Comparison of multiple estimates of efficacy for influenza vaccine.

Influenza vaccine trials typically report vaccine efficacy for infection-confirmed symptomatic illness. Data on indirect vaccine efficacy for susceptibility, the degree of vaccine protection to susceptibles, or indirect vaccine efficacy for illness given infection, are sparse. Using inactivated influenza vaccine randomized trial data, we calculated indirect vaccine efficacy for susceptibility of 20% [95% CI 9-30] and indirect vaccine efficacy for illness among infected persons 12% [95% CI 2-22], values inferior to a direct vaccine efficacy for infection-confirmed symptomatic illness of 55% [95% CI -21 to 84] and an indirect effect of 61% [95% CI 8-83]. Such data reveal variance in protective efficacy of the vaccine for multi-dimensional direct and indirect efficacy measures.