Will shortened isolation periods spread the virus?

Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said Tuesday that the new guidance had been necessitated by the volume of people about to be infected.

In a series of holiday weekend meetings, she said, agency officials pored over transmissibility data for past variants and signs that omicron caused less severe illness. But ultimately, Walensky said, she decided that rapid tests were not effective enough at diagnosing infectiousness in people.

“You don’t necessarily do a test if you don’t know what you’re going to do with the result,” she said, adding, “The anticipated number of cases that we were seeing required us to take action at this moment.”

The CDC’s recommendations cut isolation periods for infected people from 10 days to five. The agency did not recommend rapid testing before people left isolation.

But some scientists maintain that rapid tests are the most convenient indication of whether or not someone remains contagious. Regulatory delays, manufacturing problems and shortfalls in government support have left rapid tests in extremely short supply as the omicron variant has surged, pushing caseloads to near record levels in the United States.

According to a scientist who has discussed isolation policy with the CDC in recent months, officials said the agency could not recommend rapid tests while supplies were so scarce.

The scientist spoke on condition of anonymity to describe confidential discussions.

President Joe Biden has promised to make 500 million tests available free of charge, but it is not clear how quickly they will be shipped.

Walensky denied that testing availability affected the agency’s thinking and said that masking alone addressed the risk that infected people remained contagious after isolation. The CDC has asked infected people to wear a mask around others for five days after leaving isolation.

“We know that, let’s call it 85% of your transmissibility time is already behind you,” Walensky said of the five-day isolation period. “A minority of it is in front of you. And if you wear a mask, you can avert it.”

Scientists said they worried that like rapid tests, the most effective masks, known as N95s, remained out of reach for many Americans.

The Food and Drug Administration said on Tuesday that rapid tests do detect Omicron cases, though they may have reduced sensitivity. (Another type of test, known as a PCR, is not useful for releasing people from isolation because it can return positive results after someone is no longer contagious.)

One federal official said the CDC incorporated unpublished modeling on the spread of the delta variant that found the risk of transmission was 13% five days after someone tested positive. The CDC was working to make that data public, the official said.

Scientists, though, expressed concern about applying models of delta’s spread at a time when omicron was fast becoming dominant. The variant has a considerably easier time than delta infecting vaccinated people. It is also highly contagious, potentially even more so than delta.

The right isolation periods for omicron depend on when people are testing, as well as an individual's level of immunity and the properties of the variant itself, scientists said. Some people remain contagious much longer than others, and evidence from rapid tests suggests certain patients are still infectious for longer than five days.

“I’d be very wary of translating data from delta to omicron,” said Stephen Goldstein, a virus expert at the University of Utah. “I think there’s the potential for this to make things even worse or accelerate the course of the pandemic.”

Immunologists said Tuesday that there were also signs that people infected with omicron were developing symptoms earlier in the course of infections than they had with past variants — a shift that could have major ramifications for isolation periods.

Those symptoms, they said, were making people aware of their illnesses sooner and, in some cases, compelling them to get tested sooner. Those people may be starting the clock on their isolation periods at the very beginning of their infections — rather than at the middle or end, as was true earlier in the pandemic — and returning to work while they remain contagious.

“I don’t think reducing the time for isolation overall is a bad idea,” said Angela Rasmussen, a virus expert at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada. “But saying, ‘Five days is probably OK, based on delta, so let’s give it a shot and see,’ is really not what you should be doing.”

She added, “This could’ve been implemented in a much more reasonable and lower-risk way.”

The CDC has long faced criticism for issuing confusing guidance during the pandemic, particularly on the use of face masks. Its isolation advice Monday did little to allay those concerns, scientists said.

Several doctors, for example, said they were struggling to grasp what patients would fit under the CDC’s advice that infected people whose symptoms were “resolving” could leave isolation after five days.

Many people’s symptoms fluctuated over the course of a single day, they said. Other patients may seem to be feeling better before experiencing a flare-up.

“The guidance is far more confusing than it could and should be,” said Dr. Megan Ranney, an emergency physician and academic dean at Brown University’s School of Public Health.

“Front and center, this should be for people who are not symptomatic. If you have symptoms, you should not be out in public.”

Dr. Marc Boom, CEO of Houston Methodist, said he was grateful that the CDC shortened isolation periods for health workers. In the past week, he said, roughly 3% of the workforce had tested positive, putting more significant strain on the hospital than at any other point in the pandemic.

Still, he said that the mixed messages were confusing. “They advanced the public past the basic hospital rules,” Boom said. “That threw us for a loop. We looked at that and said, ‘That makes no sense.’ ”

Scientists said that they expected more data to emerge about the course of omicron infections in the coming weeks.

Ravindra Gupta, a virus expert at the University of Cambridge, said Tuesday that he did not believe people with omicron infections would shed virus for longer than people infected with earlier variants, given signs that it replicated less efficiently in certain human cells.

Infections with earlier variants appeared to last roughly nine days in vaccinated people and 11 days in unvaccinated people, according to studies of NBA personnel, though that did not necessarily mean that people were contagious for that same period.

Denis Nash, a public health researcher at City University of New York, said compliance with any isolation policy appeared low: A study he led indicated that only 29% of people with past infections had isolated, although that included some who never knew they were ill.

But he said it was not at all clear that shortening isolation periods would persuade more people to stay home.

“Not testing after five days of isolation — is that because the testing supply isn’t there?” he said. “If so, that’s no reason to make it a policy.”