The Ever-Changing Face of Influenza and the Importance of Vaccine Updates

Influenza viruses are notorious for their rapid mutation, necessitating regular updates to vaccine compositions. The WHO’s recommendations serve as a blueprint for national regulatory agencies and pharmaceutical companies globally, guiding the development and production of vaccines tailored to combat the anticipated viral strains. This proactive approach is paramount in ensuring the highest possible level of protection against severe illness and mortality.

The WHO’s Global Influenza Surveillance and Response System (GISRS) forms the backbone of this process. Established in 1952, GISRS is the longest-standing global platform for systematic disease surveillance, continuously monitoring influenza viruses circulating in both humans and animals. Twice yearly, experts from WHO Collaborating Centres and Essential Regulatory Laboratories convene to analyze data generated by GISRS and its partners, identifying emerging trends and potential pandemic threats.

“Season after season, constantly evolving influenza viruses circulate globally, showing us how connected our world is. Shared risks require shared action,” stated Dr. Tedros Adhanom Ghebreyesus, WHO Director-General. “WHO’s recommendations for influenza vaccine composition rest on the diligent, year-round work of the Global Influenza Surveillance and Response System (GISRS) and partners. Thanks to GISRS, next season’s vaccines have been updated to counter the latest strains of influenza viruses, in turn better protecting communities.”

The Rise of Subclade K and the 2025-2026 Influenza Season

The 2025-2026 influenza season was notably impacted by the emergence of a distinct variant of the A(H3N2) virus, classified as J.2.4.1, also known as “subclade K.” This variant rapidly spread across the globe, contributing to an earlier-than-usual start to the influenza season in many countries and resulting in higher-than-normal levels of activity. Subclade K became the dominant influenza virus reported across various regions.

Overall, influenza A viruses predominated, with other variants of A(H3N2) and A(H1N1) also detected. Levels of influenza B viruses (B/Victoria lineage) remained low, and no cases of B/Yamagata lineage viruses have been reported since March 2020.

Monitoring Zoonotic Influenza: A Constant Vigilance Against Pandemic Threats

Beyond seasonal influenza, the WHO’s surveillance efforts extend to influenza viruses circulating in animals – known as zoonotic influenza. These viruses pose a significant pandemic risk due to their potential to jump to humans. Experts closely monitor these animal viruses, assessing their ability to cause human infections and their potential for widespread transmission.

Since September 23, 2025, following the last consultation, 25 human infections with zoonotic influenza were reported from six countries. The majority of these cases involved exposure to infected animals or contaminated environments, with no evidence of human-to-human transmission. The ongoing assessment of these viruses is critical for the selection of candidate vaccine viruses (CVVs) that can be rapidly deployed to manufacture vaccines in the event of a pandemic threat. At the recent meeting, experts recommended the development of a new CVV for an A(H9N2) virus.

2026-2027 Influenza Vaccine Recommendations

Based on the latest surveillance data, the WHO recommends the following compositions for influenza vaccines intended for use during the 2026-2027 northern hemisphere influenza season:

Egg-Based Vaccines

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

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; and
• a B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus.

Are you prepared for the upcoming flu season? What steps will you take to protect yourself and your loved ones?

Frequently Asked Questions About Influenza and Vaccination

1. What is the primary goal of updating influenza vaccines annually?
   The primary goal is to ensure the vaccine closely matches the circulating influenza viruses, maximizing protection against illness.
2. How does the WHO’s GISRS contribute to global influenza preparedness?
   GISRS provides a continuous, worldwide surveillance network that identifies emerging influenza strains and informs vaccine development.
3. What is zoonotic influenza, and why is it a concern?
   Zoonotic influenza refers to influenza viruses that circulate in animals and have the potential to infect humans, potentially leading to pandemics.
4. What was significant about the influenza season in 2025?
   The emergence and rapid spread of the A(H3N2) subclade K contributed to an earlier and more active influenza season in many regions.
5. Where can I find more information about influenza and vaccination?
   You can find comprehensive information on the WHO website: https://www.who.int/influenza