccinated with OPV only (0 deaths) (log-rank test: P = 0.006) (Table III). Furthermore, children vaccinated with DTP only had an HR of 3.92 (95% CI, 1.78–8.62) compared with unvaccinated children and a HR of 3.38 (95% CI, 1.59–7.20) compared with children vaccinated with DTP and OPV.
      Table IIIMortality rates and HRs from 3 to 8 months of age by most recent vaccination status (MV before 9 months has been censored).
      Vaccination GroupMortality per 100 Person-Years (Deaths/Person-Years)DTP-Only HR (95% CI)
      DTP only28.4 (13/45.8)Reference
      OPV only0 (0/28.8)P < 0.01
      Log-rank test for equality.
      Unvaccinated7.4 (15/203.7)3.92 (1.78–8.62)
      DTP with OPV8.5 (14/165.7)3.38 (1.59–7.20)
      DTP = diphtheria-tetanus-pertussis vaccine; HR = hazard ratio; MV = measles vaccine; OPV = oral polio vaccine.
      Log-rank test for equality.

       MV Period

      In the third analysis, we followed up all children after 9 months of age when most children receive MV (Table IV and Supplemental Figure 2). Compared with children who received MV after DTP, DTP-vaccinated and MV-unvaccinated children had a HR of 2.76 (95% CI, 1.36–5.59). This negative effect may have been more pronounced for girls (HR = 5.13; 95% CI, 1.52–17.28) than boys (HR = 1.76; 95% CI, 0.74–4.20) (test for interaction: P = 0.16) (Supplemental Table IV). Children with out-of-sequence vaccinations (ie, DTP with MV or DTP after MV) tended to have higher mortality than the MV after DTP children (Table IV and Supplemental Table V). When we compared no OPV with OPV among children who had DTP with or after MV, the HR was 6.25 (95% CI, 2.55–15.37); this effect may have been worse for girls, who had a HR of 13.31 (95% CI, 4.21–42.03) (test of interaction: P = 0.11) (Supplemental Table VI).
      Table IVMortality rates and hazard ratios (HR) by disjoint vaccination groups exploring timing of vaccination and most recent vaccination, children aged 9–36 months of age.
      Group (Most Recent Vaccines)Mortality Rate
      Mortality rate presented as (event/person-years) × 100.
      (Deaths/Person-Years)
      HR (95% CI) With MV After DTP as ReferenceHR (95% CI) With Unvaccinated as Reference
      MV after DTP (with or without OPV)2.2 (10/448.0)Reference0.53 (0.22–1.25)
      DTP with MV (with or without OPV)4.7 (14/295.6)1.96 (0.87–4.41)1.03 (0.47–2.26)
      DTP after MV (with or without OPV)2.9 (16/554.1)1.44 (0.65–3.18)0.76 (0.34–1.67)
      DTP with or without OPV (no MV)8.5 (46/543.3)2.76 (1.36–5.59)1.45 (0.77–2.74)
      Most recent vaccine containing only live vaccines, MV, and/or OPV (no DTP)4.1 (3/73.5)1.40 (0.38–5.12)0.74 (0.21–2.61)
      Unvaccinated (no DTP, MV, or OPV)6.0 (12/199.3)1.90 (0.80–4.49)Reference
      DTP with or after MV (with or without OPV)
      All groups represented in model with the 2 groups (DTP with MV [with or without OPV] and DTP after MV [with or without OPV]) combined.
      3.5 (30/849.7)1.64 (0.80–3.36)0.88 (0.43–1.77)
      DTP = diphtheria-tetanus-pertussis vaccine; HR = hazard ratio; MV = measles vaccine; OPV = oral polio vaccine.
      Mortality rate presented as (event/person-years) × 100.
      All groups represented in model with the 2 groups (DTP with MV [with or without OPV] and DTP after MV [with or without OPV]) combined.

      Discussion

       Main Observations

      Children who received only the OPV had lower mortality than children who had received DTP only or DTP and OPV. Although unvaccinated children were increasingly frail after 6 months of age, DTP vaccination was associated with slightly higher mortality than being unvaccinated. DTP vaccinated and MV-unvaccinated children had a nearly 3-fold higher mortality than children having MV after DTP as their most recent vaccination. Nonlive DTP was associated with negative NSEs,
      • Aaby P.
      • Ravn H.
      • Benn C.S.
      The WHO review of the possible non-specific effects of diphtheria-tetanus-pertussis vaccine.
      whereas live OPV and MV were associated with beneficial effects. OPV reduced the negative effects of DTP because children who received DTP and OPV had lower mortality than children who received DTP only at 3–8 months of age before MV and when vaccines were given out of sequence with MV after 9 months of age.

       Strengths and Weaknesses

      We used 40-year-old data to assess the effects of OPV and DTP, when given separately and combined. Few sites have data from when these vaccines were first introduced in a low-income countries with high mortality. When introducing these vaccines, there were many natural experiments in which only 1 vaccine was given. Because the removal of OPV is planned, it is particularly important to understand the nonspecific and specific effects of OPV.
      To ensure comparability, we included only children weighed in community examinations (Figure 2).
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      DTP and/or OPV vaccination was associated with higher mortality than being DTP unvaccinated (HR = 1.66; 95% CI, 1.03–2.69). This is bad news. First, the 5-fold increased mortality previously reported in 3- to 5-month-old children
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      cannot be dismissed as a random result because of small numbers in that age group. DTP-vaccinated children had 1.7 times higher mortality than unvaccinated children even though the frail and malnourished remained unvaccinated. Second, the higher mortality in girls after DTP is unnatural because girls did not have higher mortality than boys in the prevaccination era.
      • Aaby P.
      • Benn C.S.
      • Nielsen J.
      • Lisse I.M.
      • Rodrigues A.
      • Ravn H.
      Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
      The finding that, from 9 months of age, DTP-vaccinated and MV-unvaccinated children had higher mortality than children vaccinated with MV after DTP could possibly be explained by bias (eg, weak children not being vaccinated with MV). However, following this logic, unvaccinated children should be even weaker than DTP-vaccinated children and should have had higher HR compared with children vaccinated with MV after DTP. That was not the case (Table IV); unvaccinated children had only slightly higher mortality than children vaccinated with MV after DTP, and DTP-vaccinated children had higher mortality than unvaccinated children.
      Most studies of vaccines and their impact on survival report only 1 mortality estimate. However, the dynamics of vaccination programs imply that the unvaccinated group will become increasingly frail with age because mothers and health care workers are reluctant to vaccinate ill children. As a result, the HR changes over time. So, if the initial estimate was negative,
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      one might actually get a positive estimate by waiting until only very frail children are left in the unvaccinated group, as done in a study from Cebu, the Philippines.
      • Chan G.J.
      • Moulton L.H.
      • Becker S.
      • Muñoz A.
      • Black R.E.
      Non-specific effects of diphtheria-tetanus-pertussis vaccination on child mortality in Cebu, the Philippines.
      We followed up DTP-vaccinated and unvaccinated children to see whether frailty bias would eventually produce a beneficial HR for DTP, but that did not happen. From 3 to 5 months of age, when the comparison was a natural experiment, DTP-vaccinated children had an HR 5 times that of unvaccinated children.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      Subsequently, the HR decreased, but it continued to be at ≥1 among older children despite frailty bias.

