Few people infected with SARS-CoV-2 asymptomatic, according to review

Most people who become infected with SARS-CoV-2 will not remain asymptomatic throughout the course of the infection, according to the results of a recently published systematic review and meta-analysis.

Much focus has been placed on the asymptomatic nature of Covid-19 since the pandemic emerged, but researchers now believe few people will remain truly symptomatic throughout the course of their infection.

Published recently in PLOS Medicine, the research found that 20 per cent of people with Covid-19 remained asymptomatic during follow up, but that biases in study designs limit the certainty of this estimate.

Titled, ‘Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis’, the review recommended that “future studies should be designed specifically to determine the true proportion of asymptomatic SARS-CoV-2 infections, using methods to minimise biases in the selection of study participants and ascertainment of symptom status during follow-up”.

“Overall, in 79 studies in a range of different settings, 20 per cent (95 per cent confidence interval [CI] 17–25 per cent, prediction interval 3–67 per cent) of people with SARS-CoV-2 infection remained asymptomatic during follow-up, but biases in study designs limit the certainty of this estimate.

“In seven studies of defined populations screened for SARS-CoV-2 and then followed, 31 per cent per cent (95 per cent CI 26–37 per cent, prediction interval 24–38 per cent) remained asymptomatic.

“We found some evidence that SARS-CoV-2 infection in contacts of people with asymptomatic infection is less likely than in contacts of people with symptomatic infection.”

According to Prof Gerry Killeen, AXA Research Chair in Applied Pathogen Ecology at University College Cork (UCC) School of Biological, Earth and Environmental Sciences, most people that have the virus are not “truly asymptomatic”, but will have some symptoms so mild that “they shrug them off”.

“For most people with Covid the symptoms will look familiar, like all the common colds. The tricky part is knowing when you don’t have Covid,” said Dr Killeen.

“Most people will have symptoms so mild that they don’t associate them with having a severe disease, but the symptoms are still recognisable, when you ask people about symptoms.”

He referred to a study at a Korean call centre, which suffered an outbreak of the virus, where close contacts were asked to monitor themselves and were on high alert for any symptoms, no matter how minor they may have appeared.

“There wasn’t a single case they didn’t find on the basis of symptoms,” said Dr Killeen.

Separately, Dr Killeen expressed grave concern at the high positivity rate of the virus in the community, currently at almost six per cent. We are currently detecting “one in 10 or one in 20” cases of the virus, he added.

“If you don’t get detection rates above 30 per cent then you tend to catch the virus too late, even with the best public health teams in the world.”