Healthy and Infected: What We Know About the Asymptomatic Spread of COVID-19

One of the mysteries of the COVID-19 pandemic has been that seemingly healthy individuals can spread the virus to others. This method of transmission, known as asymptomatic spread, has made it difficult to isolate infected individuals and prevent outbreaks. 

But how does asymptomatic spread work exactly? And what consequences will it have if we reopen schools and businesses without a vaccine? 



By the time an individual infected with COVID-19 starts showing the telltale symptoms of dry cough, fever, and fatigue, they may have already spread the virus to dozens of people. Such a patient is known as a presymptomatic transmitter. 

An asymptomatic transmitter, on the other hand, never feels sick. Asymptomatic spread becomes particularly challenging for public health officials when the virus in question is as highly infectious as SARS-CoV-2. Moreover, this mode of transmission is not as uncommon as we once thought. 



According to estimates from the CDC, up to 40% of infections in the U.S. are asymptomatic. This type of transmission isn’t exclusive to COVID-19, either—we see the same behavior in other, less lethal viruses, such as influenza and common colds. 

A study from South Korea published in early August showed that people without symptoms carry just as much virus in their nose, throat, and lungs as their symptomatic counterparts—and for nearly the same amount of time. 

The study analyzed 193 symptomatic and 110 asymptomatic patients isolated at a community treatment center in Cheonan. Of patients who were initially asymptomatic, around 30% were truly asymptomatic, while the rest turned out to be presymptomatic. 

Back in June, the WHO was criticized for, and promptly withdrew, a comment claiming that asymptomatic transmission of COVID-19 was “rare.” The 30% figure from South Korea aligns with what other scientists have modeled as a 40% asymptomatic transmission rate. In a television interview, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, referred to the same 40% figure. 

The results from South Korea confirm what we’ve long suspected—that seemingly healthy individuals can indeed spread the virus to others. 



The likelihood of asymptomatic transmission, however, is still poorly understood. While a lack of symptoms may mean less coughing and sneezing, asymptomatic people may carry on with their business, infecting others in the process.