Covid, cold or flu? Why it is getting tricky to tell them apart

In 1918 a deadly influenza pandemic swept the world, and doctors had a tough time diagnosing it. The symptoms were so unusual that many doctors, without the benefits of modern medicine, thought their patients had cholera, dengue fever or even typhoid.

Tom Calver 

The descendants of that H1N1 virus are circulating today as seasonal flu, yet mercifully the worst of the symptoms are consigned to the history books. Flu will put you in bed with a temperature and give you a cough, but it will not make your eyes bleed.

Neither will Covid, but some of its early symptoms were unusual: alongside a persistent cough, patients reported muscle aches and a lost sense of taste and smell. As other viruses make a comeback, from the “super-cold” to seasonal flu, it is becoming increasingly hard to tell what it is that we have.


Professor Tim Spector of King’s College London has been tracking the symptoms of millions of users of his Zoe Covid app since the start of the pandemic. “We’re not getting the classic symptoms nearly as much,” he says. “It’s much harder to tell between a cold and Covid now.”

That is a worry for GPs such as Simon Hodes, in Watford. “Monday was one of the busiest days I can remember in my career,” he says. “It was literally: cough, cough, cold, runny nose, temperature, fever. I don’t know what we’re dealing with when they come in.”

It would be a mistake to assume that the coronavirus is no longer causing serious illness: more than 1,100 people in Britain are dying of it every week.

The Delta variant, with which about one in 50 people are now infected, is about twice as infectious as a cold and three times as infectious as seasonal flu: research from Imperial College London suggests your chance of catching it from contact with an infectious person is one in ten. The virus is spreading quickly among households with children.

But for the majority of its carriers the cases are mild. Most who catch it are young: under-20s make up half of the caseload, and children’s immune systems are better at fighting it. And most of the adults who catch it are vaccinated, so their cases are generally less severe too.

Common cold symptoms are caused when the body’s immune system reacts to one of a number of viruses, says Dr Julian Tang, a virologist at Leicester University. Symptoms such as coughing, sneezing and a runny nose are caused by cytokines, released by the body’s virus-fighting B-cells (which make antibodies) and T-cells (which attack infected cells).

But in years to come, as newer variants emerge, experts such as Tang believe that Covid symptoms could get even milder. “We will see fewer of the unusual neurological features like altered smell and taste, tinnitus and psychological issues — though, as with flu, the muscle aches, fatigue, fever, coughs and headaches may persist.”

What remains odd about Covid is the variety of symptoms. John McCauley, director of the Worldwide Influenza Centre at the Francis Crick Institute in London, is one of the world’s leading flu experts. “The consistency between patients is not as simple as with something like measles, which has well-defined symptoms. The loss of taste and smell in some Covid patents is unusual, but many do not have this: it seems to be a whole spectrum.”

This inconsistency is not unheard of: when McCauley caught swine flu in 2009, his only symptom was muscle ache.


While 38,000 people are testing positive for Covid a day (and many more unaware of it), Spector says the number of people catching colds could be four to five times higher. Colds, which are caused by a number of viruses including rhinoviruses, seasonal coronaviruses and RSV, have been circulating for a long time in humans. “Newborn babies and children are exposed to all of these viruses from a young age,” Tang says. “But because these viruses have adapted to humans to some extent, the illnesses have been generally milder than with Covid, which is a new virus.”

Since “freedom day” on July 19, we have become reacquainted with old foes that are taking advantage of our immune systems being out of practice. Since September social media users have spoken of a “super-cold”, which is simply a made-up name for all the illnesses that we have not had for two years. Data from the UK Health Security Agency shows that calls to the NHS 111 service about cold and flu symptoms are much higher than expected for this time of year, particularly among patients aged 15 to 44.


One piece of fortune has been flu rates, which are below average for this time of year. But if levels surge, it may be confused with Covid: a high temperature, a cough and muscle aches are symptoms of both. This matters: if a flu patient is admitted, quick diagnosis is key if they are to benefit from anti-flu drugs such as oseltamivir.

McCauley has spent the past months developing a flu jab for the months ahead. The uptake is expected to be high, with about 63 per cent of over-65s having had it. With so little flu in the world, it has been hard to know what to base the flu jab on. “It’s really too early to tell [how effective] it will be this year. What we can say is that we have the viruses well covered, or pretty well covered.”

How bad a flu outbreak is in a given year mainly depends on which strain circulates. “H3N2 viruses are probably the worst overall, with the elderly being particularly vulnerable,” McCauley says. These viruses emerged from the 1968 Hong Kong flu outbreak, which killed an estimated million people worldwide.

Flu or no flu, a cocktail of respiratory viruses looks likely to dominate this winter — and to tell them apart, many countries are acknowledging the wide range of symptoms that Covid is causing. The WHO lists 13.

Britain has not followed suit, opting instead to list just the big three: temperature, cough, and loss of taste or smell. Spector has been campaigning for the NHS to expand its official symptom list for 18 months. “They’re refusing to add flu and cold-like symptoms,” he says. “Perhaps they’re worried about testing capacity or think it’ll create confusion.”

With such an overlap in symptoms, the only way to be confident that your illness is not Covid is to take a test. The coronavirus is like a cold for most, but for about 170 people a day it is a death sentence.

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