Waning vaccine effectiveness against influenza-associated hospitalizations among adults, 2015-2016 to 2018-2019, US Hospitalized Adult Influenza Vaccine Effectiveness Network

We observed decreased effectiveness of influenza vaccine with increasing time since vaccination for prevention of influenza A(H3N2), influenza A(H1N1)pdm09, and influenza B(Yamagata)-associated hospitalizations among adults. Maximum VE was observed shortly after vaccination, followed by an absolute decline in VE of about 8 to 9% per month post-vaccination.