The vast majority of people who test positive for coronavirus do not have any key symptoms on the day of their test, a study has suggested.
Some 77% of people who had a positive test had no symptoms on the day of their test, while 86% did not have a cough, temperature or loss of taste/smell.
Researchers led by Professor Irene Petersen at University College London (UCL) 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%) had a positive test result, the study found, of whom 27 (23.5%) were symptomatic and 88 (76.5%) were asymptomatic on the day of the test.
When looking at cough, fever and loss of taste/smell – seen as the three main symptoms – 86.1% of those who tested positive had none of these.
Prof Petersen said people may have had symptoms in the days before their test or developed them later, but the figures suggested large numbers may be spreading the virus while asymptomatic.
She told the PA news agency: “They may be silent transmitters and they don’t know about it. And so I think that’s a problem.
“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,” she said.
“Hopefully they can get that (testing) up and running before Christmas, I don’t think they should wait until Christmas.”
The researchers said there was a need to change testing strategies.
“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 over a week found that 85% 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% 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% 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.”
Paul Hunter, professor in medicine at the University of East Anglia, said the UCL study could not determine the proportion of people with Covid who become symptomatic or remain asymptomatic at some stage during their infection, due to the fact it looked at a fixed time point.
He said: “Anyone who was previously symptomatic and had now recovered or who were currently incubating the infection and would develop symptoms within the following hours would not be included as being symptomatic in this study.”
A Department of Health and Social Care spokesman said: “Since the beginning of this pandemic we have prioritised testing for health and care workers to ensure all NHS staff have consistent access to testing.
“NHS staff with symptoms can access testing as a priority and staff in outbreak areas can access tests if they are asymptomatic. We will continue to expand testing availability as our capacity expands to 500,000 tests a day by the end of October.”
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