Given that the symptoms associated with the original strain of Covid-19 and its first variants were so similar to the common cold, it has been difficult to tell over the last year or so whether the onset of headaches and sniffles meant you had contracted the coronavirus or just a bout of conventional winter flu - but the subsequent emergence of the Omicron variant has complicated the pictured even further.
The symptoms of the new variant are slightly different - stuffy nose, sore throat - and because it cannot yet be specifically identified by home test kits, which simply tell us whether someone is Covid-positive or negative, not which strain they have contracted.
Professor Tim Spector, from Britain’s ZOE Covid app, said it is now more important than ever to get tested - even without symptoms - as we seek to bring the new variant under control against soaring infection rates.
It comes as Dr Chaand Nagpaul, chairman of the council at the British Medical Association, said there was concern over a “significant increase” of people in hospital with coronavirus.
Professor Spector said that data from the ZOE study app suggests that about half of all cases of Delta are being “missed” because people are only on the lookout for the “classic” Covid symptoms of fever, new and persistent cough and a loss or change of smell or taste they have been told to expect by the official guidance, whereas a mild case may not result in all of the conditions on that checklist being experienced.
“Omicron is probably much more similar to the mild variants we’re seeing in people who have been vaccinated [but still subsequently taken ill] with Delta than anything else,” he said.
“And so it is going to be producing cold-like symptoms that people won’t recognise as Covid if they just believe the official government advice.”
Christina Marriott, chief executive of the Royal Society for Public Health says: “Growing evidence shows that people who’ve received two doses of the vaccine typically present with less severe symptoms, such as headache, runny nose, sneezing, sore throat and loss of smell.
“It’s important for people who’ve been fully vaccinated to stay vigilant for cold-like symptoms and get tested if they’re living or working around people who are at greater risk from the disease.”
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Professor Irene Petersen, a professor of epidemiology and health informatics at University College London, adds: “A runny nose and headache are symptoms of many infections, but may also be the first symptoms - and only symptoms - of Covid. Therefore, if you have these symptoms, I’d encourage you to use lateral flow tests for a couple of days.”
The ZOE Covid Symptom Study, funded by the government, has identified the top symptoms associated with Covid and says they differ slightly depending on whether you have been vaccinated or not.
Although headaches are a less well-known symptom of Covid, they are one of the earliest signs, according to the ZOE study, and are more common than the classic symptoms of cough, fever and loss of smell.
The study found Covid headaches tend to be moderately to severely painful, can be “pulsing”, “pressing” or “stabbing”, occur across both sides of the head rather than in one area, may last for more than three days and tend to be resistant to regular painkillers.
Last winter, the ZOE study found that a runny nose was the second most commonly reported symptom after headaches, with nearly 60 per cent of people who tested positive for Covid with loss of smell also reporting having a runny nose.
But now the data indicates that the prevalence of the disease is the most significant factor. So, when Covid rates are high, the chances of a runny nose being due to the virus are also high.
The study also stresses that when infection rates are low, a runny nose is less likely to indicate the sufferer has caught the coronavirus and is more likely to be due to a cold or even an allergy.
It concludes that while many people with Covid may report a runny nose, it is difficult to call it a definitive symptom as it is so common, especially during winter.
The ZOE study found sneezing more than usual can be a sign of Covid in people who have been vaccinated, although it stresses sneezing is much more likely to be a sign of a cold or an allergy.
It says that even though many people with Covid might sneeze, “it’s not a definitive symptom because sneezing is so common”.
Many people with Covid have reported via the ZOE Study app that they have a sore throat that feels similar to one you might experience you get when you have a cold or laryngitis.
Covid-related sore throats tend to be mild and last no more than five days so a very painful one that lasts longer is likely to be something else. If it persists, you should contact your GP.
Although it can be a Covid symptom, most people with a sore throat will probably just have a cold. According to ZOE’s data, almost half of people who are ill with Covid report having a sore throat, although this is more common in adults aged between 18-65 than the elderly or those under 18.
Loss of smell
This continues to be the strongest indicator of Covid infection, regardless of a person’s age, sex or illness severity.
While people who have Covid might not lose their sense of smell completely, it may change, so you may not be able to smell strongly-scented things, and your sense of taste may be affected too, so food may taste different or seem tasteless.
A persistent cough is widely agreed to be one of the three main symptoms of Covid but, according to the ZOE study, only around four in 10 people who are ill with the virus will experience this.
In this context, “persistent” means coughing many times a day, “for half a day or more”.
A Covid cough is usually a dry cough, compared with a chesty one that brings up phlegm or mucus and that may indicate a bacterial infection. A persistent cough tends to arrive around a few days into the illness and usually lasts for around four or five days.
Alan McNally, a professor of microbial evolutionary genomics at the University of Birmingham, who was infectious disease lead at the Milton Keynes Lighthouse Lab - the UK government’s first flagship Covid testing facility - adds: “If you have any symptoms of respiratory infection, you should stay at home to prevent transmission and get a test done.
“Trying to self-diagnose is a sure fire way to send Covid case rates soaring again.”