In mathematical modeling studies, estimated proportions of all COVID-19 infections that resulted from asymptomatic individuals were mostly below 15%, according to researchers at the University of Bern in Switzerland.
The researchers evaluated 30 studies and data on more than 28,000 people who were infected with COVID-19 from 42 countries. Nearly 12,000 people had asymptomatic infections.
There’s no clear answer as to why people with no COVID symptoms are less likely to spread the virus. One theory is that asymptomatic people may produce less infectious droplets, the authors said.
A 2021 study in China also suggested that people were more likely to develop an asymptomatic COVID-19 infection if they were exposed to an asymptomatic case. Yang Ge, PhD, an epidemiologist at the University of Georgia, and the lead author of the study, told Verywell that asymptomatic cases may be due to a lesser amount of virus shedding.
“However, even assuming the asymptomatic cases shed less, if they have high social activities, then the transmission still could be high," Ge said.
Another study found that people with asymptomatic COVID have an elevated white blood cell count—a sign of better immune function—compared to symptomatic people.
Genetics may also play a role. Research has suggested that some patients have variations of the ACE2 genes that make them more susceptible to getting infected by the spike protein on the COVID-19 virus, which could cause more severe symptoms.
Asymptomatic vs. Presymptomatic
Experts stress that asymptomatic infections are not the same as the presymptomatic phase of COVID-19.
Before people exhibit symptoms or test positive for the coronavirus, they’re considered “presymptomatic” and can still pass the virus on to others—even if they don’t yet realize they are sick.
“In our review, people with presymptomatic infection were as likely to transmit the infection to close contacts as those who had symptoms at the time of diagnosis,” the researchers of the new study told Verywell via an email. “And people without any symptoms throughout the course of infection can still pass it on."
That is why health experts recommend testing after a potential COVID exposure and isolating if you’re infected. Even if you don’t feel sick, it doesn’t mean you’re not infectious at all.
Getting vaccinated and booster is also key to reducing COVID severity. Even if you’ve already had a prior infection, vaccination may lower the risk of developing long COVID.
Researchers at the University of Bern said there are still gaps in their knowledge about how well vaccines can prevent COVID-19 transmission, especially with newer variants that are more adept at evading immune responses.
“We are working on a new update to find the proportion of asymptomatic infection in people infected with variants of concern and amongst people who have been vaccinated," they said.
The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.