       Comparison With Previous Studies of DTP and OPV

      In the WHO-sponsored meta-analysis of the NSEs of DTP, BCG, and MV, for child mortality before 5 years of age, DTP was associated with a 38% (95% CI, −8%–108%)
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      increase in mortality that was not statistically significant. However, the WHO-sponsored review included studies with survival bias. Excluding studies with survival bias, DTP-vaccinated children had a 2-fold higher mortality (HR = 2.00; 95% CI, 1.50–2.67).
      • Aaby P.
      • Ravn H.
      • Benn C.S.
      The WHO review of the possible non-specific effects of diphtheria-tetanus-pertussis vaccine.
      This unfortunate result is further strengthened by 2 recent studies of the introduction of DTP to children aged 3–5 months and 6–35 months; both found DTP to be associated with 2-fold higher mortality.
      • Aaby P.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • Benn C.S.
      Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.
      ,
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      Negative NSEs of DTP were stronger for girls than for boys.
      • Aaby P.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • Benn C.S.
      Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.
      ,
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • et al.
      The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
      The DTP estimates are consistent, but they may underestimate the harm from DTP: unvaccinated children are inherently disadvantaged; they have poorer WAZ scores, and their BHP card usually indicates that the child is sick or malnourished.
      This study found evidence that OPV has beneficial NSEs: OPV only was associated with a lower mortality than DTP only, and OPV and DTP were associated with a lower mortality than DTP only when given with or after MV. Only 2 previous studies have tested the NSEs of OPV: in an RCT in Guinea-Bissau, OPV at birth reduced infant mortality by 32% (range, 0%–57%) before the children received campaign OPV,
      • Lund N.
      • Andersen A.
      • Hansen A.S.
      • et al.
      The effect of oral polio vaccine at birth on infant mortality: a randomized trial.
      and, in natural experiments, campaigns with OPV were associated with 19% (range, 5%–32%) lower mortality.
      • Andersen A.
      • Fisker A.B.
      • Rodrigues A.
      • et al.
      National immunization campaigns with oral polio vaccine reduce all-cause mortality: a natural experiment within seven randomized trials.
      Hence, all the available studies point to OPV having beneficial NSEs.
      • Lund N.
      • Andersen A.
      • Hansen A.S.
      • et al.
      The effect of oral polio vaccine at birth on infant mortality: a randomized trial.
      • Andersen A.
      • Fisker A.B.
      • Rodrigues A.
      • et al.
      National immunization campaigns with oral polio vaccine reduce all-cause mortality: a natural experiment within seven randomized trials.
      • Aaby P.
      • Rodrigues A.
      • Biai S.
      • et al.
      Oral polio vaccination and low case fatality at the paediatric ward in Bissau, Guinea-Bissau.
      In other words, DTP and OPV have contrasting effects. When DTP was first introduced in 1984 in rural Guinea-Bissau,
      • Aaby P.
      • Jensen H.
      • Gomes J.
      • Fernandes M.
      • Lisse I.M.
      The introduction of diphtheria-tetanus-pertussis vaccine and child mortality in rural Guinea-Bissau: an observational study.
      OPV was not used, and the HR for DTP-vaccinated versus DTP-unvaccinated children was 5.00 (95% CI, 0.63–39.7). From 1985 to 1987, DTP and OPV were coadministered, and the HR was 1.90 (95% CI, 0.91–3.97). In the 3- to 8-month age group in the present study, DTP only was associated with higher mortality than DTP and OPV (HR = 3.38; 95% CI, 1.59–7.20). Likewise, when DTP was administered with MV or after MV, DTP only versus DTP and OPV was associated with higher mortality (HR = 6.25; 95% CI, 2.55–15.37).

