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People Without Symptoms Responsible for More Than 50% of COVID Cases

(Photo by engin akyurt on Unsplash)

According to new research, more than half of new COVID cases have been spread by those who don’t realize they have it.


New research, published in the Journal of the American Medical Association, has put a figure on the percentage of people who have spread COVID without actually realizing that they have it. Horrifically, the data estimates that more than half of the transmissions have been from asymptomatic sources.

Back in November, the Centers for Disease Control and Prevention (CDC), noted that the asymptomatic “estimated to account for more than 50% of transmissions.”

The lead author of the new study, Jay Butler, said the findings reinforce the importance of following public-health guidelines about mask-wearing and distancing.

“There was still some controversy over the value to community mitigation – face masks, social distancing, and hand hygiene – to limit spread,” Butler told Business Insider. “This study demonstrates that while symptom screening may have some value, mitigation, as well as strategically planned testing of persons in some settings, will be a significant benefit.”


Back in November, the Centers for Disease Control and Prevention (CDC), noted that the asymptomatic “estimated to account for more than 50% of transmissions.”


As Business Insider explained, “the researchers modeled potential COVID-19 transmitters in three groups: pre-symptomatic (people who hadn’t had symptoms yet), never symptomatic, and symptomatic. The researchers then modeled how much each group would transmit COVID-19 depending on the day people were most infectious. At baseline, they assumed people in all groups would be most infectious five days after getting exposed to the coronavirus. That’s what researchers have 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% of people were asymptomatic and that those individuals were 75% as infectious as people who were showing or would eventually show symptoms. Based on those assumptions, the results suggested that asymptomatic people alone were responsible for 24% of infections. But the researchers also modeled scenarios in which peak infectiousness occurred after three, four, six, and seven days, and they raised and lowered the percentage of asymptomatic people in the model, as well as their rate of infectiousness relative to other groups.”

As the study discovered, across most of the scenarios, those without symptoms (asymptomatic and pre-symptomatic) were found to transmit at least 50% of new infections.

“The proportion of transmissions remained generally above 50% across a broad range of base values,” Butler said, adding that the consistency of that finding was surprising, before suggesting that their model likely underestimates the real percentage of cases spread by the asymptomatic.

As Business Insider pointed out, “even in the most conservative estimate, in which peak infectiousness came seven days after exposure and asymptomatic people accounted for zero percent of transmission, the pre-symptomatic group still caused more than 25% of cases overall, according to the model.”


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