FDA vaccine advisers recommend adding subclade K to fall shots

Although the United States officially left the World Health Organization (WHO) in January, US scientists have continued to collaborate with international researchers to track the evolution of influenza viruses.

Today, the Vaccine and Related Biological Products Advisory Committee (VRBPAC) unanimously endorsed the WHO’s recommendation for viral strains to include in flu shots starting this fall. The final decision will be made by Food and Drug Administration (FDA) Commissioner Martin Makary, MD, MPH.

Seven voting members of the committee recommended that fall flu vaccines include a new variant that emerged in October, called A(H3N2) subclade K. Although the strain appeared too late to be included in last year’s vaccines, subclade K has since become the dominant flu variant in the Northern Hemisphere, Lisa Grohskopf, MD, MPH, an influenza expert at the Centers for Disease Control and Prevention’s (CDC’s) National Center for Immunization and Respiratory Diseases, told the committee.

Scientists must make decisions about flu shots every year in February or March because, using current technology, the vaccines take about six months to manufacture and distribute.

At today’s meeting, CDC experts detailed the genetic makeup of flu viruses currently in circulation, based on specimens collected around the world. But flu viruses mutate frequently. There’s no guarantee that the three variants selected today will be the same as the strains spreading next fall and winter.

“We were reminded about how difficult strain selection is” this year, said Arnold Monto, MD, acting VRBPAC chair and a professor of public health and epidemiology at the University of Michigan School of Public Health. “It’s a little bit of science, a little bit of luck.”

Vaccines offered less protection this year

The mismatch helps explains why flu vaccines were less effective than usual this season. Based on interim results from three surveillance networks published today in the CDC’s Morbidity and Mortality Weekly Report (MMWR), vaccination reduced the number of influenza-related outpatient visits among children by 38% to 41% and lowered their risk of hospitalization by 41%.

In adults, vaccine effectiveness (VE) among adults through February was 22% to 34% for outpatient visits and 30% against flu-related hospitalization. In a separate MMWR report based on data from California, VE against lab-confirmed flu for all age-groups through January was 33% (32% against influenza A and 47% against influenza B).

Influenza has taken a heavy toll on young people in recent months, leading the CDC to classify this season as “high severity” for children, Grohskopf said. 

At least 90 children have died of the flu. Among those who died, about 85% of eligible children were not fully vaccinated. The flu season was “moderate” for adults. Among adults age 65 and older, flu vaccines reduced influenza-related medical visits by 22% to 34% and hospitalizations by 30%, Grohskopf said.

The total number of flu-related hospitalizations is the third-highest since the 2010-11 season, and the second-highest among children. The current flu season is on its way out, Grohskopf said. Doctors are now treating fewer patients with influenza A and more with influenza B, a shift that occurs in many flu seasons.

The flu has caused up to 42 million illnesses since October, along with up to 19 million medical visits, up to 660,000 hospitalizations, and up to 66,000 deaths since October 1, according to CDC estimates.

Falling vaccination rates

Flu vaccination rates have fallen since the COVID-19 pandemic.

The number of flu vaccines distributed each season has decreased by 23% since the 2019-20 flu season, when more than 173.8 million doses were distributed, said Beverly Taylor, PhD, head of global influenza scientific affairs at CSL Seqirus, which makes flu shots. This season, manufacturers distributed 134.6 million flu doses.

Per the WHO’s recommendations, VRBPAC recommended that all three viral strains be swapped out for next fall’s flu vaccines. Specific recommendations of viral strains to include in this year’s flu shots include:

For egg-based vaccines 

• an A/Missouri/11/2025 (H1N1)pdm09-like virus
• an A/Darwin/1454/2025 (H3N2)-like virus 
• a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus

For cell culture-, recombinant protein-, or nucleic acid-based vaccines  

• an A/Missouri/11/2025 (H1N1)pdm09-like virus
• an A/Darwin/1415/2025 (H3N2)-like virus 
• a B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus