Depletion-of-susceptibles Bias in Analyses of Intra-season Waning of Influenza Vaccine Effectiveness

Bias arises in studies of waning vaccine effectiveness when higher-risk individuals are depleted from the at-risk population at different rates between study groups. We examined how this bias arises and how to avoid it. A reanalysis of data from California confirmed a finding of intra-season waning of influenza vaccine effectiveness.