COVID-19: Around 20 per cent of cases are asymptomatic © Getty Images

COVID-19: Around 20 per cent of cases are asymptomatic

Researchers argue that accurate estimates of true asymptomatic and presymptomatic infections are critical to understanding transmission of the virus at the population level.

Asymptomatic coronavirus cases may make up around 20 per cent of infections, a new study suggests.

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Some people who contract COVID-19 never experience any symptoms, and there remains disagreement about what proportion of total infections these cases represent.

The full spectrum of the severity of symptoms is still not well understood. Some infected people may experience severe infections resulting in viral pneumonia, respiratory distress syndrome, and death, while others remain completely asymptomatic or develop mild, nonspecific symptoms.

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 did not develop symptoms.

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Though the study couldn’t gauge the impact of false negatives, the researchers estimated that 20 per cent of infections remained asymptomatic during follow-up.

The researchers argue that accurate estimates of true asymptomatic and presymptomatic infections are critical to understanding transmission of the virus at the population level.

“The findings of this systematic review of publications early in the pandemic suggest that most SARS-CoV-2 [the virus that causes COVID-19] infections are not asymptomatic throughout the course of infection,” the authors wrote.

“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.”

While this is very interesting, it remains the case that on any given day, the majority of people with the virus will not be displaying any symptoms
Dr Simon Clarke

More accurate tests are needed to reduce the number of false negatives, the study suggests.

As each person infected with the virus 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.

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“This extensive overview of multiple studies comes to the conclusion that most people who are infected with the coronavirus causing COVID-19 eventually develop symptoms,” said Dr Simon Clarke, associate professor of cellular microbiology at the University of Reading.

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“Much has been made of the number of people with the virus, but without symptoms. While this is very interesting, it remains the case that on any given day, the majority of people with the virus will not be displaying any symptoms and these findings should not in any way detract from current infection control advice.”