Estimation of population vaccination effectiveness from HIV vaccine trials

LONGINI, DATTA, and HALLORAN (1996) proposed to design HIV vaccine trials in a way that will permit the simultaneous estimation of the vaccine effects on susceptibility to infection and on infectiousness of vaccine brak-throughs. The main feature of their design is the inclusion of steady partners of trial participants. They estimate four parameters from the vaccine trial: the probability that a susceptible person will become infected from his/her steady partner, the probability of becoming infected from outside the partnership, the vaccine efficacy for susceptibility and the vaccine efficacy for infectiousness. We show how the estimates of these parameters can be used to predict the attack rate in a given population during a specified period following mass-vaccination. This is an iterative method, as the attack rate depends on the HIV prevalence which, in turn, depends on the number of new cases during that period. The same method is also used to estimate the attack rate in that population during the same period in the absence of vaccination. The estimated attack rates allow us to estimate the population vaccination effectiveness, defined as the fraction HIV cases prevented by a vaccination program.