Vaccine effectiveness against influenza B/Victoria-associated medically attended influenza-like illness: Beijing, China, 2021-2022 influenza season.

Abstract

Influenza B/Victoria viruses predominated during the 2021-2022 influenza season in Beijing, China, unlike most northern hemisphere countries, likely due to reduced international travel. We estimated influenza vaccine effectiveness (VE) against the B/Victoria lineage to provide a more comprehensive evaluation of 2021-2022 influenza VE. Between October 2021 and April 2022, patients aged ≥6 months with influenza-like illness (ILI) visiting outpatient departments in Beijing's influenza virological surveillance system were enrolled. A test-negative design was used to assess VE against influenza B/Victoria, adjusting for sex, age groups, the presence of chronic diseases, onset-to-enrollment interval, and symptom onset timing. Of the 8,813 eligible patients, 1,787 (20.3%) tested positive for influenza B/Victoria only. 573/8813 (6.5%) were vaccinated against influenza. The adjusted effectiveness against B/Victoria for all ages was 16.6% (95% CI: -7.5% to 35.2%) overall. VE was low against influenza B/Victoria among medically attended ILI patients during the 2021-2022 season in Beijing, China.