In the 9 influenza seasons after the 2009 A(H1N1) pandemic, vaccinations provided “substantial benefit against outpatient illness,” according to a CDC study, with vaccine effectiveness of 46%. “Investigation of host-specific or virus-related mechanisms that may result in differences by age and virus type/subtype may help further efforts to promote increased vaccination coverage and other influenza-related preventative measures,” the authors conclude.
At 5 sites in the U.S. Influenza Vaccine Effectiveness Network in the 2011-2012 through the 2019-2020 influenza seasons, children aged 6 months to 17 years with acute respiratory illness were tested for influenza. Using a test-negative design to determine vaccine effectiveness, the authors report: “Among 24,148 enrolled children, 28% overall tested positive for influenza, 3,017 tested positive for influenza A(H3N2), 1,459 for influenza A(H1N1)pdm09, and 2,178 for influenza B. Among all enrollees, 39% overall were vaccinated, with 29% of influenza cases and 43% of influenza-negative controls vaccinated. Across all influenza seasons, the pooled VE for any influenza was 46% (95% confidence interval, 43–50). Overall and by type/subtype, VE against influenza illness was highest among children in the 6- to 59-month age group compared with older pediatric age groups. VE was lowest for influenza A(H3N2) virus infection.”