#18,887
Twice each year international influenza experts meet to discuss recent developments in human and animal influenza viruses around the world, and to decide on the composition of the next influenza season’s flu vaccine. Due to the time it takes to manufacture and distribute a vaccine, decisions on which strains to include must be made six months in advance.
Which means the composition of the northern hemisphere’s vaccine must be decided upon in February of each year, while decisions on the southern hemisphere’ vaccine are made in September.Yesterday the WHO released their recommendations for the composition of next year's (2026) Southern Hemisphere flu vaccine, which notably include updates for both the H1N1 and H3N2 components.
Australia has recently come off a particularly rough flu season (see Ian Mackay's A Flunami in July), and just three days ago the MMWR published a review of the interim effectiveness of this year's Southern Hemisphere Flu vaccine (excerpt below).
During the 2025 Southern Hemisphere influenza season, seasonal influenza vaccination reduced influenza-associated outpatient visits by 50.4% and hospitalization by 49.7%.
A respectable, if not spectacular, result.
But the WHO obviously sees evolutionary changes accuring in both H1N1 and H3N2, hence the tweak in next year's vaccine.
Next Feburary, when it comes time to formulate next year's Northern Hemisphere's vaccine, we'll hopefully have even more data. With influenza, the only constant is change.
First, the press release from the WHO, after which I'll have a bit more.
Recommendations announced for influenza vaccine composition for the 2026 southern hemisphere influenza season
26 September 2025
The World Health Organization (WHO) today announced its recommendations for the viral composition of influenza vaccines for use in the 2026 influenza season in the southern hemisphere. The announcement was made at an Information Meeting after a 4-day Consultation on the Composition of Influenza Virus Vaccines.
Given the constantly evolving nature of influenza viruses, regular updates to vaccine composition are essential to ensure their effectiveness and to protect public health worldwide.
WHO convenes these consultations twice annually – once for the southern hemisphere and once for the northern hemisphere, bringing together an advisory group of experts from WHO Collaborating Centres and Essential Regulatory Laboratories. The group reviews surveillance and other data provided by the WHO Global Influenza Surveillance and Response System (GISRS) and collaborators to inform its recommendations.
WHO’s recommendations serve as the basis for national and regional regulatory authorities, pharmaceutical manufacturers, and other stakeholders to develop, produce, and license influenza vaccines for the upcoming season.
WHO recommends that trivalent vaccines for use in the 2026 southern hemisphere influenza season contain the following:
Egg-based vaccines
Cell culture-, recombinant protein- or nucleic acid-based vaccines
- an A/Missouri/11/2025 (H1N1)pdm09-like virus;
- an A/Singapore/GP20238/2024 (H3N2)-like virus; and
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Consistent with the four previous WHO recommendations since September 2023, it remains the opinion of the WHO influenza vaccine composition advisory committee that the inclusion of a B/Yamagata lineage antigen is no longer warranted.
- an A/Missouri/11/2025 (H1N1)pdm09-like virus;
- an A/Sydney/1359/2024 (H3N2)-like virus; and
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
Quadrivalent vaccines, where the transition to trivalent vaccines is not yet complete, contain a 4th component – a B/Yamagata lineage virus (B/Phuket/3073/2013-like virus).
There will no longer be updated recommendations for the B/Yamagata lineage component.
WHO has also updated recommendations for the development of new candidate vaccine viruses for zoonotic influenza, with a view on pandemic preparedness.
We've just come off one of the most impactful flu seasons in 15 years, which was characterized in the CDC's MMWR two weeks ago.
Seasonal flu vaccine uptake in the United States peaked in 2019, and has begun a slow, but steady decline since.
Pandemic fatigue, plus growing anti-vaccination sentiment, and admittedly lackluster protection against influenza infection have all had some impact.
Flu vaccines, however, do a much better job reducing the severity of infection, and thereby reducing hospitalizations and death (see (ESR) Influenza Vaccine Outcomes: A Meta-Analysis Revealing Morbidity Benefits Amid Low Infection Prevention).
And the more we learn about influenza's extra-pulmonary impacts on the body (see Risk of Cardiovascular Events After Influenza: A Population-based Self Controlled Case Series Study) the more important protecting yourself becomes.
Although we've no way of knowing what this fall/winter's flu season will bring, the flu vaccine still remains one of the most effective ways to reduce your risk of severe disease from the influenza virus.
