Role of Age in the Spread of Influenza, 2011–2019: Data From the US Influenza Vaccine Effectiveness Network

Intraseason timing of inf luenza infection among persons of different ages could ref lect relative contributions to propagation of seasonal epidemics and has not been examined among ambulatory patients. Using data from the US Inf luenza Vaccine Effectiveness Network, we calculated risk ratios derived from comparing weekly numbers of inf luenza cases prepeak with those postpeak during the 2010–2011 through 2018–2019 inf luenza seasons. We sought to determine age-specific differences during the ascent versus descent of an inf luenza season by inf luenza virus type and subtype. We estimated 95% credible intervals around the risk ratios using Bayesian joint posterior sampling of weekly cases. Our population consisted of ambulatory patients with laboratory-confirmed inf luenza who enrolled in an inf luenza vaccine effectiveness study at 5 US sites during 9 inf luenza seasons after the 2009 inf luenza A virus subtype H1N1 (H1N1) pandemic. We observed that young children aged <5 years tended to more often be infected with H1N1 during the prepeak period, while adults aged ≥65 years tended to more often be infected with H1N1 during the postpeak period. However, for inf luenza A virus subtype H3N2, children aged <5 years were more often infected during the postpeak period. These results may ref lect a contribution of different age groups to seasonal spread, which may differ by inf luenza virus type and subtype.