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 how many new cases are transmitted by people without symptoms: at least 50 percent.
The findings reflect estimates that the Centers for Disease Control and Prevention took care of in November, when the agency said people without symptoms “were estimated to be responsible for more than 50 percent of transmissions.”
Jay Butler, deputy director for infectious diseases at the CDC and a lead author of the new study, said the findings underscore the importance of following public health guidelines about wearing masks and standing back.
“There was still some controversy about the value of community restriction – face masks, social aloofness and hand hygiene – to limit spread,” Butler told Business Insider. “This study shows that while symptom screening may have some value, mitigation and strategically planned testing of individuals in a particular setting will be of significant benefit.”
For the study, researchers modeled potential COVID-19 transmitters in three groups: presymptomatic (people who had no symptoms yet), never symptomatic, and symptomatic.
The researchers then modeled how much each group would transmit COVID-19, depending on the day when people were most contagious. At the outset, they assumed that people in all groups would be most contagious five days after exposure to the coronavirus. That’s what researchers have found as the median incubation time – the time it takes most people to develop symptoms after exposure.
The model initially assumed that 30 percent of people were asymptomatic and those individuals were 75 percent as contagious as people who showed or would eventually show symptoms. Based on those assumptions, the results suggested that asymptomatic people were responsible for only 24 percent of the infections.
But the researchers also modeled scenarios in which peak infection occurred after three, four, six, and seven days, and they increased and decreased the percentage of asymptomatic people in the model, as well as their infection rate compared to other groups.
In most of these scenarios, people without symptoms (asymptomatic and presymptomatic) were found to transmit at least 50 percent of new infections.
“The share of transmissions generally remained above 50 percent over a wide range of baseline values,” Butler said, adding that the consistency of that finding was surprising.
Even in the most conservative estimate, where the infectivity peak came seven days after exposure and asymptomatic people accounted for 0 percent of transmission, the presymptomatic group still caused more than 25 percent of cases overall, according to the model.
However, Butler and his co-authors cautioned that their model likely underestimates the true percentage of COVID-19 cases caused by people without symptoms, as they would calculate transmission rates if everyone were moving randomly. But in reality, many restaurants and other establishments screen for fever and other symptoms to prevent symptomatic people from entering. In addition, many people with symptoms isolate themselves at home, which also makes them less likely to spread COVID-19 than those who feel healthy.
This article was originally published by Business Insider.
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