       Interpretation

      Epidemiologic studies have found increased all-cause mortality associated with nonlive DTP vaccination compared with reduced all-cause mortality associated with live OPV and MV that is not explained by reduced mortality from polio or measles. The beneficial NSEs of live vaccines, including BCG and vaccinia, may relate to epigenetic reprogramming of the innate immune system, enhancing protection against unrelated infections.
      • Kleinnijenhuis J.
      • Quintin J.
      • Preijers F.
      • et al.
      Bacille Calmette-Guerin induces NOD2-dependent nonspecific protection from reinfection via epigenetic reprogramming of monocytes.
      • Blok B.A.
      • Jensen K.J.
      • Aaby P.
      • et al.
      Opposite effects of Vaccinia and modified Vaccinia Ankara on trained immunity.
      • Arts R.J.W.
      • Moorlag S.
      • Novakovic B.
      • et al.
      BCG vaccination protects against experimental viral infection in humans through the induction of cytokines associated with trained immunity.
      On the other hand, nonlive vaccines may have the opposite effect, including tolerance and increased susceptibility to unrelated infections.
      • Leentjens J.
      • Kox M.
      • Stokman R.
      • et al.
      BCG vaccination enhances the immunogenicity of subsequent influenza vaccination in healthy volunteers: a randomized, placebo-controlled pilot study.
      ,
      • Blok B.A.
      • de Bree L.C.J.
      • Diavatopoulos D.A.
      • et al.
      Interacting non-specific immunological effects of BCG and Tdapf vaccinations: an explorative randomized trial.
      In an experimental study among young Dutch girls, diphtheria and tetanus toxoids and acellular pertussis (DTaP)–inactivated polio vaccine (IPV) was associated with down-regulation of cytokine responses, which were abrogated if the child received BCG together with DTaP-IPV or BCG after DTaP-IPV.
      • Blok B.A.
      • de Bree L.C.J.
      • Diavatopoulos D.A.
      • et al.
      Interacting non-specific immunological effects of BCG and Tdapf vaccinations: an explorative randomized trial.
      After the WHO-sponsored review, WHO requested RCTs to settle the dispute about the importance of NSEs.
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      ,
      • Pollard A.J.
      • Finn A.
      • Curtis N.
      Non-specific effects of vaccines: plausible and potentially important, but implications uncertain.
      The Strategic Advisory Group of Experts on Immunization delegated the setting of priorities and planning of RCTs to the Immunization and Vaccines Related Implementation Research Advisory Committee; 6 years later nothing has happened in relation to assessing the potential negative NSEs of DTP in an RCT.
      Hence, we need to triangulate all available data to evaluate the impact of DTP. First, DTP would be expected to be associated with reduced mortality because inherent biases favor the DTP-vaccinated group.
      • Aaby P.
      • Ravn H.
      • Benn C.S.
      The WHO review of the possible non-specific effects of diphtheria-tetanus-pertussis vaccine.
      ,
      • Benn C.S.
      • Fisker A.B.
      • Rieckmann A.
      • Jensen A.G.
      • Aaby P.
      How to evaluate potential non-specific effects of vaccines: the quest for randomized trials or time for triangulation?.
      Second, all studies of DTP introduction have found a negative effect.
      • Aaby P.
      • Mogensen S.W.
      • Andersen A.
      • Rodrigues A.
      • Benn C.S.
      Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.
      Third, routine DTP is consistently associated with increased female mortality.
      • Aaby P.
      • Ravn H.
      • Fisker A.B.
      • et al.
      Is diphtheria-tetanus-pertussis (DTP) associated with increased female mortality? A meta-analysis testing the hypotheses of sex-differential non-specific effects of DTP vaccine.
      Fourth, DTP after MV is associated with increased female mortality.
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      ,
      • Aaby P.
      • Jensen H.
      • Samb B.
      • et al.
      Differences in female-male mortality after high-titre measles vaccine and association with subsequent vaccination with diphtheria-tetanus-pertussis and inactivated poliovirus: reanalysis of West African studies.
      Fifth, live high-titer MV (HTMV) was given at 4–5 months of age, and girls had a 2-fold increase in mortality if they received nonlive vaccines (DTP or IPV) after HTMV.
      • Aaby P.
      • Jensen H.
      • Samb B.
      • et al.
      Differences in female-male mortality after high-titre measles vaccine and association with subsequent vaccination with diphtheria-tetanus-pertussis and inactivated poliovirus: reanalysis of West African studies.
      Hence, HTMV became associated with 2-fold higher female mortality.
      • Aaby P.
      • Jensen H.
      • Samb B.
      • et al.
      Differences in female-male mortality after high-titre measles vaccine and association with subsequent vaccination with diphtheria-tetanus-pertussis and inactivated poliovirus: reanalysis of West African studies.
      Sixth, other nonlive vaccines (pentavalent, hepatitis B virus, IPV, RTS,S, and H1N1) are also associated with increased female mortality even though girls did not have higher mortality in the prevaccination era.
      • Aaby P.
      • Benn C.S.
      • Nielsen J.
      • Lisse I.M.
      • Rodrigues A.
      • Ravn H.
      Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
      ,
      • Andersen A.
      • Fisker A.B.
      • Rodrigues A.
      • et al.
      National immunization campaigns with oral polio vaccine reduce all-cause mortality: a natural experiment within seven randomized trials.
      ,
      • Aaby P.
      • Ravn H.
      • Fisker A.B.
      • et al.
      Is diphtheria-tetanus-pertussis (DTP) associated with increased female mortality? A meta-analysis testing the hypotheses of sex-differential non-specific effects of DTP vaccine.
      • Fisker A.B.
      • Biering-Sørensen S.
      • Lund N.
      • et al.
      Contrasting female-male mortality ratios after routine vaccinations with pentavalent versus measles and yellow fever vaccine: a cohort study from Guinea-Bissau.
      • Garly M.L.
      • Jensen H.
      • Martins C.L.
      • et al.
      Hepatitis B vaccination associated with higher female than male mortality in Guinea-Bissau: an observational study.
      • Klein S.L.
      • Shann F.
      • Moss W.J.
      • et al.
      RTS,S malaria vaccine and increased mortality in girls.
      • Aaby P.
      • Garly M.L.
      • Nielsen J.
      • et al.
      Increased female-male mortality ratio associated with inactivated polio and diphtheria-tetanus-pertussis vaccines: observations from vaccination trials in Guinea-Bissau.
      Seventh, a live vaccine given shortly after DTP may reduce the negative NSEs.
      • Aaby P.
      • Benn C.S.
      • Nielsen J.
      • Lisse I.M.
      • Rodrigues A.
      • Ravn H.
      Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
      With this level of consistency among independent observations, it is likely that DTP has negative effects. To paraphrase Cornfield, “If only one hypothesis can explain all the evidence, then the question is settled, even if the evidence is observational.”
      • Cornfield J.
      Principles of research.

