This year’s flu season has hit the UK earlier than expected, prompting concerns about a potentially severe outbreak. Conor Meehan, an Associate Professor of Microbial Bioinformatics at Nottingham Trent University, has cautioned that the country might face one of its most challenging flu seasons yet. Typically, flu season is considered underway when approximately 10% of suspected flu cases yield positive results.

In early November, 11% of daily flu tests returned positive, a stark increase compared to the same period last year, when only 3% tested positive. This indicates that the UK reached the critical 10% threshold a full month sooner than it did in the previous year. Writing for The Conversation, Professor Meehan noted, “It’s still early to determine the severity of this year’s flu season, but given its premature onset, the strain’s high transmission rate, and the reduced effectiveness of the vaccine, a higher number of cases than usual could be anticipated.”

Professor Meehan elaborates that in the UK and other northern hemisphere nations, the flu season generally spans from mid-November to mid-February. In contrast, the southern hemisphere experiences flu season from May to July.

Accurately determining the current number of flu cases in the UK is challenging, as many individuals do not report their illness, opting instead to rest and recover at home.

To gauge the full impact of this year’s flu season, Professor Meehan highlights the reliance on hospital admissions and general practitioner reports, which typically reflect only the most severe instances of the disease.

He explained, “We recognize the onset of flu season when around 10% of suspected cases test positive for the influenza virus.”

“The UK’s flu season is already well underway – and weeks before it usually starts. This is because at the start of November we were already seeing 11% of daily tests come back positive for the flu. 

“At the same time last year, just 3% of tests were positive. The UK crossed the 10% threshold a whole month earlier than it did last year. 

“School-aged children are currently most affected, with 38% of tests coming back positive for the flu – up from 30% just one week prior. Around this time last year, the number of children testing positive for flu was just under 7%. 

“Similar increases have been seen elsewhere, such as in Japan and across Europe.” 

What’s causing this early flu season? 

The UK’s flu vaccine uptake seems to be almost identical to previous years, so the increase in cases cannot be explained by a fall in vaccination rates. 

One likely factor contributing to the UK’s early spike in flu cases is the strain of influenza virus that’s circulating. 

Prof Meehan adds: “Flu is caused by influenza viruses – mainly the influenza A virus. There are lots of variants of this virus, so they’re usually designated by a combination of H and N numbers. 

“For example, H5N1 is the main cause of the ongoing avian flu pandemic in birds and other animals. Seasonal flu in humans is usually caused by H3N2 and H1N1. 

“The seasonal flu vaccine is designed to combat these two strains, as well as an influenza B virus alongside them. This vaccine tends to be between 20-70% effective at preventing the flu, depending on the year. 

“The vaccine tends to be most effective for school-aged children, especially in preventing severe forms of the disease. 

“A new vaccine is developed every year as the circulating strains of influenza can mutate over time, reducing vaccine efficacy. 

“Twice a year (once for each hemisphere), the World Health Organization convenes an expert panel to decide, based on the strains that circulated last year, what strains of influenza should be used to build the vaccine for the coming flu season. The vaccine almost always includes an H1N1, H3N2 and influenza B strain. 

“Generally, building these vaccines based on what circulated previously is quite effective. This is because any genetic changes that occur in these strains between flu seasons aren’t large enough to render the vaccine ineffective. 

“But this year there seems to have been an exception. A new strain of influenza, influenza A H3N2 subclade K, is now infecting the majority of people. 

“This strain has seven mutations that differentiate it from the previous H3N2 strain. This is many more genetic mutations than what’s usually seen between seasons.”

He says it’s too early to know why this strain has developed so many genetic mutations. But we do know that these changes appear to have made this strain slightly more transmissible compared to previous strains. 

The strain’s R number (the average number of people an infected person will go on to infect) increased from the usual 1.2 for influenza to 1.4. This means about 20% more people will be infected than we would normally expect. 

Early research into this strain shows that the vaccine is still very effective in children at preventing severe forms of the disease. But in adults, effectiveness has dropped to between 30% and 40%. 

However, he says we can’t say just yet whether reduced vaccine efficacy in adults and the new mutations to the H3N2 strain are the causes behind the current spike in flu cases. 

He goes on: “It’s also too soon to know whether this year’s flu season will be more severe than in previous years. 

“But based on its early start, the strain’s high R number and low vaccine effectiveness in adults, we might expect higher numbers than usual. 

“And, if we look at data from southern hemisphere’s flu season – which usually gives us a good idea of what we should expect – Australia saw its worst flu season ever. They reported 10% more cases than in the previous year.” 

Giving tips on how to protect yourself, he stresses that, especially in children, the vaccine is still the best form of protection. 

He continues: “Flu can be very severe in both the young and old, resulting in hospitalisation and sometimes death. Vaccination (including by those who regularly come in close contact with older and younger people) is key. 

“It’s also important to know how flu symptoms differ from those of the common cold so that you can recover and protect others from catching it. The presence of fever, headache and a strong cough typically indicate the flu. 

“If you have these symptoms, you should rest and follow standard flu guidance. Also remember you’re infectious for a week or so after symptoms start, so isolating at this time will stop the virus from spreading. 

“Alongside getting the jab, wearing a mask and following good hand hygiene can help you avoid getting sick and prevent you from spreading the flu if you are sick.”