From the early days of the pandemic, researchers know that people with COVID-19 can spread the disease before they develop symptoms and even if they never feel sick.
A study published in Journal of the American Medical Association on Thursday it quantifies how many new cases are transmitted from people without symptoms: at least 50 percent.
The findings echo estimates that the Centers for Disease Control and Prevention released in November, when the agency said people without symptoms were “estimated at more than 50 percent of transmissions”.
Jay Butler, deputy director of infectious diseases at the CDC and lead author of the new study, said the findings reinforce the importance of following public health guidelines on mask use and distance.
“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 be of some value, mitigation, as well as strategically planned testing of people in some setting, will be a significant benefit.”
For the study, the researchers modeled the potential COVID-19 transmitters in three groups: pre-symptomatic (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 infectious. At the beginning of the study, they assumed that people in all groups would be more infectious five days after being exposed to the coronavirus. This is what the researchers found to be the average incubation period – the time it takes most people to develop symptoms after exposure.
The model initially assumed that 30% of people were asymptomatic and that these individuals were 75% as infectious as people who had or would eventually have symptoms. Based on these assumptions, the results suggested that only asymptomatic people were responsible for 24% of infections.
But the researchers also modeled scenarios in which the peak of infectiousness 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 rate of infectivity in relation to other groups.
In most of these settings, people without symptoms (asymptomatic and pre-symptomatic) transmitted at least 50% of new infections.
“The proportion of transmissions has generally remained above 50 percent over a wide range of basic values,” said Butler, adding that the consistency of this finding was surprising.
Even in the most conservative estimate, where the peak of infectivity came seven days after exposure and asymptomatic people were responsible for 0 percent of transmission, the pre-symptomatic group still caused over 25 percent of cases in general, according to the model.
Butler and his co-authors cautioned, however, that their model is likely to underestimate the actual percentage of COVID-19 cases caused by people without symptoms, since they calculated transmission rates if everyone moved at random. But in reality, many restaurants and other establishments do tests for fever and other symptoms to keep out symptomatic people. In addition, many people with symptoms isolate themselves 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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