Vast majority infected with coronavirus are asymptomatic when tested, study finds

More than eight in ten people who test positive for coronavirus show none of the main symptoms at the time they are tested, a major study has shown.

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

They analysed data from the Office for National Statistics (ONS) coronavirus infection survey, which has been testing thousands of households every week regardless of whether people have symptoms.

The analysis looked at data for 36,061 people who had a test 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 while some people may have had symptoms in the days before their test or developed them later, the figures suggested large numbers may be spreading the virus while asymptomatic.

“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 who are out in the society and they’re not self-isolating because they didn’t know that they are positive.”

She said university students are one group who should be tested regularly, and definitely before they go 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.”

The researchers also argued for a change in testing strategy.

“Covid-19 symptoms are a poor marker of (Covid) 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 specific locations.”

Prof Petersen added: “Future testing programmes should involve frequent testing of a wider group of individuals, not just symptomatic cases, especially in high-risk settings or places where many people work or live close together such as meat factories or university halls.”

Tim Spector, professor of genetic epidemiology at King’s College London, who leads the Covid Symptom Study (CSS) app, said data from more than four million people who used the app and reported symptoms in a week found 85 per cent of adults reported fever, cough or loss of taste/smell.

“But the data on children and the over-65s from the CSS app 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.

“We need to learn from other countries and improve awareness of all the symptoms of Covid-19 to properly control the spread of the virus.”