International Journal of Epidemiology, Volume 33, Number 2, pp. 367-373
IJE vol.33 no.2 © International Epidemiological Association 2004; all rights reserved.
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Divergent female–male mortality ratios associated with different routine vaccinations among female–male twin pairs
1 Bandim Health Project, Apartado 861, Bissau, Guinea-Bissau
2 Danish Epidemiology Science Centre, Artillerivej 5, 2300 Copenhagen S, Denmark
3 IRD, UR 24 Epidemiology and Prevention Research Unit, Dakar, Senegal
Correspondence: P Aaby, Statens Serum Institut, Artillerivej 5, 2300 Copenhagen S, Denmark. E-mail: psb{at}mail.gtelecom.gw
Background Observational studies have suggested that vaccinations have non-specific effects that differ by sex. In the absence of randomized trials, studies of female–male twin pairs would allow us to investigate whether an intervention had sex-specific effects on survival. We therefore examined mortality patterns among female–male twin pairs according to vaccination status.
Design We identified female–male twin pairs using the population registers from one urban district and three rural studies from Guinea-Bissau and Senegal and examined the female–male mortality ratio (MR) according to the last vaccine received among pairs in which a death occurred before 18 months of age. As background information, we examined sex- and age-specific mortality patterns in the pre-vaccination era.
Subjects In all, 626 female–male twin pairs identified between 1978 and 2000.
Results There was no sex difference in mortality for boys and girls in the pre-vaccination era. In the combined analysis of all studies, the female–male MR was 0.25 (95% CI: 0.05, 0.93) for pairs having received Bacille Calmette-Guerin (BCG) as the last vaccine, 7.33 (95% CI: 2.20, 38.3) for pairs having received diphtheria, tetanus, pertussis (DTP) as the last vaccine, and 0.40 (95% CI: 0.04, 2.44) for pairs having received measles vaccine as the last vaccine. The female–male MR varied significantly for BCG compared with DTP (exact test of homogeneity, P < 0.001) and for DTP compared with measles vaccine (exact test of homogeneity, P = 0.001).
Conclusion Non-specific effects of routine vaccinations are likely to be important and influence sex-specific mortality patterns in areas with high mortality. The effects of vaccines need to be considered in the planning of immunization programmes for low-income countries.
Keywords BCG, DTP, female–male twins, gender-specific mortality, measles vaccine, non-specific effects of vaccines
Accepted 8 July 2003
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