Flu vaccine 36 percent effective against influenza A(H3N2)-related illnesses during 2021-2022 flu season

For the 2021 to 2022 influenza season, influenza vaccines were 36 percent effective against influenza A(H3N2)-related illnesses, according to a study published online Dec. 12 in Clinical Infectious Diseases.

Ashley M. Price, from the U.S. Centers for Disease Control and Prevention in Atlanta, and colleagues estimated influenza vaccine effectiveness against laboratory-confirmed outpatient acute illness caused by influenza A(H3N2) viruses among patients aged 6 months and older between October 2021 and April 2022. Participants who tested positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were excluded due to strong correlations between influenza and SARS-CoV-2 vaccination.

Overall, 468 (7 percent) of the 6,260 participants tested positive for influenza only, including 94 percent who tested positive for influenza A(H3N2). The researchers found that all 206 sequenced influenza A(H3N2) viruses belonged to the genetic group 3C.2a1b subclade 2a.2, which had antigenic differences from the vaccine component of the 2021 to 2022 season influenza A(H3N2) vaccine that belonged to clade 3C.2a1b subclade 2a.1. In the analyses of influenza vaccine effectiveness among 4,312 patients, 57 percent had been vaccinated against influenza. For all ages, vaccine effectiveness against influenza A(H3N2) was 36 percent overall.

"Influenza vaccination in 2021 to 2022 reduced outpatient medically attended acute respiratory illness with cough due to influenza A(H3N2) viruses by approximately one-third overall," the authors write. "Protection afforded by vaccination was comparable to previous A(H3N2)-dominant seasons before the COVID-19 pandemic."

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