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Experts comment on reports of mutations in the H3N2 flu virus
Prof Adam Finn, Professor of Paediatrics, University of Bristol, said:
“There are several current concerns relating to the 2025-26 seasonal flu epidemic. The main one is that it appears to have started earlier than usual. This is worrying because it suggests that the strain or strains in circulation are finding it easier than usual to spread about and so are doing so before the colder weather conditions that usually signal and drive the onset of the season.
“An important possible explanation for this could be that there is less immunity than usual in the population controlling the spread of flu viruses.
“When a completely novel strain of flu that can spread between people arrives against which no one has immunity, we call it a pandemic, and very large waves of infection occur at any time of the year.
But sometimes a new strain develops that isn’t entirely new but sufficiently new to enable it to spread more efficiently because our immunity from previous infections and vaccinations is not that well matched.
“This is what may be happening this year as one of the strains of influenza A that we are familiar with – called H3N2 – seems to have acquired several new mutations over the summer and is responsible for many of the flu cases showing up now.
“The other reason why the early start of this year’s flu epidemic is a worry is that there has not been enough time yet for a lot of people to get vaccinated. The children’s programme kicked off in September and the adult programme in October. Usually, November is a month when a lot of people get the vaccine before the infections start to spread in December and January.
“Even though this new strain of H3N2 has arrived and it is different from the H3N2 strain in this year’s vaccines, they should provide some degree of protection against this strain and certainly against the other flu strains that may be coming through as the winter goes on.
The bottom line is that it’s looking possible that we may be facing a very bad flu season this year and the best thing we can all do right now to tackle the problem is to get vaccinated.”
Dr Antonia Ho, Clinical Senior Lecturer and Consultant in Infectious Diseases, University of Glasgow, said:
“Of the two seasonal influenza A viruses, the current dominant circulating virus (A/H3N2) tends to cause more severe illness than A/H1N1, particularly in older adults. For example, in 2022/23 winter season when A(H3N2) was the dominant virus, there were 16K influenza-associated deaths, compared to 8K last winter.
“It has arrived 5 weeks earlier than a ‘normal’ flu season, which means a smaller proportion of the vulnerable population may have received the flu vaccine.
“From previous experience, influenza waves that start early tend to affect a larger number of people in the population.
“The current circulating influenza A(H3N2) virus acquired 7 new mutations over the summer, which means the virus is quite different to the A(H3N2) strain included in this year’s vaccine (vaccine strains are selected in February for the Northern hemisphere winter season). It also means that the virus could have changed sufficiently to escape the immunity that have been built up from previous infections and vaccinations.
“The R (reproduction) number is thought to have increased from 1.2 to 1.4, which means it is more transmissible. I.e. 100 people infected with this strain of flu can infect 140 people (rather than 110 to 120 people in other flu seasons)”
Declared interests
Dr Antonia Ho: I have previously been funded by Public Health Scotland to undertake enhanced surveillance for influenza and SARS-CoV-2 (2021-2025). I was previously employed as a clinical consultant for the WHO Global Influenza Programme (2023-24). I am currently funded by Pfizer to evaluate the real-world vaccine effectiveness of RSV vaccine.
Prof Adam Finn: AF provides paid consultancy to several flu vaccine manufacturers including GSK, Sanofi and CSL/Seqirus.