This year's flu season has arrived much earlier than expected, with cases already rising five weeks ahead of the typical schedule - raising concerns about transmissibility, reduced vaccine coverage, waning immunity and severe illness.
Experts have warned that the surge is driven by an H3N2 strain that developed seven new mutations over the summer months and made the virus significantly more transmissible than usual.
Where previously 100 infected people would pass it on to roughly 120 others, this strain can now infect around 140 people from the same starting point.
Health experts are particularly worried because the early start means the virus is spreading before many vulnerable people have had their flu jabs, presenting a challenge to protection efforts.
New strains could dodge much of the immunity Britons have built up against influenza
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The mutations have created a virus that's different enough from previous strains to dodge much of the immunity people have built up, meaning protection from past infections and vaccinations isn't as effective as it would normally be.
"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," explained Dr Antonia Ho from the University of Glasgow.
The vaccine strains are chosen in February for the winter season, so this summer's mutations have left a gap between what's in the jab and what's actually circulating.
The timing is particularly concerning given what we know about H3N2's impact. During the 2022/23 winter, when this strain dominated, there were 16,000 flu-related deaths compared to just 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," warned Dr Ho.
The children's vaccination programme only began in September, with adults following in October. Usually, people have November to get protected before infections spread in December and January.
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But with cases already climbing, many haven't had the chance to book their appointments, let alone receive their jabs.
"From previous experience, influenza waves that start early tend to affect a larger number of people in the population," noted Dr Ho.
Professor Adam Finn from the University of Bristol shares these concerns, noting: "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."
At-risk groups have been urged to attend their vaccination appointments
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Despite the vaccine mismatch, experts stressed it should still offer some protection against this strain and others that might emerge later in winter.
Dr Emma Thomson from Glasgow urged at-risk groups to attend their vaccination appointments as advised by the NHS.
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