HOUSTON – Each year, as Americans roll up their sleeves for their annual flu shot, they might be surprised to learn their protection often begins with data from our friends down under.
Medical experts say that international surveillance is essential for developing effective flu vaccines for the United States and other Northern Hemisphere nations. The work to prevent a bad flu season in the U.S. starts in Australia.
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“Every year, the World Health Organization and others will look at what flu is doing in the Southern Hemisphere because those influenza strains will then ultimately migrate north into the northern hemisphere and result in our flu season,” said Dr. Wesley Long, Medical Director of Microbiology at Houston Methodist.
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Why Australia Matters
Because the hemispheres’ winters are opposite, Australia’s winter (June–August) gives researchers an early look at which flu strains are circulating.
That surveillance, run through the World Health Organization’s Global Influenza Surveillance and Response System and its collaborating centres, including the Victorian Infectious Diseases Reference Laboratory in Melbourne, supplies virus samples, genetic data and clinical trends that help predict which strains may dominate the Northern Hemisphere season.
How Vaccine Strains Are Chosen
Each spring the WHO convenes expert consultations and issues recommendations for the Northern Hemisphere vaccine composition; for the 2025–26 season the WHO published its recommended vaccine composition on Feb. 28, 2025.
National authorities and manufacturers use that guidance to finalize vaccine formulas and begin production.
Because vaccine manufacture takes months, those February decisions are necessary to have doses ready before the U.S. flu season begins.
The U.S. regulatory authorities then align with those recommendations.
In March 2025 federal partners published the FDA’s recommended strain list for manufacturers for the 2025–26 U.S. season, specifying the exact A and B strains to be included in egg-based and cell/recombinant vaccines.
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Why The Shots Change Every Year
Influenza viruses mutate and the mix of circulating strains shifts season to season, so experts reassess which strains are most likely to circulate and update vaccine composition accordingly.
That’s why “the flu shot does tend to change every year,” as Long noted and why annual vaccination is advised.
The Advisory Committee on Immunization Practices (ACIP) and CDC continue to recommend yearly vaccination for everyone aged 6 months and older, with the timing intended to balance early protection and durability through peak season.
When To Get Vaccinated
CDC guidance for the 2025–26 season again advises people get their flu vaccine ideally by the end of October, but also emphasizes that vaccination should continue as long as influenza viruses are circulating in the community.
Because immunity wanes and circulating strains shift, a seasonal booster remains the main public-health tool for reducing severe illness, hospitalizations and deaths.
Early Signals From The Southern Hemisphere
Public-health agencies monitor vaccine performance in the Southern Hemisphere as an early indicator of how well the chosen formula matches circulating viruses.
Interim analyses comparing Southern Hemisphere vaccine performance to subsequent Northern Hemisphere expectations are already being tracked; CDC analyses note that the 2025–2026 Northern Hemisphere vaccine composition was the same as the 2025 Southern Hemisphere composition, so Southern Hemisphere vaccine effectiveness can give a preview of likely protection if similar viruses circulate here.
Making It Matter Locally
Locally, Houston has seen influenza activity rise: as you reported, cases in the area doubled weekly since mid-October, a pattern public-health officials are watching closely as vaccination campaigns ramp up.
Vaccination remains the best protection against severe outcomes from flu.