From the earliest days of the pandemic, researchers knew that people with COVID-19 could spread the disease before they developed symptoms and even if they never felt sick.
A study published in Journal of the American Medical Association Thursday quantifies how many new cases are transmitted from asymptomatic people: at least 50%.
The results of the echo estimate that the Centers for Disease Control and Prevention provided in November, when the agency said asymptomatic people were “estimated to account for more than 50% of transmissions.”
Jay Butler, deputy director of infectious disease at the CDC and lead author of the new study, said the findings reinforce the importance of following public health guidelines regarding wearing and distancing the mask.
“There was still some controversy over the value of community mitigation – face masks, social distance and hand hygiene – to limit the spread,” Butler told Business Insider. This study demonstrates that while symptom screening may have some value, mitigation, as well as strategically planned testing of people in a given setting, will be of significant benefit.
For the study, the researchers modeled potential COVID-19 transmitters into three groups: pre-symptomatic (people who had no symptoms yet), never symptomatic and symptomatic.
The researchers then modeled how much each COVID-19 group would transmit based on the day people were most infectious. Initially, they assumed that people in all groups would be most infectious five days after coronavirus exposure. This is what the researchers found to be the median incubation period – the length of time it takes for most people to develop symptoms after exposure.
The model initially assumed that 30 percent of people were asymptomatic and that those individuals were 75 percent as infectious as people who had or would eventually have symptoms. Based on these hypotheses, the results suggested that asymptomatic individuals alone were responsible for 24 percent of infections.
The researchers also modeled scenarios in which peak infectivity occurred after three, four, six, and seven days and increased and decreased the percentage of asymptomatic individuals in the model, as well as their infectivity rate compared to other groups.
In most of these scenarios, it was found that people without symptoms (asymptomatic and presymptomatic) transmit at least 50% of new infections.
“The proportion of transmissions has generally remained above 50% in a wide range of core values,” Butler said, adding that the consistency of this finding was surprising.
Even in the most conservative estimate, in which the peak of infectivity came seven days after exposure and asymptomatic individuals accounted for 0% of transmission, the pre-symptomatic group continued to cause more than 25% of cases, according to the model.
Butler and his co-authors warned, however, that their model underestimates the actual percentage of asymptomatic COVID-19 cases because they calculated transmission rates if everyone should travel at random. But in reality, many restaurants and other units are analyzing fever and other symptoms to stop the entry of symptomatic people. In addition, many people with symptoms isolate themselves at home, making them even less likely to spread COVID-19 than people who feel healthy.
This article was originally published by Business Insider.
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