The non-specific effects of vaccines and other childhood interventions: the contribution of INDEPTH Health and Demographic Surveillance Systems

Abstract

Most childhood interventions (vaccines, micronutrients) in low-income countries are justified by their assumed effect on child survival. However, usually the interventions have only been studied with respect to their disease/deficiency-specific effects and not for their overall effects on morbidity and mortality. In many situations, the population-based effects have been very different from the anticipated effects; for example, the measles-preventive high-titre measles vaccine was associated with 2-fold increased female mortality; BCG reduces neonatal mortality although children do not die of tuberculosis in the neonatal period; vitamin A may be associated with increased or reduced child mortality in different situations; effects of interventions may differ for boys and girls. The reasons for these and other contrasts between expectations and observations are likely to be that the immune system learns more than specific prevention from an intervention; such training may enhance or reduce susceptibility to unrelated infections. INDEPTH member centres have been in an ideal position to document such additional non-specific effects of interventions because they follow the total population long term. It is proposed that more INDEPTH member centres extend their routine data collection platform to better measure the use and effects of childhood interventions. In a longer perspective, INDEPTH may come to play a stronger role in defining health research issues of relevance to low-income countries.

Keywords: BCG; DTP; INDEPTH Network; childhood interventions; measles vaccine; non-specific effects of vaccines.

Figure 1.

Hazard ratios (HR) for BCG scar-positive vs scar-negative individuals and for tuberculin-skin-test- (TST)-positive vs TST-negative individuals among BCG-vaccinated children in urban Guinea-Bissau, 2000–02.

Figure 2.

Kaplan–Meier survival curves for unvaccinated children and recipients of DTP in rural areas of Guinea-Bissau, 1984–87. Note: the graph shows mortality during 6 months of follow-up for DTP-vaccinated (trace 2) and DTP-unvaccinated (trace 1) children aged 2–8 months at the initial visit to their villages. Unvaccinated children received no DTP because they were travelling on day of vaccination, were too sick to get vaccinated and had lower nutritional status than DTP vaccinated children, or were visited on days when the team for logistic reasons had no vaccines. The adjusted mortality rate ratio for DTP-vaccinated (trace 2) vs DTP-unvaccinated (trace 1) children was 1.92 (1.04–3.52).