What the data say about asymptomatic #COVID infections
Définition : Au moins 7 jours de suivi sans symptômes
20% des sujets infectés sont asymptomatiques ;
contagieux mais moindre que les symptomatiques ;
leur rôle dans la propagation n’est pas capital
Leur charge virale est initialement aussi importante que les symptomatiques (d’où la nécessité des mesures barrière malgré tout) mais diminue plus rapidement.
Research early in the pandemic suggested that the rate of asymptomatic infections could be as high as 81%. But a meta-analysis published last month1, which included 13 studies involving 21,708 people, calculated the rate of asymptomatic presentation to be 17%. The analysis defined asymptomatic people as those who showed none of the key COVID-19 symptoms during the entire follow-up period, and the authors included only studies that followed participants for at least seven days. Evidence suggests that most people develop symptoms in 7–13 days, says lead author Oyungerel Byambasuren, a biomedical researcher at the Institute for Evidence-Based Healthcare at Bond University in Gold Coast, Australia.
Byambasuren’s review also found that asymptomatic individuals were 42% less likely to transmit the virus than symptomatic people.
One reason that scientists want to know how frequently people without symptoms transmit the virus is because these infections largely go undetected. Testing in most countries is targeted at those with symptoms.
As part of a large population study in Geneva, Switzerland, researchers modelled viral spread among people living together. In a manuscript posted on medRxiv this month2, they report that the risk of an asymptomatic person passing the virus to others in their home is about one-quarter of the risk of transmission from a symptomatic person.
Although there is a lower risk of transmission from asymptomatic people, they might still present a significant public-health risk because they are more likely to be out in the community than isolated at home, says Andrew Azman, an infectious-disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland, who is based in Switzerland and was a co-author on the study. “The actual public-health burden of this massive pool of interacting ‘asymptomatics’ in the community probably suggests that a sizeable portion of transmission events are from asymptomatic transmissions,” he says.
But other researchers disagree about the extent to which asymptomatic infections are contributing to community transmission. If the studies are correct in finding that asymptomatic people are a low transmission risk, “these people are not the secret drivers of this pandemic”, says Byambasuren. They “are not coughing or sneezing as much, they’re probably not contaminating as much surfaces as other people”.
Muge Cevik, an infectious-disease researcher at the University of St Andrews, UK, points out that because most people are symptomatic, concentrating on identifying them will probably eliminate most transmission events.
To understand what is happening in people with no symptoms, Cevik and colleagues conducted a systematic review and meta-analysis3 of 79 studies on the viral dynamics and transmissibility of SARS-CoV-2, which is posted on social-sciences preprint server SSRN. Some studies showed that those without symptoms had similar initial viral loads — the number of viral particles present in a throat swab — when compared with people with symptoms. But asymptomatic people seem to clear the virus faster and are infectious for a shorter period.
The immune systems of asymptomatic individuals might be able to neutralize the virus more rapidly, says Cevik. But that doesn’t mean these people have a stronger or more durable immune response — and there is evidence that people with severe COVID-19 have a more substantial and long-lasting neutralizing antibody response, she says.
Although there is a now a better understanding of asymptomatic infections and transmission of COVID-19, Cevik says that asymptomatic people should continue to use measures that reduce viral spread, such as social distancing, hand hygiene and wearing a mask.