Loss of a sense of smell may be a more reliable indicator of Covid-19 than cough or fever, research suggests.
A study by University College London (UCL) of 590 people who lost their sense of smell or taste earlier in the year found 80% had coronavirus antibodies.
Of those people with antibodies, 40% had no other symptoms.
The research only looked at people with mild symptoms, however.
Evidence that loss of smell and taste could be signs of coronavirus began to emerge from about April, and they were added to the official list of symptoms in mid-May.
Current guidance states anyone who experiences a loss of, or change to their sense of smell or taste should self-isolate and apply for a test.
But lead author of the UCL study, Prof Rachel Batterham, says cough and fever are still seen by many as the main symptoms to look out for.
She recruited people between 23 April and 14 May by sending out texts via four GP surgeries in London, enrolling those who reported losing their smell or taste in the previous four weeks.
All of these participants were tested for antibodies, and four out of five were positive, suggesting a previous Covid-19 infection.
The study was constrained by the fact that all its participants had mild symptoms, including or limited to a loss of smell or taste, so they may not be representative of all Covid patients.
But its findings emphasise the importance of people looking out for any change to their sense of smell or taste, and self-isolating if they realise they can't smell "everyday" items like perfume, bleach, toothpaste, or coffee, Prof Batterham said.
While not all coronavirus patients will necessarily lose their sense of smell, if you do lose your sense of smell it is highly likely to be coronavirus, this research seems to suggest.
The thing to look out for is a loss of smell without having a blocked or runny nose, Prof Batterham explained.
It's thought loss of smell happens with Covid-19 because the virus invades the cells found at the back of the nose, throat and on the tongue.
This is distinct from the experience of having a cold where smell and taste might be altered because a person's airways are blocked.
King's College London researchers, who run the Covid Symptom Study app, previously estimated 60% of people with coronavirus lost their sense of smell or taste.
Although this is considered a milder symptom and unlikely to land someone in hospital, Prof Batterham points out the potential dangers of losing your sense of smell including not being able to detect smoke, leaking gas or food that has gone off.
If suffered longer term, it can also have a significant impact on people's quality of life.
Thousands of people online have reported worrying experiences including causing fires and not being able to smell the smoke. Some have noticed constantly smelling a rancid "garbage" odour or experiencing a metallic taste, while others have found themselves unable to taste food for months after being clear of the virus.
The group of people who only lose their smell without experiencing any other symptoms may also pose the "greatest risk" to others since they may feel generally well and carry on going about their daily lives, Prof Batterham pointed out.
Although the two often go together, loss of or change to smell was more common than loss of taste among people who have recovered from coronavirus, she said.
Her research took place at a time when loss of smell and taste were not recognised symptoms of the virus.
Follow Rachel on Twitter.
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