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?

Abstract

Background: Whole-cell diphtheria-tetanus-pertussis (DTP) and oral polio vaccine (OPV) were introduced to children in Guinea-Bissau in 1981. We previously reported that DTP in the target age group from 3 to 5 months of age was associated with higher overall mortality. DTP and OPV were also given to older children and in this study we tested the effect on mortality in children aged 6-35 months.

Methods: In the 1980s, the suburb Bandim in the capital of Guinea-Bissau was followed with demographic surveillance and tri-monthly weighing sessions for children under 3 years of age. From June 1981, routine vaccinations were offered at the weighing sessions. We calculated mortality hazard ratio (HR) for DTP-vaccinated and DTP-unvaccinated children aged 6-35 months using Cox proportional hazard models. Including this study, the introduction of DTP vaccine and child mortality has been studied in three studies; we made a meta-estimate of these studies.

Results: At the first weighing session after the introduction of vaccines, 6-35-month-old children who received DTP vaccination had better weight-for-age z-scores (WAZ) than children who did not receive DTP; one unit increase in WAZ was associated with an odds ratio of 1.32 (95% CI = 1.13-1.55) for receiving DTP vaccination. Though lower mortality compared with not being DTP-vaccinated was, therefore, expected, DTP vaccination was associated with a non-significant trend in the opposite direction, the HR being 2.22 (0.82-6.04) adjusted for WAZ. In a sensitivity analysis, including all children weighed at least once before the vaccination program started, DTP (±OPV) as the most recent vaccination compared with live vaccines or no vaccine was associated with a HR of 1.89 (1.00-3.55). In the three studies of the introduction of DTP in rural and urban Guinea-Bissau, DTP-vaccinated children had an HR of 2.14 (1.42-3.23) compared to DTP-unvaccinated children; this effect was separately significant for girls [HR = 2.60 (1.57-4.32)], but not for boys [HR = 1.71 (0.99-2.93)] (test for interaction p = 0.27).

Conclusion: Although having better nutritional status and being protected against three infections, 6-35 months old DTP-vaccinated children tended to have higher mortality than DTP-unvaccinated children. All studies of the introduction of DTP have found increased overall mortality.

Keywords: bias in vaccine studies; diphtheria–tetanus–pertussis vaccine; heterologous effects; measles vaccine; non-specific effects of vaccines; oral polio vaccine.

Figure 1

Flowchart of study population and children included in the analyses. DOB, date of birth; [], died during follow-up. Children were only included in the main analysis if they had taken part in a weighing session after October 1, 1980.

Figure 2

Examinations are plotted on the axes of age and calendar time. Each dot corresponds to a weighing examination of a child. The gray triangle illustrates the age groups and periods, where follow-up time was included in the survival analysis. The approximate tri-monthly regime of examinations is visible in the distribution of dots on the horizontal axis.

Figure 3

Meta-analysis of the three studies of the introduction of diphtheria–tetanus–pertussis. The fixed effects model gave an estimate of 2.14 (1.42–3.23) and the random effects model gave 2.17 (1.39–3.38).