The Woolcock Institute of Medical Research

Australia is experiencing unusually high levels of influenza activity this summer. The number of reported cases increased throughout December, a pattern driven predominantly by influenza A and in particular a new variant of subtype H3N2, subclade K. Activity stabilised in January but remains at moderate levels.

In a typical year, influenza in Australia peaks between May and September. In December and January, flu activity normally plummets as we spend more time outdoors. But this year that usual “off-peak” period has been anything but.

“One of the biggest misconceptions is that you can only catch a cold or flu in the winter,” says Distinguished Professor Brian Oliver, research leader of the Woolcock’s Respiratory Cellular and Molecular Biology group. “But, it’s possible at any time.”

Central to this year’s unseasonal activity is a variant within the influenza A (H3N2) subtype known as subclade K, sometimes dubbed “Super-K.” This virus has become the dominant circulating form in Australia and across more than 30 countries worldwide.

Subclade K is not a totally new virus – it is an evolutionary branch of the familiar H3N2 influenza A. But it carries genetic changes that appear to help it spread efficiently through communities and contribute to prolonged transmission. While it isn’t necessarily more severe, it is high transmissible so there are more cases and a greater burden on our healthcare system.

SUPER K AND VACCCINATION

One reason for the increase in flu cases this summer is that our 2025 flu vaccine was formulated before Super-K emerged in the northern hemisphere as the dominant flu strain. The vaccine strains were selected based on circulating variants identified months earlier, and by the time it became widespread, the vaccine production “lock-in” had already occurred. This sort of vaccine-virus mismatch is common with influenza because the virus is constantly evolving.

Does that mean the vaccine offers no protection? No — even a “mismatched” flu vaccine still provides meaningful protection against severe disease, hospitalisation, and complications. That’s because some immune response overlaps between related strains, and vaccines also protect against the other strains included in the formulation.

However, because Super-K differs enough, the vaccine’s effectiveness against it is reduced compared with a perfectly matched season.

OTHER FACTORS

Several factors are contributing to this prolonged influenza activity:

• Vaccination rates: During and post-COVID flu vaccination rates were relatively higher. Coverage among those aged over 65 reached almost 70 percent. By 2024, concerns were being expressed about “vaccine fatigue” as rates dropped to pre-2020 levels.
• High transmissibility and global travel: As international travel has returned to pre-pandemic levels, viruses circulating in the northern hemisphere have more opportunities to “seed” transmission chains in the southern hemisphere.
• Behavioural factors: Holiday travel, crowded events and inter-state mixing over the summer months increase chances for the virus to spread.

EXPERT VIEWS AND PUBLIC HEALTH GUIDANCE

Public health authorities emphasise that, while the timing is unusual, seasonal influenza viruses can sometimes persist outside “peak” transmission periods. The extended season this year reinforces the importance of vaccination — even if imperfect — especially for people with respiratory illness and other high-risk populations, because it reduces the chance of severe outcomes.

“Young healthy people often don’t see the need for vaccination, but they are helping to protect more vulnerable people in the community,” says Distinguished Professor Oliver.

Updated vaccine formulations for the 2026 season are already being prepared to better match currently circulating viruses, including recent H3N2 variants.

Find out more

• Our Respiratory Cellular and Molecular Biology research group