As many as 45% of people infected with COVID-19 have no symptoms, and they may play a major role in the spread of the virus, a new study says.
The findings show the need for large-scale testing and contact tracing to combat the pandemic, according to the Scripps Research investigators.
“The silent spread of the virus makes it all the more challenging to control,” said study author Dr. Eric Topol, founder and director of the Scripps Research Translational Institute in La Jolla, Calif., and professor of molecular medicine at Scripps Research.
“Our review really highlights the importance of testing. It’s clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us,” he added in a Scripps news release.
The researchers analyzed data from studies on coronavirus infection in people worldwide and in various groups, including nursing home residents, cruise ship passengers and prison inmates.
“What virtually all of them had in common was that a very large proportion of infected individuals had no symptoms,” said study co-author Daniel Oran, a behavioral scientist at Scripps.
“Among more than 3,000 prison inmates in four states who tested positive for the coronavirus, the figure was astronomical: 96% asymptomatic,” Oran said in the release.
Another finding was that asymptomatic people may be able to transmit the new coronavirus for a long time, perhaps longer than 14 days.
The researchers also said that even if they don’t have symptoms, infected people may still be at risk for harm. For example, CT scans showed that 54% of infected but asymptomatic people on a cruise ship had minor lung abnormalities, which suggests that coronavirus infection could affect lung function in a way that’s not immediately apparent.
With so many asymptomatic cases around, the researchers stressed the importance of not spreading infection.
“Our estimate of 40-45% asymptomatic means that, if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms. So to protect others, we think that wearing a mask makes a lot of sense,” Oran concluded.
The study was published recently in the journal Annals of Internal Medicine.
The U.S. Centers for Disease Control and Prevention has more on COVID-19.