How dangerous is the ‘K’ strain super-flu?

Cases of the new “subclade K” super-flu are “ballooning” in the UK, said NHS England. Its latest figures show that the number of flu patients admitted to hospital is up 50% on the same period last year, and an “incredible” 10 times higher than in 2023.

This “troublesome mutant” flu virus is a variant of influenza A H3N2, said London’s The Standard. And H3N2 generally tends to cause more severe illness and hospital admissions than influenza A H1N1, which has been more dominant in the UK in recent years. Subclade K of H3N2 is now the predominant flu virus in the UK and Japan, and samples taken in the US and Canada seem to show a similar trend.

What exactly is subclade K?

It’s part of the H3N2 flu virus “family” but it has undergone several mutations that have caused a distinct “genetic drift”. This means it’s “differentiated” from the reference strain of H3N2 chosen for use in this season’s flu vaccine – and could have “changed sufficiently to escape the immunity that has been built up from previous infections and vaccinations”, said Antonia Ho, a consultant in infectious diseases at the University of Glasgow, on VaccinesWork.

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“The good news” is that, this subclade K variant “does not seem to be more virulent or cause more severe disease” than other H3N2 strains, said microbiologist Ignacio López-Goñi on The Conversation.

So why the rise in cases?

H3N2 flu waves are “always hotter and nastier” than those caused by other strains, said The Telegraph’s science correspondent Joe Pinkstone. H3N2 is “inherently more severe and infectious than other types of flu, owing to more potent genes and a bigger ‘R rate’ – the number of people one infected person will pass the virus on to, on average”.

And then, as subclade K of H3N2 is different from previous strains and from the strain in the flu vaccine, people may be more “susceptible” to it, Giuseppe Aragona, a GP and medical adviser for an online pharmacy, told The Independent. In other words, our herd immunity and the NHS vaccine may offer us less protection than usual against this new flu strain.

Other factors that have contributed to the spike in UK cases include the flu season starting earlier this year, giving the virus more time to spread, and the fact that “fewer people have been exposed to flu in recent years, especially children, which leaves more people vulnerable”.

What should you do?

NHS bosses, warning of “a tidal wave of flu” in the run-up to Christmas, are encouraging everyone who is eligible to get the free NHS flu vaccine – including children (who can take it in the form of a nasal spray). You can also pay to get the vaccine privately at most pharmacies.

Data published by the UK Health Security Agency shows that the current vaccine is 70%-75% effective at preventing hospital attendance in children aged two to 17 years, and 30%-40% in adults. However well-matched to subclave K of H3N2 or not, it’s still “the best form of defence”, said Thomas Waite, UKHSA deputy chief medical officer.

There are three times as many people hospitalised with flu in London than at this time last year, said NHS England – and yet fewer than half of Londoners who are eligible for the flu vaccine have taken it up.