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.