Influenza remains one of the world’s most important vaccine-preventable respiratory diseases, causing substantial morbidity and mortality every year despite the availability of safe and effective vaccines. According to the World Health Organization (WHO), seasonal influenza epidemics result in approximately one billion infections annually, including 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths worldwide. Beyond its direct health impact, influenza places a considerable burden on healthcare systems through increased outpatient visits, hospitalizations, and absenteeism from work and school.
The epidemiology of influenza is highly dynamic. Influenza A viruses, particularly the A(H1N1)pdm09 and A(H3N2) subtypes, are responsible for most seasonal epidemics and are characterized by continuous antigenic drift, allowing them to evade pre-existing immunity. Influenza B viruses, now represented almost exclusively by the B/Victoria lineage following the apparent disappearance of the B/Yamagata lineage, continue to contribute significantly to disease burden, especially in children. The timing, intensity, and dominant circulating strains vary from season to season and across geographic regions, underscoring the importance of continuous global surveillance.
To address these ongoing viral changes, the WHO Global Influenza Surveillance and Response System (GISRS)—a network of laboratories operating in more than 120 countries—continuously monitors circulating influenza viruses throughout the year. Based on these data, WHO convenes experts twice annually to recommend the optimal vaccine composition for the Northern and Southern Hemisphere influenza seasons.
Updated Vaccine Composition
For the 2026–2027 Northern Hemisphere influenza season, WHO continues to recommend trivalent influenza vaccines, reflecting the global transition away from quadrivalent formulations after the apparent disappearance of the B/Yamagata lineage.
For egg-based vaccines, the recommended strains are:
- A/Missouri/11/2025 (H1N1)pdm09-like virus
- A/Darwin/1454/2025 (H3N2)-like virus
- B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus
For cell culture-, recombinant protein-, and nucleic acid-based vaccines, the recommended strains are:
- A/Missouri/11/2025 (H1N1)pdm09-like virus
- A/Darwin/1415/2025 (H3N2)-like virus
- B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus
The slight differences between egg-based and non-egg vaccine strains reflect manufacturing characteristics and antigenic optimization for the different vaccine production platforms.
Why These Changes Matter
The 2026–2027 formulation represents an important update, with all three vaccine components replaced compared with the previous Northern Hemisphere season. These changes reflect the ongoing evolution of circulating influenza viruses, particularly the emergence of new antigenic variants within the A(H3N2) lineage.
Although influenza vaccine effectiveness varies from season to season depending on the match between vaccine and circulating strains, annual vaccination consistently reduces the risk of severe disease, hospitalization, intensive care admission, and influenza-associated death. Even in years with moderate effectiveness against infection, vaccination continues to provide meaningful protection against serious clinical outcomes.
Who Should Be Vaccinated?
WHO continues to recommend annual influenza vaccination, prioritizing individuals at greatest risk of severe disease, including:
- Older adults
- Pregnant women
- Young children
- Individuals with chronic medical conditions
- Immunocompromised patients
- Healthcare workers
Countries should adapt their vaccination strategies according to local epidemiology, healthcare priorities, vaccine availability, and national immunization policies.
Looking Ahead
Influenza viruses continue to evolve rapidly, making annual surveillance and vaccine updates essential. The continued use of trivalent influenza vaccines, combined with updated viral strains selected through the WHO GISRS network, represents an evidence-based strategy to maximize protection against the viruses most likely to circulate during the upcoming season.
As influenza increasingly co-circulates with SARS-CoV-2 and respiratory syncytial virus (RSV), annual influenza vaccination remains a cornerstone of respiratory disease prevention. Maintaining high vaccine coverage among high-risk populations and healthcare workers will continue to be critical for reducing severe illness and alleviating pressure on healthcare systems.
References
- World Health Organization. Recommended composition of influenza virus vaccines for use in the 2026–2027 Northern Hemisphere influenza season. https://www.who.int/news/item/27-02-2026-recommendations-for-influenza-vaccine-composition-for-the-2026-2027-northern-hemisphere-season.
- World Health Organization. Influenza (Seasonal). https://www.emro.who.int/health-topics/influenza/influenza-seasonal.html.
- World Health Organization. Global Influenza Programme (GISRS). https://www.who.int/initiatives/global-influenza-surveillance-and-response-system.
- U.S. Food and Drug Administration. Influenza Vaccine Composition for the 2026–2027 U.S. Influenza Season. https://www.fda.gov/vaccines-blood-biologics/vaccines/influenza-vaccine-composition-2026-2027-us-influenza-season.
- Centers for Disease Control and Prevention. Seasonal Influenza (Flu). https://www.cdc.gov/flu/prevention/index.html.
- EU recommendations for 2026/2027 seasonal flu vaccine composition.
- https://www.ema.europa.eu/en/news/eu-recommendations-2026-2027-seasonal-flu-vaccine-composition.
