The head of the Centers for Disease Control and Prevention defended the latest guidelines shortening the isolation period for certain people who test positive for COVID amid criticism from public health experts who believe a negative test should also be included in the recommendation.
CDC Director Rochelle Walensky said that the new guidelines of five days isolation for asymptomatic people or people who have seen their symptoms largely resolve stemmed from "two years of understanding transmissibility," and that the period was shortened from 10 days to reduce staffing shortages in hospitals. The guidance recommends an additional five days of masking after the isolation period.
"Let me make clear that we are standing on the shoulders of two years of science, two years of understanding transmissibility, and a lot of information that we have gleaned from the wild type virus, as well as the alpha and delta variants and more that we continue to learn every single day about omicron," Walensky said on Wednesday at a White House briefing on the virus.
She also said the CDC did not recommend getting a negative test before leaving isolation because scientists aren't confident that rapid tests provide a good indication of contagiousness and PCR tests can show a positive result for months. But, she said, data shows people are largely less infectious after five days.
"We do know the vast majority of viral transmission happens in those first five days, somewhere in the 85 to 90% range. So if a person can isolate for the first five days they absolutely should," she said.
"We also don't know that antigen tests give a good indication of transmissibility at this stage of infection. On the other hand, we know that after five days people are much less likely to transmit the virus and that masking further reduces that risk. And this is why people need to mask for five days after the five days of isolation," she said.
Some infectious disease experts expressed frustration with the new guidlines on social media, with Dr. Ashish Jha, dean of the Brown University School of Public Health, saying rapid antigen tests are a good additional layer of assurance before leaving isolation.
"The CDC should have included a negative antigen tests to end isolation," Jha wrote, and rapid tests are "great when used properly."
Dr. Jerome Adams, the U.S. surgeon general under former President Donald Trump, also slammed the CDC in a Twitter thread, criticizing the agency's decision to omit the recommendation for COVID-19-positive individuals to take a test prior to ending isolation.
The current guidelines are that a person with a positive COVID-19 test should isolate for five days, regardless of their vaccination status. If a person never had or no longer has symptoms after five days, they can leave isolation. They should then wear a mask for the next five days, even at home, according to the guidance.
"After five days, if you're asymptomatic or if your symptoms have largely resolved, you may leave isolation as long as you continue to wear a mask around others, even in the home for an additional five days," Walensky said.
Rapid tests, which are a faster and often more convenient option for testing, have always been slightly less accurate than PCR tests, which can take days to get back.
Most major test makers, such as Abbott, say their rapid tests still work to detect the omicron variant. But without naming specific tests, the Food and Drug Administration said preliminary research indicates some of these tests could be less sensitive.
The FDA says rapid tests still work, and the agency is now studying how much of an impact the omicron variant might make on accuracy. Walensky and other members of the White House COVID team said Wednesday that the FDA did not intend to dissuade Americans from taking the tests but wanted to be "transparent."
"What the FDA was saying was that when they were looking at the sensitivity with regard to omicron in some of the tests, there appears to be somewhat of a diminution, not a disappearance, but a diminution of the sensitivity," Dr. Anthony Fauci, chief medical advisor to the White House, said at the briefing.
"The fact that the sensitivity is diminished somewhat does not obviate the importance of the still advantage and usefulness of these tests under different circumstances," Fauci said.
"That was the message of the FDA. They wanted to make sure they were totally transparent in saying the sensitivity might come down a bit, but they did emphasize there still is an important use of these tests," Fauci said.
Walensky added that "another important use is where we use them for serial testing in places like tests to stay, to keep children in school, in higher ed to keep our college campuses safe" -- all situations in which the quantity of tests being used can make them more useful.
Fauci also presented some data from around the world about the apparent lessened severity of omicron.
In the U.K., a study has found the risk of hospitalization admission with omicron was 40% of that for delta, and in Scotland, preliminary data suggested a two-thirds reduction in the risk of COVID-19 hospitalization with omicron, Fauci said.
Meanwhile, in the U.S., data from the last 14 days shows "the spike in cases is out of proportion to the increase in hospitalization," he said.
There has been a 126% increase in cases and an 11% increase in hospitalizations, and while deaths and hospitalizations tend to lag weeks behind cases, "the pattern and disparity between cases and hospitalization strongly suggest that there will be a lower hospitalization to case ratio when the situation becomes more clear," he said.
"So in conclusion, the data are encouraging, but still, in many respects, preliminary, yet they are getting stronger and stronger as additional data are accumulated," he said.
"All indications point to a lesser severity of omicron versus delta."
Still, Fauci warned that severity on its own is good news, but it is paired with higher transmissibility.
"Increased transmissibility of omicron resulting in an extremely high volume of cases may override some of the impact of the lower disease severity," he said.
"And so we should not become complacent since our hospital system could still be stressed in certain areas of the country."
ABC News' Arielle Mitropoulos conributed to this report.