FDA's Shocking Decision: Why Subclade K in Fall Vaccines Could Change Everything—Are You Prepared?

Despite the United States officially withdrawing from the **World Health Organization (WHO)** in January, American scientists remain committed to global health measures, collaborating with international researchers to monitor the evolution of influenza viruses. This commitment was evident this week as the **Vaccine and Related Biological Products Advisory Committee (VRBPAC)** unanimously endorsed the WHO's recommendations for the viral strains to be included in upcoming flu vaccines for the fall season. The final decision will rest with **FDA Commissioner Martin Makary, MD, MPH**.

Among the committee's recommendations is the inclusion of a new variant, known as **A(H3N2) subclade K**, which emerged in October and has since become the dominant strain in the Northern Hemisphere. According to **Lisa Grohskopf, MD, MPH**, an influenza expert at the **Centers for Disease Control and Prevention (CDC)**, this strain was not included in last year’s vaccines due to its late emergence. The process of selecting flu vaccine strains is complex, requiring decisions to be made every February or March, as the vaccines can take approximately six months to manufacture and distribute.

During the recent VRBPAC meeting, CDC experts detailed the genetic profiles of flu viruses currently circulating globally, highlighting the constant mutation of these viruses. This unpredictability means that there’s no certainty that the three variants chosen will match the strains prevalent in the following fall and winter. **Arnold Monto, MD**, the acting chair of VRBPAC and a professor at the **University of Michigan School of Public Health**, remarked, “We were reminded about how difficult strain selection is this year. It's a little bit of science, a little bit of luck.”

Impact of Vaccination Mismatches

This year's flu vaccine effectiveness was notably lower than usual, a situation exacerbated by mismatches between the circulating strains and those included in the vaccines. According to interim results from three surveillance networks published 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%, also lowering their risk of hospitalization by 41%. In adults, the effectiveness (VE) of the vaccine ranged from 22% to 34% for outpatient visits and was approximately 30% against flu-related hospitalizations. A separate MMWR report based on data from California indicated that VE across all age groups was 33%, with 32% effectiveness against influenza A and 47% against influenza B.

The current flu season has had a particularly harsh impact on younger populations, prompting the CDC to classify it as “high severity” for children. Tragically, at least 90 children have died from the flu this season, with about 85% of those eligible children not being fully vaccinated. In contrast, the flu has been categorized as “moderate” for adults. Among adults aged 65 and older, vaccines proved effective, reducing influenza-related medical visits by 22% to 34% and hospitalizations by about 30%. Overall, the total number of flu-related hospitalizations is now the third-highest since the 2010-11 season, and the second-highest among children.

Since October 1, **CDC** estimates indicate that influenza has caused up to 42 million illnesses, approximately 19 million medical visits, around 660,000 hospitalizations, and up to 66,000 deaths. This data underscores the ongoing public health challenge posed by influenza.

One concerning trend is the decline in flu vaccination rates since the onset of the **COVID-19 pandemic**. The number of flu vaccines distributed has decreased by 23% since the **2019-20 flu season**, which saw over 173.8 million doses distributed. In the current season, manufacturers distributed 134.6 million doses, as highlighted by **Beverly Taylor, PhD**, head of global influenza scientific affairs at **CSL Seqirus**.

In line with WHO recommendations, VRBPAC has recommended that next fall’s flu vaccines replace all three viral strains. For **egg-based vaccines**, the recommended strains include:

• 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, the recommendations are slightly different:

• 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**

The upcoming flu season presents a critical opportunity for public health officials to enhance vaccination efforts and better prepare for influenza outbreaks. As flu viruses continue to evolve, effective strain selection and increased vaccination coverage may prove vital in reducing the impact of future flu seasons.