The Introduction of Diphtheria-Tetanus-Pertussis and Oral Polio Vaccine Among Young Infants in an Urban African Community: A Natural Experiment

Abstract

Background: We examined the introduction of diphtheria-tetanus-pertussis (DTP) and oral polio vaccine (OPV) in an urban community in Guinea-Bissau in the early 1980s.

Methods: The child population had been followed with 3-monthly nutritional weighing sessions since 1978. From June 1981 DTP and OPV were offered from 3months of age at these sessions. Due to the 3-monthly intervals between sessions, the children were allocated by birthday in a 'natural experiment' to receive vaccinations early or late between 3 and 5months of age. We included children who were <6months of age when vaccinations started and children born until the end of December 1983. We compared mortality between 3 and 5months of age of DTP-vaccinated and not-yet-DTP-vaccinated children in Cox proportional hazard models.

Results: Among 3-5-month-old children, having received DTP (±OPV) was associated with a mortality hazard ratio (HR) of 5.00 (95% CI 1.53-16.3) compared with not-yet-DTP-vaccinated children. Differences in background factors did not explain the effect. The negative effect was particularly strong for children who had received DTP-only and no OPV (HR=10.0 (2.61-38.6)). All-cause infant mortality after 3months of age increased after the introduction of these vaccines (HR=2.12 (1.07-4.19)).

Conclusion: DTP was associated with increased mortality; OPV may modify the effect of DTP.

Keywords: DTP; Diphtheria-tetanus-pertussis vaccine; Measles vaccine; Non-specific effects of vaccines; Oral polio vaccine.

Fig. 1

Each vaccination of the specified type is plotted according age of the recipient and date of vaccination.

Fig. 2

Flowchart of study population and children included in the analyses. Notes: DOB = date of birth; [] indicates the number of deaths before 6 months of age in the group.

Fig. 3

Natural experiment study design. Note: Children were weighed every third month. After 3 months of age they received DTP and OPV on weighing days if they were healthy. Children who attended but were not vaccinated at a weighing session after 3 months of age were censored in the survival analysis comparing DTP-vaccinated and unvaccinated children.