More than eight in 10 people who tested positive for coronavirus in England had no symptoms of the illness at the time, the latest monthly study for the Government revealed today.
The research, by Imperial College London, its sister NHS trust and Ipsos MORI, found that infection rates fell between June 19 and July 8 but there was an “increased infection intensity in and near London”.
The REACT-1 study is the largest in the country, with almost 160,000 people agreeing to undertake a random nasal and throat test sent to them at home, to check for antigens showing the presence of Covid-19.
A total of 123 positive samples were found from 159,199 tests, a rate of 0.077 per cent. This is down from 0.13 per cent in May, when 120,620 tests detected 159 cases.
But 81 per cent of positive tests in the latest period were in people who had reported no symptoms, such as a new, continuous cough, high temperature or change in taste or smell, either on the day of the test or in the previous week.
This compares with 69 per cent of asymptomatic cases in the study’s first period, from May 1 to June 1.
In London, the latest prevalence rate was 0.15 per cent, with 23 positive tests from 15,750 samples.
The lack of symptoms is a concern as it means people will have the ability to spread the virus without realising they are infectious. The report said asymptomatic people were “a potential source of ongoing transmission”.
However they may have developed symptoms subsequently. The swab test does not show how infectious they were.
The second report covered a period when lockdown was eased, with more interactions outside the home and the reopening of non-essential shops.
The study said the prevalence of the virus halved every eight to nine days during this period, with an overall reproduction number of 0.58, compared with 0.57 in May.
At any point in the second study period, it is estimated there were 39,000 people in England with Covid-19, compared with 74,000 in May.
Health Secretary Matt Hancock said the research showed “we were able to keep rates of infection low as some restrictions were lifted”.
The first study raised concerns about higher infection rates in the 18-24 age group. The second study found a five-fold decrease in infection rates in this group but said people of BAME heritage were more likely to test positive.
Initial concerns about higher infection rates in health and social care workers were not detected for a second month, suggesting measures to curb transmission of the virus in hospitals and care homes were working.