While there is an increasing awareness of asymptomatic infections, experts suggest that interventions to mitigate viral transmission, including social distancing and wearing masks, should continue to be used by individuals regardless of whether they have symptoms. According to Krutika Kuppalli, an infectious disease researcher at Stanford University School of Medicine in California, the problem with placing an accurate figure on the incidence of asymptomatic COVID-19 cases is, distinguishing between people who are asymptomatic and pre-symptomatic. “Asymptomatic is someone who never developed symptoms ever throughout their disease, and pre-symptomatic is somebody who has mild symptoms before they do go on to develop symptoms,” Kuppalli says. There is also no standardized accepted definition of that, she says.

The prevalence of asymptomatic infections may be as high as 81 percent, studies early in the pandemic indicated. But a meta-analysis published last month, which included 13 studies involving 21,708 individuals, reported that 17 percent was the rate of asymptomatic presentation. Asymptomatic individuals were identified by the study, as those who displayed none of the main symptoms of COVID-19 during the entire follow-up period, and the authors included only studies that followed participants for at least seven days. Data shows that in 7-13 days most people experience symptoms, says lead author Oyungerel Byambasuren, a biomedical researcher at Bond University’s Center for Evidence-Based Healthcare in Gold Coast, Australia.

The study by Byambasuren also found that asymptomatic individuals were 42 percent less likely than symptomatic individuals to transmit the virus. One reason scientists want to know how much the virus is spread by people without symptoms is that these infections go largely undetected. Testing is aimed at people with symptoms in most countries. Researchers modelled viral spread among people living together as part of a broad population study in Geneva, Switzerland. In a manuscript posted on medRxiv this month, 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 pose 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 many 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.

Cevik and colleagues performed a systematic review and meta-analysis of 79 studies on the viral dynamics and transmissibility of SARS-CoV-2, which is posted on the SSRN preprint server for social sciences, to understand what is happening in people without symptoms. Some studies have shown that when compared to people with symptoms, those without symptoms had identical initial viral loads, the number of viral particles found in a throat swab. But asymptomatic individuals tend to clear the virus more easily and for a shorter time, are contagious.

Asymptomatic individuals’ immune systems should be able to neutralize the virus more easily, says Cevik. But that doesn’t indicate that these individuals have a better or more robust immune response, and there is evidence that there is a more significant and long-lasting neutralizing antibody response in individuals with extreme COVID-19, 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.

By Jumana Jabeer