       Implications

      More studies should investigate ways to reduce negative effects of DTP and pentavalent. One solution might be to replace the whole-cell pertussis vaccine in DTP with the live pertussis vaccine that has beneficial NSEs in animal studies.
      • Cauchi S.
      • Locht C.
      Non-specific effects of live attenuated pertussis vaccine against heterologous infectious and inflammatory diseases.
      We have previously reported that live vaccines may reduce the negative effect if given shortly after a nonlive vaccine
      • Aaby P.
      • Benn C.S.
      • Nielsen J.
      • Lisse I.M.
      • Rodrigues A.
      • Ravn H.
      Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
      ; however, when OPV campaigns were conducted before children received early MV, early MV did not reduce the mortality associated with DTP.
      • Fisker A.B.
      • Nebie E.
      • Schoeps A.
      • et al.
      A two-centre randomised trial of an additional early dose of measles vaccine: effects on mortality and measles antibody levels.
      Another strategy might be to give OPV with DTP or MV after DTP.
      • Aaby P.
      • Samb B.
      • Simondon F.
      • Coll Seck A.M.
      • Knudsen K.
      • Whittle H.
      Non-specific beneficial effect of measles immunisation: analysis of mortality studies from developing countries.
      ,
      • Benn C.S.
      • Fisker A.B.
      • Rieckmann A.
      • Sørup S.
      • Aaby P.
      Vaccinology: time to change paradigm?.
      In addition, administration of BCG with DTP may reduce the adverse effects of DTP,
      • Blok B.A.
      • de Bree L.C.J.
      • Diavatopoulos D.A.
      • et al.
      Interacting non-specific immunological effects of BCG and Tdapf vaccinations: an explorative randomized trial.
      ,
      • Aaby P.
      • Andersen A.
      • Ravn H.
      • Khalequzzaman Md
      Co-administration of BCG and diphtheria-tetanus-pertussis (DTP) vaccinations may reduce infant mortality more than the WHO-schedule of BCG first and then DTP. A Re-analysis of demographic surveillance data from rural Bangladesh.
      which suggests that it is important to conduct RCTs of giving (or not) a second dose of BCG with the third and last priming dose of DTP at 14 weeks of age. Such studies are possible and could contribute to bringing the negative effects of DTP under better control.
      Because polio is about to be eradicated, the use of OPV is planned to stop in 2024. Our findings suggest that withdrawing OPVs might increase child mortality.
      • Aaby P.
      • Benn C.
      Beneficial non-specific effects of oral polio vaccine: implications for the cessation of OPV?.
      OPV only was associated with lower mortality than DTP only, and OPV with DTP reduced the negative effects of DTP only. This possibility is supported by RCTs and observational studies of routine OPV vaccinations and OPV campaigns.
      • Lund N.
      • Andersen A.
      • Hansen A.S.
      • et al.
      The effect of oral polio vaccine at birth on infant mortality: a randomized trial.
      ,
      • Andersen A.
      • Fisker A.B.
      • Rodrigues A.
      • et al.
      National immunization campaigns with oral polio vaccine reduce all-cause mortality: a natural experiment within seven randomized trials.
      ,
      • Andersen A.
      • Fisker A.B.
      • Rodrigues A.
      • et al.
      National immunisation campaigns with oral polio vaccine may reduce all-cause mortality: an analysis of 13 years’ of demographic surveillance data from an urban African area.
      Hence, we need to mitigate the potential negative effects of removing a vaccine with highly beneficial NSEs.
      • Aaby P.
      • Benn C.
      Beneficial non-specific effects of oral polio vaccine: implications for the cessation of OPV?.
      ,
      • Aaby P.
      • Benn C.S.
      Stopping live vaccines after disease eradication may increase mortality.
      For example, we need to test whether more frequent use of MV and BCG can replace the beneficial effects of OPV when it is withdrawn.

      Conclusions

      It is a sad conclusion that DTP is associated with increased female mortality. No data, without survival bias, contradict this statement. Although the WHO-sponsored review concluded 6 years ago that DTP was associated with 38% (95% CI, −8%–108%) higher mortality and had completely different effects than BCG and MV,
      • Higgins J.P.
      • Soares-Weiser K.
      • Lopez-Lopez J.A.
      • et al.
      Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
      there has been no attempt to define how this issue should be resolved.
      DTP is the most commonly used vaccine, and the possibility that it might increase mortality demands that we urgently obtain more information about the effect of DTP on all-cause mortality.
      Given the beneficial NSEs of OPV, OPV should be tested to see if it reduces the risk of severe COVID-19 infection.
      • Chumakov K.
      • Benn C.S.
      • Aaby P.
      • Kottilli S.
      • Gallo R.
      Can existing live vaccines prevent COVID-19?.
      If such RCTs find that OPV reduces the risk of COVID-19, it may lead to a reconsideration of the current plans to stop using OPV.

      Funding Sources

      The present study and cleaning of the original data were supported by a common grant from DANIDA and the Novo Nordisk Foundation . The work on nonspecific effects of vaccines was supported by grant 104. Dan.8. f. from the Danish Council for Development Research , Ministry of Foreign Affairs , Denmark, Novo Nordisk Foundation , and grant Health-F3-2011-261375 from European Union FP7 support for OPTIMUNISE. CSB held starting grant ERC-2009-StG-243149 from the ERC. Research Centre for Vitamins and Vaccines is supported by grant DNRF108 from the Danish National Research Foundation. Peter Aaby, DMSc, held a research professorship grant from the Novo Nordisk Foundation. The funding agencies had no role in the study design, data collection, data analysis, data interpretation, or the writing of the report.

      Disclosures

      The authors have indicated that they have no conflicts of interest regarding the content of this article.

       Editor’s Note

      At the outset of the COVID-19 pandemic and while we await formal approval and widespread distribution of a specific vaccine, there has been a revival in the concept of the “collateral benefit” that other live vaccines could offer against SARS-CoV2.
      • Rabin R.C.
      Can an Old Vaccine Stop the New Coronavirus?.
      • Rabin R.C.
      Old Vaccines May Stop the Coronavirus, Study Hints. Scientists Are Skeptical.
      • Williams S.
      How Some Vaccines Protect Against More than Their Targets.
      This could be particularly useful in resource-limited settings where, due to disparities between rich and poor countries, the wait for COVID-19 vaccines may be several years.
      • Doucleff M.
      Developed Countries Plan To Start Vaccination Soon. What About The Rest Of The World?.
      In this issue of Clinical Therapeutics, we feature a paper by Dr. Peter Aaby and co-authors that describes work to support this hypothesis. They studied children who received oral polio vaccine in the country of Guinea-Bissau and demonstrated a decreased overall mortality beyond the direct attributable effect of the vaccine. This research is difficult to perform and often met with concerns about confounding factors that could account for the effect.
      • Aaby P.
      • Bukh J.
      • Lisse I.M.
      • Smits A.J.
      Measles vaccination and child mortality.
      However, we commend the authors for their efforts and are pleased to feature this content for our readers.
      Ravi Jhaveri
      Co-Editor-in-Chief

      Acknowledgment

      C. Bjerregård Øland and P. Aaby proposed the study. P. Aaby collected the original data. A. Rodrigues is responsible for the demographic surveillance system. SWM and P. Aaby cleaned the data. C. Bjerregård Øland and S. Wengel Mogensen conducted the statistical analyses. The first draft was written by P. Aaby and C. Bjerregård Øland; all authors contributed to the final version of the paper. C. Bjerregård Øland and P. Aaby will act as guarantors of the study.

      Appendix.

