People who were asymptomatic accounted for 86% of the people who tested positive for Covid-19 in a UK sample population during lockdown, a study showed, meaning the current policy of testing people with symptoms might miss many cases.
In England, people are encouraged to get tested for Covid-19 only if they have symptoms of a persistent cough, fever, or loss of taste or smell, with suspected contacts of positive cases told to self-isolate in the first instance.
But epidemiologists at University College London found that such an approach might miss the vast majority of cases, complicating the UK prime minister Boris Johnson’s attempts to clamp down on a second wave of the virus.
UCL scientists used the Office for National Statistics Infection Survey, which looks at the prevalence of Covid-19 in the community and not only those who get a test because they have symptoms.
The pilot study sampled 36,061 people living in England, Wales and Northern Ireland who were tested between 26 April and 27 June.
Of the 115 with a positive result, only 16 reported symptoms, with 99 not reporting any specific symptoms on the day of the test. Moreover, 142 people who reported symptoms on the day of the test did not test positive for Covid-19, vastly outnumbering those who tested positive.
“The fact that so many people who tested positive were asymptomatic on the day of a positive test result calls for a change to future testing strategies,” said Irene Petersen of UCL Epidemiology & Health Care.
“More widespread testing will help to capture ‘silent’ transmission and potentially prevent future outbreaks.”
The authors noted that other studies showed different results, with one in China suggesting just 5% of cases were asymptomatic, and a study in Iceland suggesting 43 cases out of 100 had no symptoms.
They added that the sampling used in any study was likely to be a factor in its findings.