Every spring scientists pick which flu viruses to include in the Northern Hemisphere’s fall influenza vaccine. Today we’ll cover the latest influenza surveillance and the content of the 2025-2026 influenza vaccines in the US.

How is the content of the fall influenza vaccine decided? It’s a process that focuses on using the best evidence – but it has challenges because of the wildcards the Southern hemisphere can play during their winter (our summer).

Each spring CDC experts look at flu viruses from all around the world and decide which strains are most likely to circulate during our (Northern hemisphere) influenza season. They check:

  • which flu strains are spreading most,
  • how the viruses’ surface proteins (especially something called hemagglutinin, or HA) have changed,
  • whether existing immunity (from prior infection or vaccination) might not work as well.

Once the strain is selected (usually in March) vaccine makers start growing those picked strains (often in eggs). Because growing and distributing takes time, the vaccine formula has to be selected in the spring.

What Happened in the Southern Hemisphere

The wildcard is in the Southern hemisphere. Flu viruses keep evolving in places like Argentina, Brazil and Australia during our summer – even as US vaccine manufacturers are already making the fall season vaccine. That means a new strain can pop up down under during our summer – but it’s found too late to change what’s in our fall vaccine.

Because the vaccine choice has to be made before all possible changes occur, this means there’s always a bit of “leap of faith” involved.

What’s in the 2025‑26 Vaccine for the Northern Hemisphere

Here are the viruses the CDC recommended for this season (back in March):

  • An A/Victoria/4897/2022 (H1N1)pdm09‑like virus
  • An A/Darwin/9/2021 (H3N2)‑like virus
  • A B/Austria/1359417/2021‑like (B/Victoria lineage) virus

These were picked because they represented what scientists believed would be the most likely flu types to spread.

The New H3N2 Strain: What’s Going On?

As sometimes is the case – the virus evolved in the Southern Hemisphere over our summer (their winter). The new strain isn’t a huge change from what was predicted – but it is a bit different. The new strain is an H3N2 “subclade K” virus – and it’s recently popped up in Canada, the UK, Japan and Europe.

  • Subclade K has several new mutations compared to the H3N2 strain that was chosen for the vaccine.
  • Early data suggest this variant is already dominant in some regions (for example, the UK found high proportions of H3N2 being subclade K).
  • Because it appeared after the vaccine strain was picked—at the tail end of the Southern Hemisphere season—it may mean this year’s vaccine isn’t perfectly matched with what’s likely to circulate in AZ this winter.

What It Means for Vaccine Effectiveness

So, what do these changes mean for how well the vaccine will work? Here’s the assessment:

  • Because subclade K has drifted (i.e., mutated) away from the strain used in the vaccine, there’s a reduced match. That means the vaccine might be less effective in preventing mild or moderate illness compared to a perfectly matched season.
  • But… the influenza vaccines in pharmacies and doctors’ offices still offer important protection, especially against severe illness and hospitalization.
  • For example, early data from the UK show around 70‑75% effectiveness in children and 30‑40% in adults, even with this new K strain dominating circulation.

Bottom Line

  • The vaccine for 2025‑26 was chosen carefully, based on the best information that was available in March.
  • A new H3N2 variant (subclade K) showed up after the CDC’s recommendation for this year’s vaccine. It has more mutations than expected, so it might reduce vaccine effectiveness somewhat, but we don’t know that for sure yet.
  • If this year’s vaccine ends up being less effective than usual it doesn’t appear to be the fault of CDC or Kennedy this time – it’s because the new strain evolved after the vaccine decision was made (and had to be made).
  • Even if there’s some ‘immune escape’ because of the new subclade K virus, early data from the UK suggest it’s likely that the vaccine will still provide strong protection especially for children and seniors and for reducing hospitalizations.

See all the recommendations: Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2025–26 Influenza Season | MMWR

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