      Supplementary Figure 1
      Supplementary Figure 1Visual presentation of the mortality rates and the HR estimates for comparing different vaccines in different age groups. Mortality rates of the two groups compared are overlaid over each other (blue represent the comparator groups (i.e. DTP ± OPV or DTP-only) and black represent the baseline groups (i.e. unvaccinated or OPV-only)); mortality rate presented as (event/pyrs)∗100; the hazard ratio estimates of the Cox proportional hazards model are then presented graphically in a log transformed axis; Log-rank test for equality in case one arm had no fatalities.
      Supplementary Figure 2
      Supplementary Figure 2Visual presentation of the mortality rates and HR estimates for comparing different vaccines after introduction of measles vaccine. Mortality rate presented as (event/pyrs)∗100; Hazard ratio estimates of the Cox proportional hazards model are presented graphically in a log transformed axis.
      Supplementary Table 1Accumulated vaccination coverages up to 3 years of age.
      Year of birth1980

      N = 32
      1981

      N = 433
      1982

      N = 354
      1983

      N = 398
      1980–1983

      N = 1217
      Median age [IQR]
      DTP1182 (160–292)184 (126–350)157 (110–256)134 (101–203)155 (112–261)
      DTP3503 (397–731)519 (363–731)386 (265–580)353 (253–497)412 (283–603)
      OPV1182 (166–222)152.5 (113–263)155 (110–248.5)175 (115–284)161 (113–264)
      OPV3301 (267–488)386 (264–599)362 (263–574)498 (367–683)425 (280–633)
      MV1389 (344–713)387 (305–570)323 (285–433)349 (301–445)355 (298–487.5)
      BCG296.5 (292–301)66 (29–113)34 (10–64)33 (13–108.5)39 (15–94)
      Vaccine coverage at 6 months of age. (censoring children ending observation time before 6 months of age)
      DTP148.39%42.92%54.94%66.06%54.09%
      DTP33.23%2.59%3.20%7.51%4.39%
      OPV148.39%53.77%56.10%45.34%51.56%
      OPV33.23%3.07%3.20%3.11%3.12%
      MV10.00%2.83%0.00%0.26%1.10%
      BCG0.00%14.15%17.15%24.35%17.97%
      Vaccine coverage at 1 year of age. (censoring children ending observation time before 1 year of age)
      DTP186.21%67.26%75.87%88.51%77.07%
      DTP310.34%16.50%34.29%42.53%29.83%
      OPV189.66%73.60%79.37%73.56%75.69%
      OPV351.72%30.46%35.56%16.95%28.18%
      MV124.14%32.23%47.30%47.70%41.34%
      BCG6.90%14.47%18.10%26.15%19.06%
      Vaccine coverage at 3 years of age. (censoring children ending observation time before 3 years of age)
      DTP195.83%94.60%96.97%97.41%96.26%
      DTP362.50%75.18%85.28%86.30%81.44%
      OPV195.83%93.17%95.67%94.44%94.40%
      OPV387.50%73.02%80.09%78.15%77.21%
      MV179.17%80.58%85.28%91.48%85.55%
      BCG4.17%16.91%18.18%31.11%21.67%
      Note: N denotes the total number of children in the group.
      Supplementary Table 2Background factors for different most recent vaccination groups observed between 12 and 35 months of age.
      Unvaccinated

      N = 86
      DTP-only [no MV]

      N = 94
      DTP + OPV [no MV]

      N = 186
      OPV-only [no MV]

      N = 48
      MV-after-DTP

      N = 314
      MV- with-DTP

      N = 229
      DTP-after-MV

      N = 439
      MV ± OPV

      No DTP

      N = 41
      Birth WAZ (SD) [N]
      Birth WAZ defined as the WAZ measured at or before 14 days of age.
      −0.02 (0.95) [25]−0.50 (1.21) [40]−0.22 (1.06) [74]−0.41 (1.35) [22]−0.23 (0.96) [146]−0.28 (1.03) [93]−0.18 (0.89) [172]−1.03 (1.22) [14]
      Mean WAZ (SD) [N] at 12–23 months of age−1.44 (1.50) [45]−1.20 (1.31) [62]−1.17 (1.15) [132]−1.33 (1.30) [36]−0.83 (1.06) [270]−1.16 (1.17) [179]−1.04 (1.10) [337]−0.94 (1.48) [30]
      Mean WAZ (SD) [N] at 24–35 months of age−0.66 (0.59) [4]−0.94 (1.67) [17]−1.29 (1.22) [38]−1.07 (0.81) [11]−1.03 (0.99) [212]−1.17 (1.01) [124]−1.10 (0.96) [330]−1.12 (0.59) [7]
      Male sex59.3%52.1%49.5%41.7%50.3%49.8%51.3%43.9%
      Twin4.7%4.3%3.2%4.2%2.9%3.1%2.1%4.9%
      Ethnic group
       Pepel50.0%51.1%52.2%60.4%50.6%53.3%54.4%61.0%
       Balanta15.1%13.8%15.1%12.5%15.9%11.8%13.2%4.9%
       Other34.9%35.1%32.8%27.1%33.4%34.9%32.3%34.1%
      Examination rate
      Examination rate calculated as the number of all observations starting the observation with the child being present divided by PYRS in the specified age period.
      [event/PYRS]
      0.34 [46/136.2]0.93 [57/61.4]0.60 [120/199.1]0.57 [26/45.7]1.77 [201/113.5]1.59 [172/107.9]2.26 [237/104.7]1.21 [29/24.0]
      Note: N denotes the number of children having been in the group; only one landmark outside these definitions due to most recent vaccine being OPV with no DTP but having received MV (did not die); only using one observation per child but allowing a child to be included in all groups once.
      a Birth WAZ defined as the WAZ measured at or before 14 days of age.
      b Examination rate calculated as the number of all observations starting the observation with the child being present divided by PYRS in the specified age period.
      Supplementary Table 3Mortality rates and hazard ratios (HR) by most recent vaccination, sex and age; MV has been censored.
      GirlsMortality rate [Deaths/Pyrs]HR (CI)
      Age groupsUnvaccinatedDTP + OPVDTP-OnlyOPV-onlyDTP ± OPVDTP ± OPV vs. UnvaccinatedDTP-only vs. UnvaccinatedDTP ± OPV vs. OPV-onlyDTP-only vs.

