Since the start of the pandemic, millions of people around the world have become sick — some critically so — with COVID-19, the disease caused by the new coronavirus. But not everyone with a coronavirus infection develops symptoms and falls ill. Researchers are learning that a significant portion of people who test positive for the virus never exhibit the warning signs.
"I think that's one of the reasons why transmission of this virus has been so hard to control and contain,” says Michael Mina, M.D., assistant professor of epidemiology at the Harvard T. H. Chan School of Public Health.
If you don't know you have the virus and otherwise feel healthy, you're less likely to change your behavior “in a way that's commensurate with reducing transmission.” Mina says this creates a massive problem when it comes to controlling the outbreak, and is one reason why “this virus will continue to transmit.”
Contagious without coughing
People who have coronavirus infections but do not show symptoms fall into two categories: There are presymptomatic individuals, who don't have symptoms at the time they are tested but go on to develop them a few days later; and asymptomatic individuals, who remain symptomless throughout the course of the infection. And emerging data show that a lot more people than previously thought fit into these two groups.
A population-based study in Iceland, for example, found that 43 percent of participants who tested positive for the virus reported no symptoms at the time of the test. Similarly, about 45 percent of positive cases in an Indiana-based population survey were asymptomatic when tested, and about 18 percent of people onboard the Diamond Princess cruise ship never developed symptoms of a coronavirus infection.
"Now that doesn't prove that they were transmitting” the virus just because they had it, says Charles E. Davis, M.D., professor emeritus of pathology and medicine at the University of California, San Diego, and director emeritus of microbiology at the UCSD Medical Center. “But there's no question in my mind whatsoever that there is transmission from the people who have been asymptomatic.” Pinpointing just how big of a role people without symptoms play in the pandemic, however, is difficult to do.