Covid-19 coronavirus: Vast majority of infected people have none of the main symptoms

People wearing masks to stop the spread of Covid-19 get out of a bus in Rome, Italy. Photo / AP

More than eight in 10 people who tested positive for Covid-19 showed none of the main symptoms when they were screened, a major study showed.

Scientists at University College London found that 86 per cent did not have a cough, temperature or loss of taste or smell, while some 77 per cent showed no symptoms at all.

They analysed data from the Office for National Statistics coronavirus infection survey, which has tested thousands of households every week regardless of whether people had symptoms.

The analysis looked at data for 36,061 people tested between the end of April and the end of June. Some 115 (0.32 per cent) had a positive test result, the study found, of whom 27 (23.5 per cent) were symptomatic and 88 (76.5 per cent) were asymptomatic on the day of the test.

Professor Irene Petersen, who led the research, said that while some people might have had symptoms before their test or developed them later, the data suggested that large numbers may spread the virus while asymptomatic.

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"They may be silent transmitters and they don't know about it - and so I think that's a problem," she said.

"You may have a lot of people out in society and they're not self-isolating because they didn't know that they are positive."

University students, she said, should be tested regularly and definitely before they went home for Christmas.

"I think you could seed a lot of new infections around Christmas - you're indoors, you sit around the table. Hopefully they can get that [testing] up and running before Christmas. I don't think they should wait until Christmas."

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The researchers argued for a change in testing strategy.

"Covid-19 symptoms are a poor marker of infection," they wrote in the journal Clinical Epidemiology.

"In order to capture 'silent' transmission and potentially prevent future outbreaks, test programmes should involve frequent and widespread (Covid-19) testing of all individuals, not just symptomatic cases, at least in high-risk settings or speciic locations."

Professor Tim Spector, a genetic epidemiologist at King's College London, who leads the Covid Symptom Study app, said data from more than four million people who used the app and reported symptoms over a week found that 85 per cent of adults reported fever, cough or loss of taste/smell.

"But the data on children and the over-65s tell us a different story," he added.

"Only using the UK's three classic symptoms will miss around 50 per cent of cases in these important groups that were included in the ONS survey.

"In a sub-study at King's College London of twins using antibody testing and the ability to report 20 different symptoms, we showed that only 19 per cent of people are truly asymptomatic."