      OPV-only
      3–5 months1.7 [1/58.4]11.6 [3/25.8]27.2 [2/7.4]0 [0/4.1]15.1 [5/33.1]11.07 (1.23–99.31)20.05 (1.74–230.62)P = 0.43aP = 0.33a
      6–11 months10.2 [6/58.7]13.1 [14/107.2]11.8 [3/25.5]0 [0/19.6]12.8 [17/132.7]1.17 (0.46–2.98)1.07 (0.27–4.29)P = 0.10aP = 0.15a
      12–23 months4.3 [2/46.3]7.6 [7/92.0]0 [0/23.6]4.2 [1/24.0]6.1 [7/115.6]1.37 (0.29–6.61)P = 0.30a1.43 (0.18–11.63)P = 0.29a
      24–35 months0 [0/15.5]7.6 [3/39.7]7.5 [1/13.3]7.6 [1/13.2]7.5 [4/53.0]P = 0.28aP = 0.24a0.97 (0.11–8.64)0.98 (0.06–15.61)
      3–35 months5.0 [9/179.0]10.2 [27/264.7]8.6 [6/69.8]3.3 [2/60.9]9.9 [33/334.4]2.13 (1.00–4.54)1.90 (0.67–5.43)2.81 (0.67–11.71)2.51 (0.51–12.45)
      BoysMortality rate [Deaths/Pyrs]HR (CI)
      Age groupsUnvaccinatedDTP + OPVDTP-OnlyOPV-onlyDTP ± OPVDTP ± OPV vs. UnvaccinatedDTP-only vs. UnvaccinatedDTP ± OPV vs. OPV-onlyDTP-only vs.

      OPV-only
      3–5 months7.5 [5/66.9]15.7 [4/25.5]46.3 [4/8.6]0 [0/4.1]23.4 [8/34.2]3.79 (1.16–12.39)7.31 (1.86–28.76)P = 0.33aP = 0.19a
      6–11 months8.3 [6/72.5]10.6 [11/103.8]16.2 [5/30.8]4.6 [1/21.6]11.2 [16/134.6]1.36 (0.53–3.48)1.86 (0.57–6.09)2.66 (0.35–20.09)3.64 (0.42–31.15)
      12–23 months8.2 [5/60.9]6.5 [6/91.6]7.8 [3/38.4]0 [0/21.3]6.9 [9/130.0]0.84 (0.28–2.51)0.94 (0.22–3.93)P = 0.22aP = 0.19a
      24–35 months4.2 [1/23.7]4.2 [2/47.2]6.4 [1/15.7]11.3 [1/8.9]4.8 [3/62.9]1.15 (0.12–11.03)1.55 (0.10–24.77)0.41 (0.04–3.93)0.55 (0.03–8.81)
      3–35 months7.6 [17/224.0]8.6 [23/268.1]13.9 [13/93.6]3.6 [2/55.9]10.0 [36/361.7]1.43 (0.78–2.59)2.03 (0.97–4.26)2.80 (0.67–11.64)4.00 (0.90–17.73)c
      Note: Mortality rate presented as (event/Pyrs)∗100; a: log-rank test for equality; b: children with no registered sex included; c: HR fails the Schoenfeld residual test of the proportional hazards assumption (p < 0.05).
      Supplementary Table 4Mortality rates and hazard ratios (HR) by disjoint vaccination groups exploring timing of vaccination and most recent vaccination, and sex, children aged 9–36 months of age.
      GroupsSexMortality rate [deaths/Pyrs]HR (CI) [With MV-after-DTP as reference]HR (CI) [With unvaccinated as reference]
      MV-after-DTP (±OPV)Girl1.3 [3/234.0]Ref.0.39 (0.09–1.78)
      Boy3.3 [7/214.0]Ref.0.65 (0.23–1.84)
      DTP-with-MV (±OPV)Girl4.5 [7/156.3]3.18 (0.82–12.30)1.25 (0.36–4.31)
      Boy5.0 [7/139.3]1.44 (0.50–4.10)0.94 (0.34–2.62)
      DTP-after-MV (±OPV)Girl2.9 [8/271.9]2.53 (0.67–9.55)
      HR fails the Schoenfeld test for the proportional hazards assumption.
      1.00 (0.29–3.40)
      Boy2.8 [8/282.2]0.97 (0.35–2.69)0.64 (0.23–1.75)
      DTP ± OPV (no MV)Girl9.1 [24/262.8]5.13 (1.52–17.28)2.02 (0.70–5.82)
      Boy7.8 [22/280.4]1.76 (0.74–4.20)1.15 (0.51–2.59)
      Most recent vaccine containing only Live vaccines, MV and/or OPV (no DTP)Girl5.2 [2/38.4]3.07 (0.51–18.50)1.21 (0.22–6.60)
      Boy2.9 [1/34.3]0.69 (0.08–5.65)0.45 (0.06–3.62)
      Unvaccinated (No DTP, MV or OPV)Girl4.7 [4/85.0]2.54 (0.56–11.51)Ref.
      Boy7.0 [8/114.3]1.52 (0.54–4.29)Ref.
      Note: Mortality rate presented as (event/Pyrs)∗100; There were no registered sex for one landmark/sub-child (did not die).
      a HR fails the Schoenfeld test for the proportional hazards assumption.
      Supplementary Table 5Mortality rates and hazard ratios (HR) by disjoint vaccination groups exploring timing of vaccination and most recent vaccination groups, and OPV, children aged 9–36 months of age.
      Most recent vaccination(s)Mortality rate [Deaths/Pyrs]HR (CI)
      1MV-after-DTP and OPV2.0 [9/445.6]Ref.
      2MV-after-DTP-only41.9 [1/2.4]18.72 (2.36–148.30)
      3DTP-with-MV and OPV3.6 [10/278.8]1.65 (0.67–4.07)
      4DTP-with-MV, no OPV23.8 [4/16.8]10.18 (3.12–33.20)
      5DTP-after-MV and OPV2.6 [14/541.9]1.41 (0.61–3.28)
      6DTP-after-MV, no OPV16.4 [2/12.2]8.22 (1.77–38.11)
      7DTP-only7.0 [4/57.6]2.48 (0.75–8.19)
      8DTP-only, OPV previously, no MV3.3 [2/59.9]1.30 (0.28–6.05)
      9DTP and OPV, no MV9.8 [36/367.1]3.52 (1.66–7.49)
      10OPV-only, DTP previously, no MV6.8 [4/58.6]2.65 (0.81–8.68)
      11OPV-only0 [0/29.4]P = 0.28
      Log-rank test for equality.
      12MV and OPV, no DTP8.0 [2/25.1]3.06 (0.65–14.28)
      13MV, no DTP and OPV5.4 [1/18.7]2.44 (0.31–19.31)
      14Unvaccinated (No DTP, MV and OPV)6.0 [12/199.3]2.13 (0.88–5.17)
      Note: Mortality rate presented as (event/Pyrs)∗100; one landmark not included since the landmark had last vaccine VP but had MV and BCG and no DTP (did not die); using only the first dose of MV.
      a Log-rank test for equality.
      Supplementary Table 6Mortality rates and hazard ratios (HR) for out-of-sequence Vaccinations with MV and DTP, children aged 9–36 months of age
      Groups from 9 to 36 months of age; most recent vaccinationGroupMortality rate (deaths/100 person-years)HR (CI)
      DTP-with-or-after-MV and OPVAll2.9 (24/820.8)Ref
      Female2.6 (11/417.9)Ref
      Male3.2 (13/402.9)Ref
      DTP-with-or-after-MV, no OPVAll20.7 (6/29.0)6.25 (2.55–15.37)
      Female38.6 (4/10.4)13.31 (4.21–42.03)
      Male10.7 (2/18.6)2.92 (0.66–12.96)

