US Study Finds Variable Influenza B Vaccine Effectiveness Across Eight Seasons

Researchers analyzed ambulatory patients aged 6 months or older with acute respiratory illness enrolled at US Flu VE Network sites across eight influenza seasons from 2016–17 through 2024–25; 2020–21 was excluded because influenza circulation was negligible. Participants were tested by RT-PCR. In a test-negative design, logistic regression adjusted for age, sex, race and ethnicity, calendar time, and study site. The analysis included 65,773 participants, with 3,629 influenza B cases used for lineage-specific estimates.

Six of 10 season-lineage strata met the minimum threshold of 50 cases for reporting. Estimated vaccine effectiveness against B/Victoria was 58% (95% CI, 28%–77%) in 2016–17, 51% (43%–57%) in 2019–20, 78% (71%–84%) in 2023–24, and 39% (14%–56%) in 2024–25. Against B/Yamagata, effectiveness was 55% (46%–63%) in 2016–17 and 49% (40%–56%) in 2017–18.

The findings indicate that effectiveness against medically attended outpatient influenza B illness varied across seasons. The authors call for continued effectiveness monitoring and genetic surveillance to inform vaccine strain selection. Estimates are limited to reported season-lineage strata meeting the case threshold and do not establish effectiveness against outcomes beyond outpatient illness.