WHO advisers swap out H3N2 strains for next Northern Hemisphere flu vaccines

The World Health Organization (WHO) today announced its advisory committee’s recommendations on strains to include for the Northern Hemisphere’s 2025-26 flu season, which swap out the H3N2 components but keep the current 2009 H1N1 and influenza B strains the same.

The three strains recommended for the trivalent vaccine are also the same as those recommended for the Southern Hemisphere’s 2025 season vaccine, which the group weighed in on at its meetings in September 2024. 

Today’s recommendations have separate H3N2 recommendations for egg-based and cell-based flu vaccines. Though the WHO recommends trivalent vaccines, some companies include a second influenza B strain targeting both lineages. The Yamagata lineage hasn’t circulated since 2020, and the recommendation for that strain remains the same as for previous seasons.

H3N2 pick often a challenge

At a WHO briefing today, Ian Barr, PhD, deputy director of the WHO Collaborating Centre for Reference and Research on Influenza at the Doherty Institute at the University of Melbourne, said the H3N2 pick is always a challenge, because it changes more quickly than the other strains. “It’s always the bane of our existence.” He added that this season’s H3N2 vaccine strain for the Northern Hemisphere didn’t turn out to be a perfect match but has been a reasonable one. 

The severe flu season under way in the United States has come with a higher proportion of H3N2 activity than in other regions of the world such as Europe and China, where H1N1 has been predominant, Barr said. South America has seen a mix of H1N1 and influenza B, while Australia—like the United States—is experiencing a mix of H3N2 and H1N1.

Maria Van Kerkhove, PhD, WHO’s director of epidemic and pandemic preparedness and prevention, said officials from the US Centers for Disease Control and Prevention (CDC) participated actively in this week’s strain selection meeting and that the United States has been sharing genetic sequences from both people and animals. 

Country regulatory authorities and flu vaccine manufacturers take the WHO recommendations into account when starting the 6-month process for making the next season’s flu vaccines.

The US Department of Health and Human Services (HHS) has canceled a March 13 Food and Drug Administration (FDA) vaccine advisory meeting to weigh in on the flu vaccine strain picks. However, HHS officials said the FDA would make its recommendations in time for manufacturers to update the vaccines for the next flu season. 

Two new picks for pandemic preparedness

In its twice-yearly flu vaccine strain consultations, the WHO advisers also comb through the latest zoonotic flu strains to see if any new candidate vaccine viruses are needed for pandemic preparedness.

Richard Webby, PhD, director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds and a researcher at St. Jude Children’s Research Hospital, in Memphis, Tennessee, said the group recommended two new avian flu strains, one targeting an H5N1 clade 2.3.2.1a virus identified in Australia in a child who had returned from a trip to India. 

In its zoonotic flu candidate virus report, the group said the clade 2.3.2.1a viruses were detected in poultry in Bangladesh and in wild birds and poultry in India, where it also turned up in captive tigers, a captive leopard, and domestic cats. “The circulation of clade 2.3.2.1a viruses in these countries has continued despite the introduction of clade 2.3.4.4b viruses,” the group wrote.

Webby said the second newly recommended candidate strain targets an H5N6 clade 2.3.4.4h virus once dominant in Southeast Asia that has reemerged in poultry in a few provinces in southeastern China. Two human H5N6 illnesses involving clade 2.3.4.4h were reported in 2024.