      References

        • Expanded Programme on Immunization
        The optimal age for measles immunization.
        Weekly Epidemiol Rec. 1982; 57: 89-91
        • Aaby P.
        • Bukh J.
        • Lisse I.M.
        • Smits A.J.
        Measles vaccination and child mortality.
        Lancet. 1981; ii: 93
        • Aaby P.
        • Bukh J.
        • Lisse I.M.
        • Smits A.J.
        Measles vaccination and reduction in child mortality: a community study from Guinea-Bissau.
        J Infect. 1984; 8: 13-21
        • Aaby P.
        • Bhuyia A.
        • Nahar L.
        • Knudsen K.
        • de Francisco A.
        • Strong M.
        The survival benefit of measles immunization may not be explained entirely by the prevention of measles disease: a community study from rural Bangladesh.
        Int J Epidemiol. 2003; 32: 106-115
        • Holt E.A.
        • Boulos R.
        • Halsey N.A.
        • et al.
        Childhood survival in Haiti: protective effect of measles vaccination.
        Pediatrics. 1990; 86: 188-194
        • The Kasongo Project Team
        Influence of measles vaccination on survival pattern of 7-35-month-old children in Kasongo, Zaire.
        Lancet. 1981; i: 764-767
        • Aaby P.
        • Samb B.
        • Simondon F.
        • Coll Seck A.M.
        • Knudsen K.
        • Whittle H.
        Non-specific beneficial effect of measles immunisation: analysis of mortality studies from developing countries.
        Br Med J. 1995; 311: 481-485
        • Higgins J.P.
        • Soares-Weiser K.
        • Lopez-Lopez J.A.
        • et al.
        Association of BCG, DTP, and measles containing vaccines with childhood mortality: systematic review.
        BMJ. 2016; 355: i5170
      1. Strategic advisory group of experts on immunization.
        Week Epidemiol Rec. 2014; 89: 233-235
        • Aaby P.
        • Jensen H.
        • Samb B.
        • et al.
        Differences in female-male mortality after high-titre measles vaccine and association with subsequent vaccination with diphtheria-tetanus-pertussis and inactivated poliovirus: reanalysis of West African studies.
        Lancet. 2003; 361: 2183-2188
        • Aaby P.
        • Jensen H.
        • Gomes J.
        • Fernandes M.
        • Lisse I.M.
        The introduction of diphtheria-tetanus-pertussis vaccine and child mortality in rural Guinea-Bissau: an observational study.
        Int J Epidemiol. 2004; 33: 374-380
        • Aaby P.
        • Benn C.S.
        • Nielsen J.
        • Lisse I.M.
        • Rodrigues A.
        • Ravn H.
        Testing the hypothesis that diphtheria-tetanus-pertussis vaccine has negative non-specific and sex-differential effects on child survival in high-mortality countries.
        BMJ Open. 2012; 2e000707
        • Aaby P.
        • Mogensen S.W.
        • Andersen A.
        • Rodrigues A.
        • Benn C.S.
        Evidence of increase in mortality after the introduction of diphtheria-tetanus-pertussis vaccine to children aged 6-35 months in Guinea-Bissau: a time for reflection?.
        Front Public Health. 2018; 6: 79
        • Benn C.S.
        • Fisker A.B.
        • Rieckmann A.
        • Sørup S.
        • Aaby P.
        Vaccinology: time to change paradigm?.
        Lancet Inf Dis. 2020; 20: e274-e283
        • Aaby P.
        • Fisker A.B.
        • Björkman
        • Benn C.S.
        Rolling out the RTS,S malaria vaccine: to test or not to test the effect on mortality?.
        BMJ. 2020; 368: l6920
        • Mogensen S.W.
        • Andersen A.
        • Rodrigues A.
        • et al.
        The introduction of diphtheria-tetanus-pertussis and oral polio vaccine among young infants in an urban African community: a natural experiment.
        EBioMedicine. 2017; 17: 192-198
        • Smedman L.
        • Aaby P.
        • Lindeberg A.
        • Zetterstrom R.
        Survival 0-6 years of age in a periurban community in Guinea-Bissau: a longitudinal assessment.
        Ann Trop Pediatr. 1986; 6: 67-72
        • Aaby P.
        • Bukh J.
        • Lisse I.M.
        • Smits A.J.
        Measles mortality, state of nutrition, and family structure: a community study from Guinea-Bissau.
        J Infect Dis. 1983; 147: 693-701
        • Mogensen S.W.
        • Aaby P.
        • Smedman L.
        • et al.
        Introduction of standard measles vaccination in an urban African community in 1979 and overall child survival: a reanalysis of data from a cohort study.
        BMJ Open. 2016; 6e011317
        • Jensen H.
        • Benn C.S.
        • Lisse I.M.
        • Rodrigues A.
        • Andersen P.K.
        • Aaby P.
        Survival bias in observational studies of the impact of routine vaccinations on childhood survival.
        Trop Med Int Health. 2007; 12: 5-14
        • Aaby P.
        • Ravn H.
        • Benn C.S.
        The WHO review of the possible non-specific effects of diphtheria-tetanus-pertussis vaccine.
        Pediatr Infect Dis J. 2016; 35: 1247-1257
        • Chan G.J.
        • Moulton L.H.
        • Becker S.
        • Muñoz A.
        • Black R.E.
        Non-specific effects of diphtheria-tetanus-pertussis vaccination on child mortality in Cebu, the Philippines.
        Int J Epidemiol. 2007; 36: 1022-1029
        • Lund N.
        • Andersen A.
        • Hansen A.S.
        • et al.
        The effect of oral polio vaccine at birth on infant mortality: a randomized trial.
        Clin Infect Dis. 2015; 61: 1504-1511
