A recently published analysis by the Centers for Disease Control and Prevention found that 59% of COVID-19 cases result from transmission of the virus by asymptomatic people: either those who have yet to show symptoms or those who never do.
The CDC scientists modeled multiple scenarios of virus transmission, looking at different incubation and infectious periods as well as varying numbers of days from the time a person is infected to the time symptoms develop. Based on past data, the models included a median incubation period of five days, and an infectious period of 10 days. The peak infectious period in their calculations varied between three and seven days.
Their combined models predict that 35% of infections originate from people who eventually develop symptoms of COVID-19, and 24% are transmitted by people who remain completely without symptoms despite being infected.
At a time when infections continue at high levels, more contagious variants of the virus are now spreading in the U.S. and vaccines have not yet been widely distributed, these findings suggest that identifying and isolating people with symptomatic COVID-19 cannot control the pandemic, they said.
Instead, control of the virus can only happen through continuing protective measures such as masking and social distancing, according to the CDC. These steps should be supplemented with strategic testing of those who are asymptomatic but have potentially been exposed.
The study was published in JAMA Network Open.