In spite of fears of immune evasion by a widely circulating H3N2 influenza variant called subclade K, a Public Health Alerts report published today shows that the current flu vaccine produces antibodies that efficiently recognize subclade K in almost 40% of people.

“Thus, the current seasonal influenza vaccine likely will be partially effective at preventing illness associated with H3N2 subclade K virus infections,” wrote the authors, from the University of Pennsylvania’s Perelman School of Medicine.

Public Health Alerts, a new collaboration between NEJM Evidence and CIDRAP, fills a gap in reliable data, offering expert-reviewed reports that translate frontline observations into actionable public health evidence. An NEJM Evidence editorial  explains the initiative further.

Percent with antibodies against subclade K rose from 11% to 39%

The researchers analyzed blood sample from 76 people before and 27 to 30 days after they received a standard dose of egg-based 2025-26 flu vaccine (GSK’s Flulaval Trivalent). They used Hemagglutination inhibition (HAI) assays to measure immune response specific to both the H3N2 strain and subclade K before and after vaccination.

“Antibody titers against both the 2025–2026 H3N2 vaccine strain and H3N2 subclade K virus increased in sera from most individuals following vaccination; however, antibody geometric mean titers were approximately twofold higher to the 2025–2026 H3N2 vaccine strain compared to the H3N2 subclade K virus after vaccination,” the authors noted.

Before vaccination, 39% of participants (30 of 76) were seropositive (had an HAI titer of 40 or greater) against the 2025-26 H3N2 vaccine strain and 11% of participants (8 of 76) were seropositive against the H3N2 subclade K virus. After vaccination, those numbers rose to 71% and 39%, respectively.

The results did not vary substantially by the age of the volunteer.

These data suggest that the 2025–2026 influenza vaccine induces antibodies in many vaccine recipients that are considered, from a regulatory perspective, likely to provide protection against H3N2 subclade K viruses.

The authors added, “These data suggest that H3N2 subclade K viruses are antigenically advanced compared to the 2025–2026 H3N2 vaccine strain; however, the antigenic differences that we observed in sera from some humans are not as large as previously reported in ferrets.”

They conclude, “Overall, these data suggest that the 2025–2026 influenza vaccine induces antibodies in many vaccine recipients that are considered, from a regulatory perspective, likely to provide protection against H3N2 subclade K viruses.”