The control of whooping cough remains a world-wide concern. There is an ongoing controversy concerning whether pertussis vaccination has any effect on disease transmission. Infectiousness of cases holds a central role in the dynamics of transmission of a disease. We have estimated the efficacy of pertussis vaccine in reducing infectiousness of cases, and thus person-to-person transmission, in a sub-Saharan rural community, the Niakhar study area (Senegal) under longitudinal surveillance. Considering a whole spectrum of case definitions, vaccine efficacy was estimated as 1 - the ratio of the secondary attack rates in households for the relevant comparison groups. Both non-parametric and model-based (taking into account correlation within houses) estimates were computed. Precision of the estimations was calculated using the bootstrap and the multivariate delta methods.
Vaccine efficacy for infectiousness was found to be very high whatever the considered definition. It was 87% (95% CI 63-93) for completely vaccinated children (3 doses of pertussis vaccine) with the key case definition and increased slightly with the specificity of definition. Partial vaccination already afforded some protection for infectiousness with vaccine efficacy reaching 48% (95% CI 3-76) after 2 doses of vaccine.
Pertussis vaccination reduces transmission and the effect of the vaccine on infectiousness appears to be a major component of a vaccination program effect. The implications of these novel findings will be discussed.
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