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 conducted in the Journal of the American Medical Association determine Thursday how many new cases of people without symptoms are transmitted: at least 50 percent.
The findings reflect the estimate that the Centers for Disease Control and Prevention was offered in November, when the agency said people without symptoms “make up more than 50 percent of the transmissions.”
Jay Butler, deputy director of infectious diseases at the CDC and a lead author of the new study, said the findings reinforce the importance of following public health guidelines on wearing and distributing masks.
“There was still controversy about the value of community mitigation – face masks, social distance and hand hygiene – to limit the spread,” Butler told Business Insider. “This study shows that while symptom screening can have some value, mitigation as well as strategic planned testing of individuals in a given environment will be a significant benefit.”
For the study, researchers modeled potential COVID-19 transmitters in three groups: pre-symptomatic (people who have not yet had symptoms), never symptomatic, and symptomatic.
The researchers then modeled how much each group would transmit COVID-19, depending on the day people were most contagious. At first, they assumed people in all groups would be most contagious five days after being exposed to the coronavirus. According to researchers, this is the median incubation period – the time it takes for most people to develop symptoms after exposure.
The model initially assumed that 30 percent of the people were asymptomatic, and that the individuals were 75 percent as contagious as people who show or would eventually show symptoms. Based on the assumptions, the results indicated that asymptomatic people alone were responsible for 24 percent of the infections.
But the researchers also modeled scenarios in which peak infections 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 relative to other groups.
In most of these scenarios, asymptomatic and asymptomatic people were found to transmit at least 50 percent of the new infections.
“The share of broadcasts generally remains above 50 percent over a wide range of base values,” Butler said, adding that the consistency of the finding was surprising.
Even in the most conservative estimate, in which the highest infection occurred seven days after exposure and asymptomatic people were responsible for 0 percent of the transmission, the pre-symptomatic group according to the model still caused more than 25 percent of the cases.
However, Butler and his co-authors warned that their model was likely to underestimate the actual percentage of COVID-19 cases driven by asymptomatic people, as they calculated the transmission rate if everyone were to move randomly. But in reality, many restaurants and other businesses screen for fever and other symptoms to prevent symptomatic people from entering. In addition, many people with symptoms isolate at home, which also makes them less likely to spread COVID-19 than people who feel healthy.
This article was originally published by Business Insider.
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