Up to 40% of COVID-19 cases are asymptomatic, analysis finds

As many as 40% of COVID-19 cases experience no symptoms, but they still can spread the virus, according to a new study. File photo by Debbie Hill/UPI
1 of 3 | As many as 40% of COVID-19 cases experience no symptoms, but they still can spread the virus, according to a new study. File photo by Debbie Hill/UPI | License Photo

NEW YORK, Dec. 14 (UPI) -- Up to 40% of people infected with COVID-19 globally experience no symptoms early in their illness, a study published Tuesday by JAMA Network Open found.

However, these asymptomatic individuals still may be susceptible to "long-haul" COVID-19 complications, such as severe chronic fatigue, and also may pass the virus to others unknowingly, the researchers said.


In the analysis of data from 95 studies that collectively included 30 million participants, 0.25% of those tested for the virus were positive but reported no symptoms, the data showed.

Nearly 5% of nursing home residents or staff tested were positive, but without symptoms, while just over 2% of airline and cruise-ship travelers and pregnant women assessed for the virus were.

Among confirmed cases of COVID-19, though, more than 40% of those with the virus were asymptomatic, according to the researchers.


"The high percentage of asymptomatic infections highlighted the potential transmission risk of asymptomatic infections in communities," study co-author Min Liu told UPI in an email.

"Screening for asymptomatic infection is required especially for countries and regions that have successfully controlled [the virus]," said Liu, a professor of epidemiology and biostatistics at Peking University in Beijing.

The findings highlight the importance of widespread, comprehensive testing, as asymptomatic infections should also be isolated and undergo contact tracing, he said.

Research published in January also suggested that as many as 40% of COVID-19 cases are asymptomatic.

This may mean that case totals are underestimated, perhaps by as much as 80%, the January study indicates.

"This is a highly important finding because this substantial group of people can transmit the virus to others," molecular medicine specialist Dr. Eric Topol, who published similar findings last year, told UPI in an email.

"It is essential that we have pervasive rapid testing to determine a person's infectiousness to limit transmission for people without symptoms," said Topol, director of Scripps Research Translational Institute in La Jolla, Calif., who was not part of the JAMA study.

For this study, Liu and his colleagues compiled data on asymptomatic COVID-19 cases from 95 studies published in 2020 or early 2021.


Collectively, the studies included data for 29.8 million people, from countries such as China and the United States, as well as several countries in Asia, Europe and the Middle East.

Of confirmed cases of COVID-19 among pregnant women in these studies, 54% were asymptomatic, the data showed.

Similarly, 53% of confirmed cases in airline and cruise-ship passengers were asymptomatic, as were 48% of nursing home residents and staff.

However, most of the included studies, at least those conducted in the United States, were published before the more contagious Delta variant of the virus emerged and the availability of vaccines became widespread, according to infectious disease specialist Dr. Ross M. Boyce.

This is significant because the rate of transmission from asymptomatic Delta to an uninfected individual is likely higher than that of earlier variants, Boyce, an assistant professor at the University of North Carolina-Chapel Hill who was not part of the JAMA study, told UPI by email.

Still, he cautioned against reading too much into the results of this analysis "because the included studies vary so much in design and the underlying population tested."

"I think, and hope, that the message that [COVID-19] can be spread by relatively asymptomatic individuals has hit the mainstream already," Boyce said.


However, this is "pretty good evidence that even if infected, those who have been vaccinated have lower levels of virus and thus are less likely to transmit to others -- so take care of those around you and get vaccinated," he said.

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