At least 50% of new coronavirus infections are spread from people without symptoms, according to a study published this week.
People who are asymptomatic or never experience symptoms made up about 24% of all transmissions, while presymptomatic individuals accounted for 35% for a total of 59%, researchers from the Centers for Disease Control and Prevention found.
Jay Butler, CDC deputy director for infectious diseases and a lead author of the study, told Business Insider that the findings back up public health guidelines about social distancing and mask wearing.
“There was still some controversy over the value of community mitigation — face masks, social distancing, and hand hygiene — to limit spread,” Dr. Butler said. “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.”
The researchers split COVID-19 transmitters into three categories: presymptomatic (those who didn’t have symptoms yet), never symptomatic and symptomatic. They then modeled how each group would spread COVID-19 based on the day people were most infectious, using five days after coronavirus exposure as the baseline, which was the median incubation period.
The model the researchers used assumed that 30% of individuals with COVID-19 never develop symptoms and are 75% as infectious as those who do develop symptoms as its baseline.
Even with changing peak infectiousness and the proportion of COVID-19 transmitters without symptoms, the researchers found that both asymptomatic and presymptomatic people transmitted at least 50% of new infections under their model.
With the most conservative estimate — which set peak infectiousness at seven days and 0% transmission from asymptomatic people — the model still found that more than 25% of spread came from presymptomatic individuals.
“In the absence of effective and widespread use of therapeutics or vaccines that can shorten or eliminate infectivity, successful control of SARS-CoV-2 cannot rely solely on identifying and isolating symptomatic cases; even if implemented effectively, this strategy would be insufficient,” the CDC researchers wrote in their study.
“These findings suggest that effective control also requires reducing the risk of transmission from people with infection who do not have symptoms.”
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