There are growing fears of a new influenza pandemic as the rapid mutations of the current strain is threatening to evade vaccination efforts.

The latest strain, known as H3N2, is said by scientists to have mutated several times in recent months and from the UK, where urgent  vaccination warnings have been issued to the rising crisis in Japan, there is a belief that this year’s flu season will be the worst in over a decade.

Professor Adam Finn, professor of Paediatrics, University of Bristol, said there have been worrying signs for what the current season may have in store.

“There are several current concerns relating to the 2025-26 seasonal flu epidemic,” he explained. “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.”

Finn continued: “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 the speed of the current strain’s mutations has impacted vaccine production and efficiency.

“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,” she added. “For example, in 2022/23 winter season when A(H3N2) was the dominant virus, there were 16,000 influenza-associated deaths, compared to 8,000 last winter.

“It has arrived five 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.”

Ho explained: “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.”