Vaccine

Background:Epidemics of seasonal influenza vary in intensity annually, and influenza vaccine effectiveness (VE) fluctuates based in part on antigenic match to circulating viruses. We estimated the incidence of influenza and influenza cases averted by vaccination in four ambulatory care sites in the United States, during seasons when overall influenza VE ranged from 29% to 40%.Methods:We conducted active surveillance for influenza at ambulatory care settings at four sites within the United States Influenza Vaccine Effectiveness Network. We extrapolated the total number of influenza cases in the source populations served by these organizations based on incidence of medically attended acute respiratory illness in the source population and influenza test results in those actively tested for influenza. We estimated the number of medically attended influenza cases averted based on incidence, vaccine coverage, and VE.Results:From 2016/17 through 2018/19, incidence of ambulatory visits for laboratory-confirmed influenza ranged from 31 to 51 per 1,000 population. Incidence was highest in children aged 9\u201317 years (range, 56 to 81 per 1,000) and lowest in adults aged 18\u201349 years (range, 23\u201332 per 1,000). Medically attended cases averted by vaccination ranged from a high of 46.6 (95% CI, 12.1\u2013 91.9) per 1,000 vaccinees in children aged 6 months to 8 years, to a low of 6.9 (95% CI, 125.1\u2013 27.3) per 1,000 vaccinees in adults aged 6565 years.Discussion:Even in seasons with low vaccine effectiveness for a particular virus subtype, influenza vaccines can still lead to clinically meaningful reductions in ambulatory care visits for influenza.U01 IP001034/IP/NCIRD CDC HHSUnited States/UL1 TR001857/TR/NCATS NIH HHSUnited States/U01IP001037/ACL/ACL HHSUnited States/CC999999/ImCDC/Intramural CDC HHSUnited States/U01IP001038/ACL/ACL HHSUnited States/U01 IP001039/IP/NCIRD CDC HHSUnited States/U01IP001034/ACL/ACL HHSUnited States/U01IP001039/ACL/ACL HHSUnited States/U01 IP001035/IP/NCIRD CDC HHSUnited States/U01 IP001037/IP/NCIRD CDC HHSUnited States/U01 IP001038/IP/NCIRD CDC HHSUnited States