Estimating influenza vaccine efficacy from challenge and community-based study data

In this paper, the authors provide estimates of 4 measures of vaccine efficacy for live, attenuated and inactivated influenza vaccine based on secondary analysis of 5 experimental influenza challenge studies in seronegative adults and community-based vaccine trials. The 4 vaccine efficacy measures are for susceptibility (VE_S), symptomatic illness given infection (VE_P), infection and illness (VE_SP), and infectiousness (VE_I). The authors also propose a combined (VE_C) measure of the reduction in transmission in the entire population based on all of the above efficacy measures. Live influenza vaccine and inactivated vaccine provided similar protection against laboratory-confirmed infection (for live vaccine: VE_S = 41%, 95% confidence interval (CI): 15, 66; for inactivated vaccine: VE_S = 43%, 95% CI: 8, 79). Live vaccine had a higher efficacy for illness given infection (VE_P = 67%, 95% CI: 24, 100) than inactivated vaccine (VE_P = 29%, 95% CI: -19, 76), although the difference was not statistically significant. VE_SP for the live vaccine was higher than for the inactivated vaccine. VE_I estimates were particularly low for these influenza vaccines. VE_SP and VE _C can remain high for both vaccines, even when VE_I is relatively low, as long as the other 2 measures of vaccine efficacy are relatively high.