Study claims up to 45pc of coronavirus cases may be asymptomatic

15 Jun 2020

Image: © Microgen/

A study on the number of asymptomatic coronavirus cases showed that up to 45pc of those with Covid-19 could be silent carriers.

Up to 45pc of those with the SARS-CoV-2 (the novel coronavirus) could be asymptomatic, helping to spread the virus unknowingly through a population, according to a new study. Published to Annals of Internal Medicine by a team from the Scripps Research Institute, the study analysed public datasets on asymptomatic infections.

Its authors highlighted the need for expansive testing and contact tracing to help mitigate the pandemic. They also stressed the need for further research to build upon their early discoveries.

“The silent spread of the virus makes it all the more challenging to control,” said Eric Topol, founder and director of the Scripps Research Translational Institute.

“Our review really highlights the importance of testing. It’s clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us.”

The study looked at datasets from 16 diverse groups of people across the world, including the population of Iceland, Japanese citizens evacuated from Wuhan, inmates in a number of American prisons, and a navy crew aboard the French aircraft carrier Charles de Gaulle.

Behavioural scientist Daniel Oran said that in some cases the number of asymptomatic cases was “astronomically” high, including 96pc of 3,000 prison inmates in four US states.

Almost like ‘a flip of a coin’

The viral loads of asymptomatic cases may be longer than 14 days, the team added. However, it remains unclear whether their infectiousness is of the same magnitude of those with symptoms, meaning that more large-scale studies of asymptomatic cases will be needed.

Another finding that requires further research is that absence of symptoms may not imply an absence of harm. While the sample size was small, CT scans conducted on 54pc of 76 asymptomatic individuals on the Diamond Princess cruise ship appeared to show significant subclinical lung abnormalities.

Longitudinal studies will also be needed to distinguish between asymptomatic and presymptomatic individuals, the researchers said. A presymptomatic person is someone who is infected and seems healthy, but will eventually develop symptoms.

Oran said of the latest findings: “Our estimate of 40 to 45pc asymptomatic means that, if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms. So to protect others, we think that wearing a mask makes a lot of sense.”

Last week, the World Health Organisation said asymptomatic spread of the coronavirus is still a “big unknown”.

Colm Gorey was a senior journalist with Silicon Republic