INFLUENZA vaccinations in the US this season have been less effective against a particularly fast-moving flu strain that is now spreading in Australia, according to research published in JAMA today.
Influenza A (H3N2) subclade K (J.2.4.1) - known as 'Super-K' - is highly infectious, spreading faster and earlier in Australia than during typical flu seasons.
First detected in Australia and New Zealand, it emerged towards the end of last winter, and was a leading cause of respiratory-related deaths in Australia in Aug (PD 26 Feb).
The rapidly mutating virus has since spread, and was the dominant strain in the US 2025-26 flu season, representing a major public health concern.
The researchers assessed the antibody responses of 46 people who received a flu shot in late 2025 against a series of strains, including Super-K and the other strains the vaccine was developed to target.
They found there was a weaker antibody response to Super-K compared to other strains, and it also appears that Super-K can partially evade immunity from prior infection with similar flu strains.
The researchers say there was still a modest response to Super-K, so while flu shots were less effective than normal, they provided some protective benefit.
"The lower antibody titers to H3N2 subclade K compared with other recent influenza strains, both before and after vaccination, have important implications for the likely continued spread of this new influenza strain and for selection of future vaccine strains," the team concluded.
The researchers cautioned that the study was only small, and it used antibody titres rather than clinical disease to measure the flu vaccine effectiveness.
Read the study HERE. KB
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