Britons could be at risk of triple-infections this winter amid fears the country may face its 'worse flu season on record', scientists have warned. 

Leading virologists fear a bad flu outbreak could leave people vulnerable to other seasonal illnesses at the same time, such as norovirus and cold-like respiratory syncytial virus (RSV). 

Covid is also expected to continue circulating. 

A troublesome mutant strain of the flu is believed to be behind a concerning early spike in flu cases, that experts say may be more severe than usual.

This is because genetic mutations to the H3N2 strain have made it 'hotter' and 'nastier', leaving people more vulnerable to infections. 

Officials have urged those with coughs, sore throats or a runny nose to limit contact with vulnerable groups — such as the elderly, pregnant women and those with underlying conditions — over fears they could become seriously unwell with one or more winter viruses.

This 'quad-demic' of four winter illnesses — flu, Covid, norovirus and RSV — could also pile pressure on an already stretched health service, they said. 

Dr Simon Clarke, an infectious disease expert at the University of Reading, told the Daily Mail: 'There is very good evidence that flu and Covid can simultaneously infect someone.

'Because the symptoms can be similar, it can be very difficult to distinguish between the different viruses without proper testing. 

'Equally, there's little reason why someone infected with a respiratory virus like flu, RSV or Covid could not be co-infected with norovirus.' 

But, he also added that evidence also suggests a flu infection may provide people with 'decent protection against RSV for a few months'. 

People, however, could still be infected with both RSV and flu at the same time and experiencing both viruses together could lead to more severe symptoms. 

One 2024 study, published by German researchers in the journal Nature, found that a flu or RSV infection ''yields between a 40 to 100 per cent protection infection with the other virus for one to five months'. 

Professor Paul Hunter, a renowned infectious diseases expert from the University of East Anglia, also told the Daily Mail: 'You can certainly catch more than one infection at the same time or close to the same time. These are called co-infections.'

He added: 'You can sometimes tell the difference clinically but not often because they are essentially all viruses that cause gastrointestinal and respiratory symptoms. 

'The difference in symptoms between the difference virus infections are not always reliable'.  

It comes as health chiefs last week sounded the alarm over a surge in flu cases, that appear to be spreading more quickly than usual. 

Experts believe this may be because the flu strain H3N2 mutated seven times over the summer, making it more severe, particularly for older people. 

Latest UK Health Security Agency (UKHSA) data shows that of influenza viruses subtyped, the majority were H3N2.

The rate of hospital admissions for flu in England stood at 3.8 per 100,000 people last week, up from 2.4 the previous week. 

It is a level of admissions usually not reached until the start of December, the UKHSA said. 

Flu rates also remain over three times higher than normal for this time of year, with the highest rates seen in children and young people. 

Prof Nicola Lewis, the director of the World Influenza Centre at the Francis Crick Institute in London, told BBC News: 'H3 is always a hotter virus, it's a nastier virus, it's more impactful on the population. 

'We haven't seen a virus like this for a while, these dynamics are unusual. It does concern me, absolutely.' 

Anecdotal reports suggest H3N2 can cause more severe body aches, longer-lasting tiredness and symptoms come on more suddenly. 

Last year saw one of the UK's worst flu seasons for a decade, with the outbreak having greater impact due to lower population immunity.

Since September, health officials have urged the millions eligible to come forward for their Covid and flu jabs, to reduce their risk of becoming severely unwell and needing hospital care.

They also advised people to turn to pharmacies and local urgent treatment centres where appropriate.

Among those invited for top-up doses are the over-65s, care home residents, frontline health and social care workers and pregnant women.

As per protocol, the decision on the design of the vaccine in the UK was made back in February to ensure there was enough time to produce the millions of flu jab doses necessary. 

But given the mutant variant did not appear until June, the vaccine is not expected to offer full protection against the strain.

Experts have cautioned, however, that the vaccine is still vital because it can lessen severe symptoms. 

Professor Adam Finn, a professor of paediatrics at the University of Bristol, said: 'One 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.'

Latest UKHSA data shows Covid levels are still low and at expected levels.

Among all tested samples from GP surgeries, just 3.5 per cent were positive for Covid, while hospital admissions stood around 2.22 per 100,000 patients. 

Covid infection rates, however, are nowhere near levels seen earlier in the pandemic.

Infections climbed as high as 4.3million in 2022 during the spread of the original Omicron — but this was topped a few months later, when the number reached a record 4.9million.

The threat of Covid for many people has been drastically reduced by the wall of immunity built up by vaccines and infections.

Norovirus, meanwhile, can show similar symptoms to Covid, with both viruses causing chills, fever and headaches. But it also causes explosive vomiting and diarrhoea.

The stomach bug usually goes away on its own within two to three days. But sufferers, who may also experience aching arms and legs, are advised to stay at home until 48 hours after their symptoms have stopped.

In severe cases, however, some people may need to be hospitalised with the elderly and children most at-risk.

It is spread through close contact with someone who is infected; touching surfaces or objects that have the virus on them, and then touching the mouth; as well as eating food that's been handled by someone with norovirus.