The decision by federal health officials to shorten isolation periods for Americans with the coronaviruses drew both support and opposition from scientists on Tuesday, particularly over the absence of a testing requirement and fears that the omission could accelerate the spread of the Omicron variant.
The new guidance seemed to some scientists to be a necessary step to shore up work forces in essential industries. Encouraging people to leave isolation early after testing negative could spare them the hardship of long periods at home.
If hundreds of thousands of people forgo those tests, it could lead to new cases and put more pressure on already overburdened health systems.
An associate professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, Grad said that he felt this was more about economics than science.
He said that it would result in some people emerging from isolation more quickly, and that there would be more opportunities for transmission, and that it would accelerate the spread of Covid-19.
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 number of people about to be infectious.
She said that agency officials looked over transmissibility data for past variants and signs that Omicron caused less severe illness in a series of holiday weekend meetings. She decided that rapid tests were not enough to detect infectiousness in people.
She said that if you don't know what you're going to do with the result, you don't necessarily do a test.
The C.D.C. recommended that the isolation periods for people with the disease be cut from 10 days to five. The agency didn't recommend rapid testing before people left isolation.
Some scientists think rapid tests are the most convenient way to determine if someone is infectious. 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.
President Biden convened a virtual meeting of the White House Covid response team on Monday.
The scientist said that the C.D.C. could not recommend rapid tests while supplies were low. The scientist spoke on the condition of anonymity.
500 million tests will be free of charge, but it is not clear how quickly they will be shipped.
Dr. Walensky said that masking alone addressed the risk that people would remain infectious after being isolated. The C.D.C. wants people with the disease to wear a mask around others for five days.
The five-day isolation period is behind you, according to Dr. Walensky. There is a minority of it in front of you. If you wear a mask, you can avoid it.
The most effective masks, known as N95s, were out of reach for many Americans.
The FDA said on Tuesday that rapid tests do detect Omicron cases, though they may have reduced sensitivity. The P.C.R. is not useful for releasing people from isolation because it can return positive results after someone is no longer contagious.
The risk of transmission was found to be 13 percent five days after someone tested positive for the Delta variant. The data was being made public by the C.D.C.
Scientists were concerned about applying models of Delta's spread at a time when Omicron was growing. The variant has a quicker time than Delta. It's also highly contagious, potentially more so than Delta.
Scientists said that the right isolation periods for Omicron depends on a number of factors, including when people are testing, immunity and the variant itself. Some people remain infectious for longer than others, and evidence from rapid tests shows that certain patients are still infectious for longer than five days.
The University of Utah's Stephen Goldstein said he would be wary of translation of data from Delta to Omicron. I think that this could make things worse or accelerate the course of the epidemic.
Covid P.C.R. was testing at a free site in Washington, D.C.
There were signs that people with Omicron were developing symptoms earlier in the course of infections than they had in the past, which could have major ramifications for isolation periods.
They said that the symptoms made people aware of their illnesses sooner and that some cases made them 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 the case earlier in the Pandemic.
Reducing the time for isolation is not a bad idea, according to a researcher at the Vaccine and Infectious Disease Organization. It is not what you should be doing if you say, 'Five days is probably OK, based on Delta, so let's give it a shot and see.'
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She said that this could have been implemented in a more reasonable and lower-risk way.
The C.D.C. has been criticized for issuing confusing guidance during the Pandemic. On Monday, its isolation advice did not allay those concerns.
Doctors were struggling to understand what patients would fit under the C.D.C. advice that people with symptoms could leave isolation after five days.
They said that many people had different symptoms on a single day. Some patients may be feeling better before they have a flare-up.
The guidance is more confusing than it should be, according to Dr. Megan Ranney, an emergency physician and academic dean at Brown University.
This should be for people who are not sick. If you have symptoms, you shouldn't be out in public.
The chief executive of Houston Methodist said that he was grateful that the C.D.C. shortened isolation periods for health workers. He said that in the last week, 3 percent of the work force had tested positive, putting more strain on the hospital than at any other point in the Pandemic.
He said that the messages were confusing. Dr. Boom said that they advanced the public past the basic hospital rules. We were thrown for a loop. We looked at that and said it was not logical.
More data about the Omicron infections is expected in the coming weeks.
The University of Cambridge's Gupta said on Tuesday that he didn't believe people with Omicron infections would shed virus for longer than people with earlier versions.
The N.B.A. personnel studies showed that the infections with earlier variant appeared to last about nine days in vaccineed people and 11 days in unvaccinated people.
Denis Nash, an epidemiologist at the City University of New York, said that a study he led showed that only 29 percent of people with past infections had been isolated.
He said it was not clear if shortening isolation periods would convince people to stay home.
He said that not testing after five days of isolation is because the testing supply isn't there. If that is the case, it is not a reason to make a policy.
Noah gave reporting.
