Efficiency of estimating vaccine efficacy for susceptibility and infectiousness: Randomization by individual versus household

In designing vaccine efficacy studies based on the secondary attack rate (S AR) or transmission probability in which both vaccine efficacy for suscepti bility, VES, and vaccine efficacy for infectiousness, VEI, are estimated, t he allocation of vaccine and placebo within transmission units has an impor tant influence on the efficiency of the study. We compared the following ra ndomization schemes that result in different allocations of vaccine and pla cebo within two-member: households: (1) randomization by individual for a m ixed allocation, (2) randomization by transmission unit for concordant allo cation, and (3) randomization of only one individual in each transmission u nit to either vaccine or placebo. There is a complex interaction among the VES, VEI, and the SAR that determines which allocation of vaccine and place bo within households provides the most information. In general, individual randomization with a mixed allocation of vaccine and placebo is better for estimating both VES and VEI than is randomizing by household. However, for estimation of VEI. at very low SARs and low VES, randomization by household is slightly more efficient than randomization by individual.