Estimation of the direct and indirect effects of vaccination.

Indirect effects play a major role in the protection afforded by a vaccination programme. In this work we define new measures of direct, indirect and total (direct + indirect) effects of a vaccination programme in terms of the protection they provide to unvaccinated and vaccinated individuals, and to the entire population. We show how these measures can be estimated using data from a vaccine trial or an observational study. The bias and standard errors of these estimates can be evaluated via stochastic simulations. Examples from a mumps outbreak and a (hypothetical) HIV vaccine trial are used to illustrate the estimation of these new measures of vaccination effectiveness.