A C.D.C. study raises questions about the agency’s own isolation guidelines.

A C.D.C. study raises questions about the agency’s own isolation guidelines.

More than half of people who took a rapid antigen test five to nine days after first testing positive for the coronavirus or after developing Covid-19 symptoms tested positive on the antigen test, according to a new study from the Centers for Disease Control and Prevention.

The finding raises more concerns about the agency’s revised isolation guidelines, which say that many people with Covid can end their isolation periods after five days, without a negative coronavirus test, if they wear masks and take other precautions for another five days.

A C.D.C. scientist who was an author of the study said that he did not believe the agency’s isolation guidelines needed to change. But the results suggest that many people with the virus may still be infectious during this period, scientists said.

The study “demonstrates what a lot of people have suspected: that five days is insufficient for a substantial number of people,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, said in an email. “The bottom line,” she added, “is that this absolutely should lead to a change in isolation guidance.”

The research was conducted after Omicron became the dominant variant in the United States and as cases were surging nationwide. Cases have since fallen precipitously, reducing the risk of infection and the number of Americans who are in isolation.

The C.D.C. shortened the isolation period to five days from 10 in December as the Omicron variant spread. Many public health experts criticized the move, noting that people might still be infectious after five days and that allowing them to end isolation without testing might help the new variant spread faster.

Dr. Ian Plumb, a medical epidemiologist at the C.D.C. and an author of the new study, said that he believed the study “basically supported” the agency’s current isolation guidance, which asks people to mask and refrain from traveling until 10 full days have passed.

“I honestly don’t think that it means that the current guidance needs to” change, he said.

Instead, he said, the study supports the idea that antigen tests can be successfully integrated into isolation guidelines.

“I think the biggest takeaway is that it’s possible to incorporate antigen tests into the guidance for isolation because they provide additional information about someone’s risk of being potentially infectious,” he said.

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