More than 80 percent of people who tested positive in a UK COVID-19 survey had none of the core symptoms of the disease the day they took the test, scientists said.
The finding has prompted fears that future COVID-19 outbreaks would be hard to control without more widespread testing in the community to pick up “silent transmission,” particularly in universities and high-risk workplaces, such as meat processing facilities.
University College London researchers said that 86.1 percent of infected people picked up by the British Office of National Statistics (ONS) survey between April and June had none of the main symptoms of the illness, namely a cough, a fever, or a loss of taste or smell the day they had the test.
Three-quarters who tested positive had no notable symptoms at all, the scientists found when they checked whether people reported other ailments such as fatigue and breathlessness on the day of testing.
Unlike COVID-19 testing in the community which focuses on people with symptoms, the ONS survey routinely tests tens of thousands of households whether the occupants have symptoms or not.
“At the moment, the focus is on people who have symptoms, but if you are not catching all those who are asymptomatic or presymptomatic it may be really difficult to get outbreaks down in time, before they get out of control,” said Irene Petersen, an author of the study and a professor of epidemiology and health informatics.
While those who tested positive in the ONS survey might have gone on to develop a fever, cough or other common symptoms, Petersen believes there is a risk of “silent transmission” by people who are unaware they are infected.
The study, reported in Clinical Epidemiology, analyzed the symptoms described by more than 36,000 people who were tested between April and June. Only 115 tests came back positive and of those only 27 people, or 23.5 percent, had symptoms of any description.
When the scientists narrowed the symptoms down to the main three for COVID-19 infections, namely a cough, a fever, or a loss of taste or smell, the number reporting the ailments fell to 16, or 13.9 percent.
On the back of the findings, Petersen said that universities and high-risk workplaces, such as meat processing facilities, should do regular testing to pick up people who might be infectious, but are not displaying symptoms.
She urged universities to ramp up testing capacity so that students could be tested through the autumn and crucially before they return home at Christmas.
“Anybody who’s had students coming home at Christmas knows they often bring some sort of bug with them, and this Christmas in particular they could bring COVID home and potentially seed new outbreaks,” she said.
The government’s Scientific Advisory Group for Emergencies last month issued a similar warning, saying that there was a “critical risk” of large numbers of infected students sparking outbreaks across the UK when they returned home at the end of term.
Given the pressure on testing capacity, Petersen said that pooled testing was needed, where swabs are grouped together and tested as one batch.
Since most people do not have COVID-19, most pooled tests would be negative, but when a batch tests positive, the contributors would need to be tested individually to identify the infected person.
Patrick Maxwell, regius professor of physic at the University of Cambridge, said that the study underscored that many people who are infected are asymptomatic.
“There will be great public health benefit in terms of reducing transmission if we can reliably identify asymptomatic individuals and they then self-isolate,” Maxwell said.
He said that Cambridge was piloting an approach that uses pooled samples to enable a “mass asymptomatic testing program” for students.
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