Vaccine Effects on Post-Infection Outcomes
A common question of interest is whether clinical cases in infected vaccinated people are less severe than clinical cases in infected unvaccinated people. As early as 1939, Kendrick and Eldering described less severe disease in children with pertussis who had been inoculated with pertussis vaccine compared to children with pertussis who had not been inoculated. Children vaccinated against chickenpox who develop clinical symptoms have less severe disease than unvaccinated children (Vazquez et al 2001). In evaluating malaria vaccine candidates, a question of interest might be whether the density of malaria blood-stage parasites is lower in infected vaccinated children than in infected unvaccinated children. In assessing an HIV vaccine, a scientific question of interest might be whether infected vaccinated people have a slower progression to clinical AIDS disease than infected unvaccinated people. In such a study, because the clinical endpoint of AIDS could take years to develop, the post-infection outcomes viral load and CD4 count could be used as surrogates for the clinical endpoint of interest.
Common to all of these questions is that the comparison is not between outcomes in uninfected vaccinated and unvaccinated individuals, but in people who have either become infected or developed clinical symptoms. We denote the vaccine effects on post-infection or post-clinical symptom outcomes broadly as VE P .
KeywordsCausal Effect Pertussis Vaccine Infected People Vaccine Effect Unvaccinated Child
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