Sex-differential and non-specific effects of routine vaccinations in a rural area with low vaccination coverage: an observational study from Senegal.

BACKGROUND

We examined the potential sex-differential and non-specific effects of bacille Calmette-Guérin (BCG), diphtheria-tetanus-pertussis (DTP) and measles vaccine (MV) in a rural area of Senegal.

METHODS

The 4133 children born in the area between 1996 and 1999 were included in the study. Vaccinations were provided at three health centres. Vaccine information was collected through 3-monthly home visits. The survival analysis compared the effects of BCG and DTP according to the following sequence of vaccinations: BCG-first, BCG+DTP1-first, or DTP1-first. We compared DTP and MV between 9 and 24 months of age, as 9 months is the minimum age for MV.

RESULTS

At 12 months the vaccination coverage was 44%, 46% and 9%, respectively, for BCG, DTP1 and MV. Most children received BCG+DTP1-first and this combination was associated with a significantly lower mortality rate ratio (MRR) of 0.69 (0.53-0.89) compared with unvaccinated children. There was no benefit for children receiving BCG-first or DTP1-first. The female-male MRR was 0.79 (0.64-0.96) among unvaccinated children, but was significantly inversed with 1.45 (1.00-2.10) for children receiving DTP vaccination (test of homogeneity, p=0.006). Children who had received DTP simultaneously with MV or DTP after MV had significantly higher mortality (MRR=2.59 [1.32-5.07]) compared with children having MV-only as their most recent vaccination. After 9 months, the female-male MRR was 0.61 (0.31-1.19) for measles-vaccinated children but remained 1.54 (1.03-2.31) for DTP-vaccinated children who had not received MV (p=0.01).

CONCLUSIONS

The sequence of routine vaccinations is important for the overall impact on child survival and these vaccines are associated with sex-differential effects.