  This study estimates the overall effect of two influenza vaccination programs
consecutively administered in a cluster-randomized trial in western Senegal
over the course of two influenza seasons from 2009-2011. We apply cutting-edge
methodology combining social contact data with infection data to reduce bias in
estimation arising from contamination between clusters. Our time-varying
estimates reveal a reduction in seasonal influenza from the intervention and a
nonsignificant increase in H1N1 pandemic influenza. We estimate an additive
change in overall cumulative incidence (which was 6.13% in the control arm) of
-0.68 percentage points during Year 1 of the study (95% CI: -2.53, 1.18). When
H1N1 pandemic infections were excluded from analysis, the estimated change was
-1.45 percentage points and was significant (95% CI, -2.81, -0.08). Because
cross-cluster contamination was low (0-3% of contacts for most villages), an
estimator assuming no contamination was only slightly attenuated (-0.65
percentage points). These findings are encouraging for studies carefully
designed to minimize spillover. Further work is needed to estimate
contamination, and its effect on estimation, in a variety of settings.
