More than half of COVID-19 transmission comes from people without symptoms, CDC model shows

A recent model developed by researchers at the U.S. Centers for Disease Control and Prevention found that people who do not exhibit COVID-19 symptoms transmit more than half of all viral cases. 

Researchers published their findings on Jan. 7 in the medical journal JAMA Network Open. 

They used data from several Chinese studies in order to determine the proportion of transmission from presymptomatic individuals — those who shed the virus before eventually developing symptoms — asymptomatic and symptomatic individuals. 

Using the model, researchers said 59% of all transmissions of the novel coronavirus came from asymptomatic individuals, which included 35% of new cases from people who infect others before they show symptoms, or presymptomatic individuals. 

"30% of individuals with infection never develop symptoms and are 75% as infectious as those who do develop symptoms," study authors wrote. 

Signage is in place to direct people to COVID-19 vaccinations at the Yes We Can Community Center on January 10, 2021 in New Cassel, New York.

"These findings suggest that measures such as wearing masks, hand hygiene, social distancing, and strategic testing of people who are not ill will be foundational to slowing the spread of COVID-19 until safe and effective vaccines are available and widely used," researchers added. 

The CDC previously noted that some people who contract COVID-19 may not show any symptoms until 14 days after being infected.

The recent CDC model echoes findings of previous research, which indicates that asymptomatic individuals are believed to be contributing to the rapid spread of COVID-19. 

A July 6 study published in Proceedings of the National Academy of Sciences refers to the phenomena of COVID-19 infections by asymptomatic and presymptomatic individuals as "silent transmission."

The study said that even if symptomatic cases are isolated — such as a person being quarantined until they have recovered or tested negative for the virus — "a vast outbreak may nonetheless unfold."

A separate study published on March 12 in "Eurosurveillance," a peer-reviewed medical journal, found that 17.9 percent of the 3,711 passengers and crew members aboard the Diamond Princess cruise ship, which experienced a COVID-19 outbreak, were asymptomatic.