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Big Mass. Study Finds You Could Be Symptom-Free But Still Carry High Load Of Coronavirus02:31
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Staff at the CRSP (Clinical Research Sequencing Platform) lab at the Broad Institute of MIT and Harvard process COVID-19 tests in July. (Courtesy Scott Sassone for the Broad Institute of MIT and Harvard)
Staff at the CRSP (Clinical Research Sequencing Platform) lab at the Broad Institute of MIT and Harvard process COVID-19 tests in July. (Courtesy Scott Sassone for the Broad Institute of MIT and Harvard)

Many people who have no symptoms can still carry very high levels of the coronavirus and potentially spread it, according to preliminary research from the Broad Institute in Cambridge that was based on mass testing in Massachusetts nursing homes.

This spring, at the peak of the state’s coronavirus epidemic, the Broad Institute performed thousands of tests as part of a state effort to check residents and staff from over 300 nursing homes and other long-term-care centers.

Now, Broad researchers say their data confirm that many people carry the virus but have no symptoms: "Of those without symptoms, at this time, at the peak in our epidemic, 12% of residents and 4% of staff at these nursing homes were positive," says Dr. Roby Bhattacharyya from the Broad and Massachusetts General Hospital. "That’s a large percentage of people without symptoms who tested positive and were at risk to transmit."

Most striking of all, the study shows that the amount of virus people carry varied widely, but the range was about the same whether people had symptoms or not.

"The viral loads really spanned a huge range: It's about a hundred-million-fold difference from the least to the most," Bhattacharyya says. During the April to June period the tests were done, "there's a lot of people with billions of virus per milliliter who don't know it."

He adds: "And there's a lot of people with billions of virus per milliliter who have symptoms, too." But there's just as high a percentage among those who test positive "who don't have symptoms at all, at the time they were tested."

It’s believed that higher viral loads of the coronavirus can make a person more contagious, he says. So the findings — which have yet to undergo peer review — could mean coronavirus transmission by people without symptoms is even more common than previously thought.

The data includes nearly 17,000 residents of Massachusetts nursing homes and more than 15,000 staffers. That makes this study by far the biggest yet to quantify the levels of coronavirus detected with testing, Bhattacharyya says.

The study did not follow the people who were tested over time, so it did not determine whether high viral loads without symptoms tended to predict the development of symptoms later on.

The New York State Department of Health says the Broad study is significant because it confirms in a very large group that viral loads are similar whether people have symptoms or not.

The study "demonstrates the importance of testing broadly in the community," the department says in a statement, as well as the "urgent need" for rapid, frequent testing to pick up anyone who could spread the virus.

This segment aired on August 6, 2020.

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Carey Goldberg Twitter Editor, CommonHealth
Carey Goldberg is the editor of WBUR's CommonHealth section.

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