Prediction: Flu could be severe in 2025|2026

Seasonal influenza is an annually recurring outbreak of Influenza, which occurs during the cold half of the year in each hemisphere. Annually, about 290,000 to 650,000 deaths from seasonal flu occur worldwide. Influenza A(H3N2) virus is one of the prevalent subtypes of the Influenza virus which contribute to the seasonal outbreaks, and a strain of Influenza A(H3N2) virus is included in the biannual reformulation of the Influenza vaccine.

In June 2025, seven mutations were found in a new strain of Influenza A(H3N2) virus[1]. This variant emerged at the tail end of the 2025 flu season in the Southern Hemisphere, after the WHO had already selected the J.2 subclade to use for the Influenza A(H3N2) virus component, for the upcoming Northern Hemisphere winter's flu vaccine.

This led to an earlier than usual start to the flu season in the United Kingdom, Canada, and Japan, and unusually severe Influenza activity in those countries. This in turn triggered fears that the new Influenza A(H3N2) virus, belonging to subclade K virus, could cause the worst flu season in at least a decade, which would produce two severe back-to-back flu seasons. Subclade K was formerly known as subclade J.2.4.1.

The new Influenza A(H3N2) virus, subclade K variant, has an estimated basic reproduction number (R₀) around 1.4, slightly higher than the typical R₀ of 1.2 for the currently circulating seasonal Influnza strains.

It means that your vaccination against Influenza might not be as perfect as it could have been.

There is a lot of nuance and uncertainty, but leading experts have told me they would not be shocked if this was the worst flu season for a decade.

"We haven't seen a virus like this for a while, these dynamics are unusual," says Prof Nicola Lewis, the director of the World Influenza Centre. "It does concern me, absolutely," she adds. "I'm not panicking, but I am worried."

[1] Sabaiduc et al: Emergence of seasonal influenza A(H3N2) variants with immune escape potential warrants enhanced molecular and epidemiological surveillance for the 2025–2026 season in Journal of the Association of Medical Microbiology and Infectious Disease Canada - 2025