Because testing for covid is no longer mandatory for certain events, nor is it as widespread as it was at the height of the pandemic, people no longer have a reason to get a test done to find out what is making them feel ill. 

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Those of us unlucky enough to fall ill at Christmas time know the hardship of trying to maintain festive cheer whilst silently suffering with whichever winter pathogen has invaded your body. Whether it is a cold, flu or even Covid, doctors are becoming increasingly unable to tell the difference as symptoms are blending and immunity is going down.

"There aren't really any symptoms we can look out for anymore that definitely tell us it's Covid," says Dr Helen Wall, the clinical director of population health for NHS Greater Manchester.

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"It can literally be any respiratory symptoms, and many others as well - Covid can masquerade as any respiratory illness.

"I have heard so many of my colleagues in the past week testing positive for Covid without any of the original symptoms we heard so much of during the peak of the pandemic. It can be really hard to distinguish whether it is coronavirus - or what it might be instead."

Because testing for covid is no longer mandatory for certain events, nor is it as widespread as it was at the height of the pandemic, people no longer have a reason to get a test done to find out what is making them feel ill.

As a result, people are staying at home suffering with a mixture of symptoms but have no specific sickness to attribute them to. The by-product of this is the difficulty in monitoring swells of Covid cases across the community.

Prof Eleanor Riley, an immunologist at the University of Edinburgh, said that two factors are contributing to what many would class as unexpectedly heavy bouts of sickness in recent times: diminished immunity against Covid strains and a lack of understanding of the blend of symptoms which people are experiencing .

"People's antibody levels against Covid are probably as low now as they have been since the vaccine was first introduced," Prof Riley said.

Antibodies are highly precise as they rely on a close match between the antibody and the part of the virus to which they stick. (Image: Getty Images)

The function of antibodies is to latch onto the surface of a virus to prevent it from infecting our body's cells and replicating. This is why you only suffer relatively mild symptoms over few days with Covid or the flu as your immune system fends off the foreign bodies.

However, Prof Riley explains that general levels of Covid-specific antibodies are lower across the population, and so experiences of harsh symptoms are more common because many people have only received their initial two vaccinations and a booster around two years ago - some have not been vaccinated at all.

Prof Riley added: "Now, because antibodies are lower, a higher dose [of the virus] is getting through and causing a more severe bout of disease."

Antibodies are highly precise as they rely on a close match between the antibody and the part of the virus to which they stick. The more a virus evolves to change its appearance, the less effective the antibodies become.

Prof Openshaw said: "The viruses circulating now are pretty distant immunologically from the original virus which was used to make the early vaccines, or which last infected them. A lot of people have very little immunity to the Omicron viruses and their variants."

So if you notice that you or someone you know has experienced a heavy bout of flu recently, it could be down to the phenomenon of waning antibodies and evolving viruses. However, the possibility of a heavy out does not necessarily mean hospitalisation will be required.

A different part of our immune system - called T-cells - kick in once an infection is already under way and they have been trained by past infections and vaccines. T-cells are less easily befuddled by mutating viruses as they spot cells that have been infected with Covid and kill them.

"They will stop you getting severely ill and ending up in hospital, but in that process of killing off the virus there's collateral damage that makes you feel pretty rough," says Prof Riley. Relying on your T-cells to clear out Covid is what results in the muscle pain, fever and chills.

Covid aside, another virus high in circulation right now is Norovirus, the highly contagious winter vomiting and diarrhoea bug, is also affecting all ages as it does every year.

Dr Wall explains: "There are lots of respiratory viruses doing the rounds at the moment - that means sore throats, coughs, colds. But it's not always just respiratory symptoms, you might find yourself feeling more generally unwell, feeling sore, fatigued.

"And it's really hard to get a definitive answer on exactly which virus it might be, but there are things that make us sure it is a virus. Viruses attack every cell in the body, whereas if you've got an infection, it's usually confined to one place."

Treating these winter viruses cannot be done with a "quick fix", said Dr Wall, who added: "It's about getting lots of rest, staying hydrated, taking paracetamol if you can."