A meta-analysis of 130 studies published last week in PLOS Medicine suggests that symptomatic Covid-19 infections are responsible for higher proportions of viral transmission than asymptomatic infections—a finding that contradicts previous research on Covid-19 transmissibility.

Study details and key findings

For the meta-analysis, researchers from the University of Bern in Switzerland analyzed databases to find studies published through July 2021 that focused on Covid-19 patients with documented symptoms at the start and end of a follow-up period. The paper served as an update of a living systematic review that was initially published in April 2020.

In total, the meta-analysis included data for 28,426 Covid-19 patients across 42 countries. Of those, 11,923—about 42%—were asymptomatic.

Nineteen percent of the Covid-19 cases were asymptomatic in 46 contact-tracing or outbreak studies. When compared with symptomatic infections, the rate of viral spread from asymptomatic patients to contacts was roughly two-thirds lower in eight of the studies. In addition, 13 modeling studies suggested that pre-symptomatic patients had higher rates of viral transmission than asymptomatic patients.

Notably, the meta-analysis did not calculate a single estimate for the asymptomatic infection rate because of heterogeneity among the included studies. However, the researchers estimated that roughly 14% to 50% of infections were asymptomatic. In addition, the researchers found that the secondary attack rate—the measure of the risk of coronavirus transmission—was roughly two-thirds lower among individuals without symptoms than those with symptoms.

"If both the proportion and transmissibility of asymptomatic infection are relatively low, people with asymptomatic SARS-CoV-2 infection should account for a smaller proportion of overall transmission than presymptomatic individuals," the authors said.

How these findings compare with previous research

A 2021 modeling study published in JAMA Network Open found that most coronavirus transmission—potentially as much as 60%—may be from people who are asymptomatic or pre-symptomatic.

According to Helen Chu, an infectious diseases physician at the University of Washington School of Medicine, the rate of asymptomatic infections may be even higher now after the winter omicron surge significantly boosted population immunity, reducing the risk of severe symptoms.

"Spaces that are not well ventilated, have large numbers of individuals, and places where people are speaking loudly or eating, those are the spaces that we should be most concerned about," she said. These high rates of asymptomatic transmission underscore a need for continued safety precautions, particularly to protect vulnerable populations like the immunocompromised and children under 5 who cannot yet be vaccinated.

"Given that community transmission levels are rising in most of the U.S., it is important we continue to layer mitigation measures to prevent further spread and exposure to the virus even if you are vaccinated," said Syra Madad, from Harvard University's Belfer Center for Science and International Affairs. "While the risk at the individual level may be low if you are not in the high-risk category, and we have more tools to manage the disease including antivirals, the risk to the overall community is still high."

Similarly, the meta-analysis authors wrote, "[w]hen SARS-CoV-2 community transmission levels are high, physical distancing measures and mask-wearing need to be sustained to prevent transmission from close contact with people with asymptomatic and presymptomatic infection."

Ultimately, study coauthor Nicola Low noted, "[t]he true proportion of asymptomatic SARS-CoV-2 infection is still not known, and it would be misleading to rely on a single number because the 130 studies that we reviewed were so different. People with truly asymptomatic infection are, however, less infectious than those with symptomatic infection." (CIDRAP News, 5/27; EurekAlert!, 5/26; Buitrago-Garcia et al., PLOS Medicine, 5/26)