It may only be early October, but flu season appears to be already under way – months earlier than usual. Official data released last week shows cases of respiratory viruses are on the rise across Britain and health chiefs are warning of ominous signs of an unusually difficult winter ahead.

Monthly Covid infections are at their highest since October last year and are rising rapidly, with 1,374 cases recorded in August and 4,589 in September.

Flu cases, meanwhile, are triple what they were this time last year – with NHS England’s national medical director Professor Frankie Swords saying monthly hospitalisation figures are ‘the highest I can remember seeing at this time of year’.

‘Something is afoot,’ wrote prominent epidemiologist Dr Caitlin Rivers in a recent edition of her influential medical newsletter, Force Of Infection.

Dr Rivers, who is based at John Hopkins University in the US, where a similar surge in viruses is occurring, says she plans to get her flu vaccination as soon as possible, months earlier than usual.

So what’s going on? Experts say the flu virus now spreading appears to be more contagious than last year’s strain, making an early spike in cases more likely.

Preliminary data from the US suggests the predominant strain this year contains mutations that could be more likely to evade the immune system. As a result, analysis published in the journal Nature concluded that fewer people will already have immunity to this year’s flu. But, intriguingly, other experts say this year’s early flu season could reflect genuine changes in the way that respiratory virus waves spread.

‘We don’t usually see flu going up in that noticeable a way until December,’ says Dr Adam Finn, emeritus professor of paediatrics at the University of Bristol. ‘So the visible increase in flu recorded in September really is quite early.’

This could be partially due to the lack of immunity to flu caused by the Covid lockdowns, he says. ‘Covid changed all respiratory infections because no one was really socialising for around two years, so no one caught flu those winters,’ adds Prof Finn. ‘As a result, many people – particularly younger adults and children – have far less immunity to flu than they did before.’

But other shifts can’t be as easily explained. Several Asian countries in the northern hemisphere – which typically experience a similar flu season to the UK – have been seeing an extraordinary phenomenon over the past few years.

Rather than hitting hardest in the winter, when more people are inside and their immune systems, lacking vitamin D, tend to be weaker, flu has begun to strike at the height of summer.

Japan declared a flu epidemic in early October last year, after cases began to spike as early as July. And in Taiwan and Hong Kong, this summer saw a similar increase in flu cases, which reached a ten-year high.

‘It’s caused a lot of consternation,’ says Prof Finn.

It is unclear why the shift has occurred, and whether it marks a permanent change. Hotter summers due to climate change – causing people to spend more time in air-conditioned, poorly ventilated indoor spaces – could have a role, Prof Finn theorises.

This may also, in part, explain the UK’s early start to flu season, following the record hot summer.

Prof Finn argues that Asia’s early flu season could also be due to low immunity, meaning major outbreaks are more likely to occur. And if these changes persist, it could require an adjustment to vaccination timings globally. ‘It’s a very big change that has lots of consequences,’ says Prof Finn.

‘If the flu now begins in April and continues until September, vaccines being offered in October are useless,’ he says. ‘Instead, they may need to roll them out considerably earlier. Experts are scratching their heads about what to do.’

In the meantime, however, the advice remains the same – get the flu vaccine if eligible for it and make sure to wash your hands as much as possible.

Dr John Tregoning, professor of vaccine immunology at Imperial College London, says: ‘We’re not sure why flu season started earlier – but getting vaccinated and reducing risk-taking behaviour is the best thing you can do to not get sick.’