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Influenza vaccine components are selected through year-round monitoring of flu viruses, followed by expert review and recommendations for each hemisphere’s next season. Those recommendations guide national decisions and vaccine production; they are not a single worldwide product approval. The final formulation can also differ by country and manufacturing platform.
How vaccine components are selected
The process begins well before a seasonal vaccine is used. The World Health Organization (WHO) coordinates the Global Influenza Surveillance and Response System (GISRS), a network that monitors influenza activity and gathers viruses and other evidence from around the world. National partners and laboratories contribute to that work.
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- Monitor circulating viruses. Surveillance helps experts track which influenza viruses are spreading and where. CDC describes representative viruses being sent by laboratories to WHO Collaborating Centres for characterization.
- Characterize the viruses. Experts examine genetic and antigenic characteristics, along with patterns of influenza activity and epidemiological and clinical information. Vaccine-serology studies provide evidence about how vaccine-induced responses relate to circulating viruses.
- Assess how well current components represent circulating viruses. At twice-yearly consultations, experts review the surveillance and characterization evidence alongside vaccine-serology results to decide whether the components should be changed.
- Recommend components for the next season. WHO consultations take place in February for the Northern Hemisphere and September for the Southern Hemisphere. WHO issues recommendations; national or regional authorities decide which compositions and products to authorize in their jurisdictions.
- Turn recommendations into vaccines. National agencies and manufacturers use the recommendations in vaccine development, production, and licensing work. The official sources describe this pathway, but do not establish a complete manufacturing timeline or a precise interval from recommendation to released batches.
WHO Director-General Dr Tedros Adhanom Ghebreyesus described the recommendations as resting on the year-round work of GISRS and its partners in WHO’s February 27, 2026 announcement. In the United States, the Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) reviews and recommends domestic vaccine composition, and FDA informs approved manufacturers. CDC then publishes seasonal guidance.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsWhy the composition is reviewed regularly
Influenza viruses evolve, and the viruses circulating in one season may not be represented as well by the same components in a later season. Regular review lets experts compare current surveillance and vaccine-response evidence with the components in use, then recommend updates when warranted. It is a recurring selection process, not a guarantee that every vaccine will match every virus circulating during the season.
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What is in the 2026–2027 Northern Hemisphere recommendations?
WHO’s 2026–2027 Northern Hemisphere recommendation names three virus-like components for each production-platform group. The H1N1 component is the same in both groups, while the listed H3N2 and B/Victoria components differ.
| Platform | H1N1 component | H3N2 component | B/Victoria component |
|---|---|---|---|
| Egg-based | A/Missouri/11/2025 (H1N1)pdm09-like | A/Darwin/1454/2025 (H3N2)-like | B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like |
| Cell culture, recombinant protein, or nucleic acid-based | A/Missouri/11/2025 (H1N1)pdm09-like | A/Darwin/1415/2025 (H3N2)-like | B/Pennsylvania/14/2025 (B/Victoria lineage)-like |
CDC reports that all three components of U.S. seasonal vaccines changed from 2025–2026 to 2026–2027. The H3N2 update was intended to address subclade K, which spread widely during the 2025–2026 season. These are recommendations and U.S. seasonal-composition details, not evidence of published efficacy results for the newly selected components.
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Why the hemisphere and country matter
WHO makes separate recommendations for the Northern and Southern Hemisphere seasons. Its 2027 Southern Hemisphere recommendation was published on September 25, 2026, and, like the Northern Hemisphere recommendation, lists three components for egg-based and cell culture, recombinant protein, or nucleic acid-based vaccines. The named viruses should not be assumed to be identical across hemispheres.
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WHO’s recommendation is not itself a universal approval or a promise that every country will use the same product. National or regional authorities determine the compositions and approvals applicable in their jurisdictions. To identify a vaccine’s components, check the information for the relevant season and country rather than relying on a formulation listed for another region.
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Why production platform can change a recommended component
Egg-based production and cell culture, recombinant protein, or nucleic acid-based approaches do not always use the same recommended virus-like component. The 2026–2027 recommendation illustrates this with different listed H3N2 and B/Victoria components between the egg-based and other-platform groups. The platform difference is part of the composition recommendation; it does not by itself establish that one platform or formulation is universally preferable.
What the U.S. supply figures do—and do not—tell you
For 2026–2027, CDC reported manufacturer projections of up to 135 million U.S. doses. This is a projected supply figure, not a count of doses ultimately administered. CDC also reported that 100% of projected U.S. supply would be trivalent, meaning it includes three components; 27% was projected to be non-egg-based; and 99% was projected to be without thimerosal as a preservative. These supply characteristics describe the projected U.S. market, not the global supply or the composition rules for every country.
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How to interpret seasonal figures
WHO’s 2026 announcement estimates around 1 billion seasonal influenza cases globally each year, including 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths annually. These are annual global estimates, not counts for a single season or a measure of how many cases a particular vaccine prevents.
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