More than half of coronavirus cases spread by people without symptoms, CDC model shows

More than half of coronavirus cases are transmitted from people without symptoms of the virus, according to a new model from the Centers for Disease Control and Prevention. 

“In this base case, 59% of all transmission came from asymptomatic transmission, comprising 35% from presymptomatic individuals and 24% from individuals who never develop symptoms,” the model says. 

Presymptomatic individuals are those who did get symptoms of the virus, but were asymptomatic and infectious before developing their symptoms. 

The authors concluded “that the identification and isolation of persons with symptomatic COVID-19 alone will not control the ongoing spread of SARS-CoV-2.”

According to the model, which was posted on JAMA Network on Thursday, to slow the spread of the virus there needs to be more focus on people who do not show symptoms.

“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,” the authors said. 

Vaccines are rolling out in the United States at a pace slower than anticipated. President-elect Joe Biden has been touting 100 million vaccines distributed in his first 100 days in office. 

“The bottom line is controlling the COVID-19 pandemic really is going to require controlling the silent pandemic of transmission from persons without symptoms,” Jay Butler, the CDC deputy director for infectious diseases and a co-author of the study, told The Washington Post. “The community mitigation tools that we have need to be utilized broadly to be able to slow the spread of SARS-CoV-2 from all infected persons, at least until we have those vaccines widely available.”