Most people who are infected by the SARS-CoV-2 virus, that causes Covid-19, are not asymptomatic throughout the course of infection, a review of studies carried early in the pandemic suggests.

While some people who contract the novel coronavirus never experience any symptoms, there remains disagreement about what proportion of total infections these cases represent.

The latest study, published in the journal PLOS Medicine, suggests that true asymptomatic cases of SARS-CoV-2 comprise a minority of infections.

According to the researchers at the University of Bern, Switzerland and colleagues, the full spectrum and distribution of the severity of Covid-19 symptoms are not well understood.

Some people with the virus may experience severe infections resulting in viral pneumonia, respiratory distress syndrome, and death, while others remain completely asymptomatic or develop mild, nonspecific symptoms.

The researchers wanted to better understand the proportion of people who become infected with SARS-CoV-2 and never develop any symptoms, as well as the proportion of people who are asymptomatic at the time of diagnosis but develop symptoms later.

They systematically reviewed literature using a database of SARS-CoV-2 evidence between March and June, 2020.

The researchers analysed 79 studies reporting empirical data on 6,616 people, 1,287 of whom were defined as asymptomatic, in order to determine the proportion of infected people who never developed symptoms.

While the study was limited by its inability to ascertain the impact of false negatives, the researchers were able to estimate that 20 per cent of Covid-19 infections remained asymptomatic during follow-up.

Accurate estimations of true asymptomatic and presymptomatic infections are critical to understanding SARS-CoV-2 transmission at the population level and for populations to adopt appropriately tailored public health strategies, the researchers said.

Improved accuracy of serological tests is also needed to reduce the number of false negatives, they said.

Since each person infected with SARS-CoV-2 is initially asymptomatic, the proportion that will go on to develop symptoms is estimated to be around 80 per cent, suggesting that presymptomatic transmission may significantly contribute to overall SARS CoV-2 epidemics.

“The findings of this systematic review of publications early in the pandemic suggests that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection,” said Diana Buitrago-Garcia at the University of Bern.

“The contribution of presymptomatic and asymptomatic infections to overall SARS-CoV-2 transmission means that combination prevention measures, with enhanced hand and respiratory hygiene, testing and tracing, and isolation strategies and social distancing, will continue to be needed,” Buitrago-Garcia added.