        • Andersen A.
        • Fisker A.B.
        • Rodrigues A.
        • et al.
        National immunization campaigns with oral polio vaccine reduce all-cause mortality: a natural experiment within seven randomized trials.
        Front Public Health. 2018; 6: 13
        • Aaby P.
        • Rodrigues A.
        • Biai S.
        • et al.
        Oral polio vaccination and low case fatality at the paediatric ward in Bissau, Guinea-Bissau.
        Vaccine. 2004; 22: 3014-3017
        • Kleinnijenhuis J.
        • Quintin J.
        • Preijers F.
        • et al.
        Bacille Calmette-Guerin induces NOD2-dependent nonspecific protection from reinfection via epigenetic reprogramming of monocytes.
        Proc Natl Acad Sci U S A. 2012; 109: 17537-17542
        • Blok B.A.
        • Jensen K.J.
        • Aaby P.
        • et al.
        Opposite effects of Vaccinia and modified Vaccinia Ankara on trained immunity.
        Eur J Clin Microbiol Infect Dis. 2019; 38: 449-456
        • Arts R.J.W.
        • Moorlag S.
        • Novakovic B.
        • et al.
        BCG vaccination protects against experimental viral infection in humans through the induction of cytokines associated with trained immunity.
        Cell Host Microbe. 2018; 23: 89-100 e5
        • Leentjens J.
        • Kox M.
        • Stokman R.
        • et al.
        BCG vaccination enhances the immunogenicity of subsequent influenza vaccination in healthy volunteers: a randomized, placebo-controlled pilot study.
        J Infect Dis. 2015; 212: 1930-1938
        • Blok B.A.
        • de Bree L.C.J.
        • Diavatopoulos D.A.
        • et al.
        Interacting non-specific immunological effects of BCG and Tdapf vaccinations: an explorative randomized trial.
        Clin Inf Dis. 2019; https://doi.org/10.1093/cid/ciz246
        • Pollard A.J.
        • Finn A.
        • Curtis N.
        Non-specific effects of vaccines: plausible and potentially important, but implications uncertain.
        Arch Dis Child. 2017; 102: 1077-1081
        • Benn C.S.
        • Fisker A.B.
        • Rieckmann A.
        • Jensen A.G.
        • Aaby P.
        How to evaluate potential non-specific effects of vaccines: the quest for randomized trials or time for triangulation?.
        Expert Rev Vac. 2018; 17: 411-420
        • Aaby P.
        • Ravn H.
        • Fisker A.B.
        • et al.
        Is diphtheria-tetanus-pertussis (DTP) associated with increased female mortality? A meta-analysis testing the hypotheses of sex-differential non-specific effects of DTP vaccine.
        Trans R Soc Trop Med Hyg. 2016; 110: 570-581
        • Fisker A.B.
        • Biering-Sørensen S.
        • Lund N.
        • et al.
        Contrasting female-male mortality ratios after routine vaccinations with pentavalent versus measles and yellow fever vaccine: a cohort study from Guinea-Bissau.
        Vaccine. 2016; 34: 4551-4557
        • Garly M.L.
        • Jensen H.
        • Martins C.L.
        • et al.
        Hepatitis B vaccination associated with higher female than male mortality in Guinea-Bissau: an observational study.
        Pediatr Infect Dis J. 2004; 23: 1086-1092
        • Klein S.L.
        • Shann F.
        • Moss W.J.
        • et al.
        RTS,S malaria vaccine and increased mortality in girls.
        MBio. 2016; 7
        • Aaby P.
        • Garly M.L.
        • Nielsen J.
        • et al.
        Increased female-male mortality ratio associated with inactivated polio and diphtheria-tetanus-pertussis vaccines: observations from vaccination trials in Guinea-Bissau.
        Pediatr Infect Dis J. 2007; 26: 247-252
        • Cornfield J.
        Principles of research.
        Am J Ment Defic. 1959; 64: 240-252
        • Cauchi S.
        • Locht C.
        Non-specific effects of live attenuated pertussis vaccine against heterologous infectious and inflammatory diseases.
        Front Immunol. 2018; 9: 2872
        • Fisker A.B.
        • Nebie E.
        • Schoeps A.
        • et al.
        A two-centre randomised trial of an additional early dose of measles vaccine: effects on mortality and measles antibody levels.
        Clin Infect Dis. 2017 Nov 21; https://doi.org/10.1093/cid/cix1033
        • Aaby P.
        • Andersen A.
        • Ravn H.
        • Khalequzzaman Md
        Co-administration of BCG and diphtheria-tetanus-pertussis (DTP) vaccinations may reduce infant mortality more than the WHO-schedule of BCG first and then DTP. A Re-analysis of demographic surveillance data from rural Bangladesh.
        EBioMedicine. 2017; 22: 173-180
        • Aaby P.
        • Benn C.
        Beneficial non-specific effects of oral polio vaccine: implications for the cessation of OPV?.
        Clin Inf Dis. 2017; 65: 420-421
        • Andersen A.
        • Fisker A.B.
        • Rodrigues A.
        • et al.
        National immunisation campaigns with oral polio vaccine may reduce all-cause mortality: an analysis of 13 years’ of demographic surveillance data from an urban African area.
        Clin Infect Dis. 2020 Sep 19; (in press)https://doi.org/10.1093/cid/ciaa1351/5909021
        • Aaby P.
        • Benn C.S.
        Stopping live vaccines after disease eradication may increase mortality.
        Vaccine. 2020; 38: 10-14
        • Chumakov K.
        • Benn C.S.
        • Aaby P.
        • Kottilli S.
        • Gallo R.
        Can existing live vaccines prevent COVID-19?.
        Science. 2020; 368: 1187-1188