Influenza B Circulation and Vaccine Effectiveness Against Medically Attended Outpatient Influenza B Illness, United States, 2016–17 Through 2024–25
Jessica R Meeker, Ashley M Price, Emily Martin, Ivana Vaughn, Karen J Wernli, C Hallie Phillips, Manjusha J Gaglani, Chandni Raiyani, Huong Q Nguyen, Jennifer P King, Stacey House, Jamie Mills, Vel Murugan, Lora Nordstrom, Elie Saade, Kiran Faryar, Richard K Zimmerman, Krissy Moehling Geffel, Emmanuel B Walter, Natalie A B Bontrager, Jessie R Chung, Brendan Flannery, Sascha Ellington, , Spencer Rose, Amanda McKillop, Nicole Calhoun, Britain Fairall, Marcus Volz, Manohar Mutnal, Arundhati Rao, Jason Ramm, Tresa McNeal, Keith Stone, Madhava Beeram, Eric Hoffman, Dedra Preece, Kayan Dunnigan, Leah Odame-Bamfo, Jason Ettlinger, Courtney Shaver, Monica Bennett, Elisa Priest, Jennifer Thomas, Javed Butler, Muralidhar Jatla, Jesus VallejoAbstract
Background
Influenza B vaccine effectiveness (VE) has varied by season, lineage, antigenic drift, and vaccine composition. After B/Yamagata detection ceased globally, trivalent vaccines containing B/Victoria were reintroduced in 2024–25. We evaluated influenza B circulation and VE across eight analyzed seasons spanning this transition.
Methods
Ambulatory patients aged ≥6 months with acute respiratory illness were enrolled at U.S. Flu VE Network sites during eight influenza seasons from 2016–17 through 2024–25; 2020–21 was excluded because influenza circulation was negligible. Participants were tested by RT-PCR. Lineage-specific VE was estimated using a test-negative design and logistic regression adjusted for age, sex, race/ethnicity, calendar time, and study site. VE was reported for season-lineage strata with ≥50 influenza B cases.
Results
Among 65,773 participants, 3,629 influenza B cases were included in lineage-specific VE analyses (1,507 B/Yamagata; 2,122 B/Victoria). Six of 10 season-lineage strata met the reporting threshold. B/Victoria VE 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. B/Yamagata VE was 55% (46%–63%) in 2016–17 and 49% (40%–56%) in 2017–18.
Conclusions
Influenza vaccines protected against medically attended outpatient influenza B illness, although VE varied by season. Continued VE monitoring and genetic surveillance are needed to detect emerging influenza B viruses and inform vaccine strain selection.