Symptoms are a poor marker of Covid-19, finds UK study; experts say important to catch 'silent' transmission

LONDON: Nearly 86 per cent of people who tested positive for COVID-19 during lockdown did not have any of the known coronavirus symptoms such as cough, fever and loss of taste or smell, a new UK study revealed on Thursday.

The University College London (UCL) authors behind the analysis, published in ‘Clinical Epidemiology', conclude that a more widespread testing programme is therefore needed to catch “silent” transmission.

“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. More widespread testing will help to capture ‘silent' transmission and potentially prevent future outbreaks,” said Professor Irene Petersen, from UCL Epidemiology & Health Care.


“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. In the case of university halls, it may be particularly relevant to test all students before they go home for Christmas,” she said.

The researchers used data from the UK's Office for National Statistics (ONS) Coronavirus (COVID-19) Infection Survey pilot study – a large population based survey looking at the association between COVID-19 symptoms and COVID-19 test results.

The data showed 115 (0.32 per cent) people out of the total 36,061 people in the pilot study had a positive test result.

Focusing on those with COVID-19 specific symptoms (cough, and/or fever, and/or loss of taste/smell), there were 158 (0.43 per cent) with such symptoms on the day of the test.

Of the 115 with a positive result, there were 16 (13.9 per cent) reporting symptoms and in contrast, 99 (86.1 per cent) did not report any specific symptoms on the day of the test.

“Pooled testing could be one way to help implement a widespread testing strategy where several tests are pooled together in one analysis to save time and resources on individual testing. This strategy would be an efficient way to test when the overall prevalence is low as negative pooled samples can quickly show a large group of people are not infectious,” notes Prof Petersen.

The study also includes data on people reporting a wider range of symptoms such as fatigue and shortness of breath. Of the sample who tested positive, 27 (23.5 per cent) were symptomatic and 88 (76.5 per cent) were asymptomatic on the day of the test. The authors believe the findings have significant implications for ongoing and future testing programmes.

“When considering SARS Cov 2 [Covid-19] testing it is important to consider the purpose of the test. A test done to indicate whether a person currently has virus levels that are likely to mean they are infectious, and not to rule in or rule out any presence of virus, does not require such a high sensitivity and cheaper rapid tests are more feasible,” said co-author Professor Andrew Phillips, from UCL Institute of Global Health.

The study included data from a representative population sample of 36,061 people living in England, Wales and Northern Ireland who were tested between April 26 and June 27 and provided information of whether they had any symptoms.

The authors of the study did note that several studies have highlighted a lower proportion of individuals testing positive for COVID-19 are asymptomatic, however, the prevalence of asymptomatic cases varies substantially, possibly due to the sampling and the settings of the study.

For example, the study references how among 262 confirmed cases admitted to hospitals in Beijing 13 (5 per cent) were asymptomatic. In contrast, reports from a small village in Italy suggest that up to 40-75 per cent were asymptomatic. A study of 13,000 residents in Iceland found 43 out of 100 with a positive Covid-19 test were asymptomatic.