The decision by federal health officials to shorten isolation periods for Americans infected with the coronavirus drew both tempered support and intense opposition from scientists on Tuesday, particularly over the absence of a testing requirement and fears that the omission could hasten the spread of the highly contagious Omicron variant.
The new guidance, coming amid a crush of new infections that has starved many hospitals of workers, seemed to some scientists like a necessary step to shore up work forces in essential industries. And encouraging people to leave isolation early after testing negative could spare them the hardships of prolonged periods at home.
But letting hundreds of thousands of infected people forgo those tests — even if, crucially, their symptoms were not entirely gone — risks seeding new cases and heaping even more pressure on already overburdened health systems, experts said in interviews on Tuesday.
“To me, this feels honestly more about economics than about the science,” said Yonatan Grad, an associate professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, who has tracked coronavirus infections in the National Basketball Association.
“I suspect what it will do is result in at least some people emerging from isolation more quickly, and so there’ll be more opportunities for transmission and that of course will accelerate the spread of Covid-19,” he added, noting that people were unlikely to adhere strictly to masking advice after leaving isolation.
Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, said in an interview on 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, Dr. 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 C.D.C.’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.
A scientist who has discussed isolation policy with the C.D.C. in recent months said that officials said the agency could not recommend rapid tests while supplies were so scarce. The scientist spoke on the condition of anonymity to describe confidential discussions.
President Biden has promised to make 500 million tests available free of charge, but it is not clear how quickly they will be shipped.
Dr. 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 C.D.C. has asked infected people to wear a mask around others for five days after leaving isolation.
“We know that, let’s call it 85 percent, of your transmissibility time is already behind you,” Dr. 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.
One federal official said the C.D.C. incorporated unpublished modeling on the spread of the Delta variant that found the risk of transmission was 13 percent five days after someone tested positive. The C.D.C. 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 for much longer than others, and evidence from rapid tests suggests that 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 virologist 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 on 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 remained contagious.
“I don’t think reducing the time for isolation overall is a bad idea,” said Angela Rasmussen, a virologist 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 C.D.C. has long faced criticism for issuing confusing guidance during the pandemic, particularly on the use of face masks. Its isolation advice on Monday did little to allay those concerns, scientists said.
The Coronavirus Pandemic: Key Things to Know
New C.D.C. guidelines Hoping to prevent further disruptions to daily life, the C.D.C. reduced the period that certain infected Americans must sequester to five days from 10. This change applies only to those without symptoms, or those without fevers whose other symptoms are improving.
Several doctors, for example, said they were struggling to grasp what patients would fit under the C.D.C.’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, the chief executive of Houston Methodist, said that he was grateful that the C.D.C. shortened isolation periods for health workers. In the last week, he said, roughly 3 percent of the work force 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,” Dr. 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 virologist at the University of Cambridge, said on 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 N.B.A. personnel, though that did not necessarily mean that people were contagious for that same period.
Denis Nash, an epidemiologist at City University of New York, said compliance with any isolation policy appeared low: A study he led indicated that only 29 percent of people with past infections had isolated, though 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.”
Noah Weiland contributed reporting.