Role of Age in Spread of Influenza, 2011-2019, U.S. Influenza Vaccine Effectiveness Network

Intra-season timing of influenza infection among persons of different ages could reflect relative contributions to propagation of seasonal epidemics and has not been examined among ambulatory patients. We calculated risk ratios derived from comparing weekly influenza cases pre-peak versus post-peak during the 2010-2011 through 2018-2019 influenza seasons using data from the US Influenza Vaccine Effectiveness network. We sought to determine age specific differences during the ascent versus the descent of a season by influenza virus types and subtypes. We estimated credible intervals around the risk ratios using Bayesian joint posterior sampling of weekly cases. Our population consisted of ambulatory patients with laboratory-confirmed influenza enrolled at five study sites during nine influenza seasons after the 2009 influenza A virus subtype H1N1 (H1N1) pandemic. We observed that young children aged <5 years tended to be more often infected with H1N1 during the pre-peak period while adults aged ≥65 years tended to be more often infected with H1N1 during the post-peak period. However, for influenza A virus subtype H3N2 children aged <5 years were more often infected during the post-peak period. These results may reflect a contribution of different age groups to seasonal spread, which may differ by influenza virus type and subtype.