At least 50% of COVID-19 cases spread from people without symptoms, a new study found

• At least 50% of new COVID-19 cases are transmitted by people who don’t show symptoms, according to a new study.
• That includes people who never display symptoms and those whose symptoms hadn’t started yet. 
• A study authors said findings underscore the need for people who feel healthy to follow public-health guidance on mask wearing and social distancing.
• Visit Business Insider’s homepage for more stories.

Since the early days of the pandemic, researchers have known that people with COVID-19 can spread the disease before they develop symptoms and even if they never feel sick. 

A study published in the Journal of the American Medical Association on Thursday quantifies just how many new cases are transmitted from people without symptoms: at least 50%. 

The findings echo estimates that the Centers for Disease Control and Prevention provided in November, when the agency said people without symptoms were “estimated to account for more than 50% of transmissions.”  

Jay Butler, deputy director for infectious diseases at the CDC and a lead author of the new study, said the findings reinforce the importance of following public-health guidelines about mask wearing and distancing. 

“There was still some controversy over the value to community mitigation – face masks, social distancing, and hand hygiene – to limit spread,” Butler told Business Insider. “This study demonstrates that while symptom screening may have some value, mitigation, as well as strategically planned testing of persons in some setting, will be a significant benefit.”

For the study, researchers modeled potential COVID-19 transmitters in three groups: pre-symptomatic (people who hadn't had symptoms yet), never symptomatic, and symptomatic. 

The researchers then modeled how much each group would transmit COVID-19 depending on the day people were most infectious. At baseline, they assumed people in all groups would be most infectious five days after getting exposed to the coronavirus. That's what researchers have found to be the median incubation period - the length of time it takes for most people to develop symptoms after exposure. 

The model initially assumed that 30% of people were asymptomatic, and that those individuals were 75% as infectious as people who were showing or would eventually show symptoms. Based on those assumptions, the results suggested that asymptomatic people alone were responsible for 24% of infections.

But the researchers also modeled scenarios in which peak infectiousness occurred after three, four, six, and seven days, and they raised and lowered the percentage of asymptomatic people in the model, as well as their rate of infectiousness relative to other groups.

Across most of these scenarios, people without symptoms (asymptomatic and presymptomatic) were found to transmit at least 50% of new infections. 

"The proportion of transmissions remained generally above 50% across a broad range of base values," Butler said, adding that the consistency of that finding was surprising. 

Even in the most conservative estimate, in which peak infectiousness came seven days after exposure and asymptomatic people accounted for 0% of transmission, the pre-symptomatic group still caused more than 25% of cases overall, according to the model. 

Butler and his coauthors cautioned, however, that their model likely underestimates the real percentage of COVID-19 cases driven by people without symptoms, since they calculated the transmission rates if everyone were to move around at random. But in reality, many restaurants and other establishments screen for fevers and other symptoms to stop symptomatic people from entering. Additionally, many people with symptoms isolate at home, which also makes them less likely to spread COVID-19 than people who feel healthy.