Model confirms ‘silent’ spread
People with no symptoms transmit more than half of all cases of the novel corona virus, according to a new model developed by researchers at the Centers for Disease Control and Prevention.
Their findings reinforce the importance of following the agency’s guidelines: Regardless of whether people feel ill, they should wear a mask, wash their hands, stay socially distant and get a coronavirus test. That advice has been a constant refrain in a pandemic responsible for more than 350,000 deaths in the United States.
Fifty-nine percent of transmission came from people without symptoms in the model’s baseline scenario. That includes 35 percent of new cases from people who infect others before they show symptoms and 24 percent that come from people who never developed symptoms.
“The bottom line is controlling the COVID-19 pandemic really is going to require controlling the silent pandemic of transmission from persons without symptoms,” said Jay Butler, the CDC deputy director for infectious diseases and a co-author of the study. “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.”
The emergence of a more contagious variant, first detected in the United Kingdom and since found in several U.S. states, throws the significance of those guidelines into even starker relief. “Those findings are now in bold, italics and underlined,” Butler said. “We’ve gone from 11-point font to 16-point font.”
The model, published Thursday in the journal JAMA Network Open, comports with earlier estimates of the contribution of asymptomatic spread.
“It’s certainly confirmatory, but it’s nice to see confirmation,” said epidemiologist Richard Menzies, who directs the Mcgill International TB Centre in Canada and was not affiliated with this research. “These are pretty believable, solid results.”
The model consistently predicted that asymptomatic spread accounted for about half of viral transmission. “I was a bit surprised howwell it held up under a broad range of base assumptions,” Butler said, such as shifting the timing of peak contagiousness from four days after infection to five or six.
But Mug eC evik, an infectiousdisease expert at Scotland’ s University of St. Andrews, argued some of the model’s assumptions are flawed. Cevik noted that the study does not account for the environment where the spread occurs. “Maybe asymptomatic transmission is important, but it may be much more important in longterm care facilities and households,” she said. “That might mean that we need to do much more targeted testing for high-risk populations,” as opposed to mass screening.