People who have the coronavirus without symptoms appear to have similar levels of the virus in their noses and throats to people with mild symptoms, a study has found. However, this doesn’t necessarily mean they are as likely to spread covid-19 as those who are sick.
After much speculation early in the pandemic, a growing body of evidence suggests people can pass on the coronavirus even if they don’t have any symptoms. But it still isn’t clear how much virus these individuals carry in their upper airways and how infectious they might be. In particular it isn’t clear whether they are as contagious as those with symptoms.
Sung-Han Kim at the University of Ulsan College of Medicine in South Korea and his colleagues took nose and throat swabs from 39 people with asymptomatic coronavirus infections as well as from 144 people with covid-19Ěýwho had mild symptoms. All the individuals had first tested positive for the virus an average of 13 days earlier.
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The researchers found that 64 per cent of the covid-19 patients and 54 per cent of those who were asymptomatic still had coronavirus-positive nose and throat swabs. They also discovered that levels of viral RNA were almost identical between these two groups.
This was unexpected, says Kim, because for other viruses, such as influenza, the amount of virus an infected person harbours – known as the viral load – usually increases with the severity of symptoms. The detection of similar levels of viral RNA in mildly symptomatic and asymptomatic individuals suggests the coronavirus may be “unique”, he says.
The study supports the idea that asymptomatic individuals have the potential to spread the virus regardless of symptoms, says Kim.
“Normally with a virus infection, the symptoms give a way for the virus to exit your body and spread,” says Lucy Thorne at University College London, who wasn’t involved in the study. “But it seems from this and other studies that it’s still possible to spread [the coronavirus] when you don’t have symptoms,” she says. “I think the really strong public health message has to be that everyone has to take precautions even if they don’t have symptoms.”
But the presence of viral RNA alone isn’t enough to confirm that asymptomatic individuals are as infectious as people with mild symptoms, says Thorne. Some of the RNA detected in the study may be from “debris” from virus-infected cells, for instance, rather than from infectious virus, she says. It will therefore be important to investigate just how infectious virus particles from asymptomatic individuals are, by testing how well they grow in cells in the laboratory.
Studies monitoring viral load in symptomatic and asymptomatic people in the days directly after diagnosis will also be informative, as the peak of infection is likely to occur earlier than 13 days after diagnosis, says Thorne.
If asymptomatic people do harbour levels of infectious virus similar to people with mild symptoms, it will be important to determine what proportion of coronavirus infections are asymptomatic and thus the extent to which this group might be contributing to the spread of the virus within communities.Ěý
An of 21 studies conducted early in the pandemic suggested that between 5 and 80 per cent of people testing positive for the coronavirus don’t have any symptoms. A separate study, published today in , suggests asymptomatic cases may only account for about 20 per cent of infections. “Although the percentages are massively variable, the key is that it is possible,” says Thorne.
Clever testing
Current advice in the UK is that anyone who has symptoms resembling covid-19 should self-isolate. But if people are carrying on as normal because they are asymptomatic, they may still be contributing to the spread of the virus within communities, Thorne points out. “If you don’t have symptoms, you could still have it and you should be cautious about not spreading it.”
Ideally, routine surveillance testing could be used to identify asymptomatic infections in high-risk groups such as healthcare workers or people who come into contact with those who are shielding, says Thorne. “I think it’s about maybe using our testing capacity more cleverly than we currently are.”
“Wearing masks, washing hands and keeping social distancing are the only currently available measures to block viral spreading from asymptomatic individuals,” says Akiko Iwasaki at Yale University. “If everyone behaves as if they are infectious when interacting with others, this will go a long way in preventing asymptomatic spread,” she says.
Thorax
