The agency also recommended indoor masks for all teachers, staff, students and visitors to schools, regardless of vaccination status, citing new information about the ability of the Delta variant to spread among vaccinated people.
The CDC released a U.S. map with four designations of transmission: High, substantial, moderate, and low, and parts of New York and New Jersey are identified as areas of substantial transmission, which means the recommendations would apply to those two states.
Connecticut is considered moderate.
The map can also be broken down by county, which shows that in New Jersey, Monmouth County has "high" transmission, while Atlantic, Bergen, Burlington, Essex, Middlesex, Ocean, and Passaic counties have "substantial" transmission.
The other 13 counties are either moderate or low.
In New York, New York City appears to be "substantial," with Staten Island "high," but with the exception of Green, Nassau and Suffolk counties being "substantial," the rest of the state is moderate or low.
"New Yorkers beat back COVID before -- going from the highest positivity rate on the globe to one of the lowest -- by staying smart, following the science, and having each other's backs, and that's exactly what we'll keep doing in this next phase of the pandemic," Gov. Andrew Cuomo said. "We are reviewing the CDC's new recommendations closely in consultation with federal and state health experts."
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Most new infections in the U.S. continue to be among unvaccinated people, but "breakthrough" infections, which generally cause milder illness, can occur in vaccinated people.
"This moment, and most importantly, the associated illness, suffering, and death, could have been avoided with higher vaccination coverage in this country," CDC Director Dr. Rochelle Walensk said.
When earlier strains of the virus predominated, infected vaccinated people were found to have low levels of virus and were deemed unlikely to spread the virus much, Walensky said. But with the Delta variant, the level of virus in infected vaccinated people is "indistinguishable" from the level of virus in the noses and throats of unvaccinated people.
The data emerged over the last couple of days from 100 samples, though it is unpublished and the CDC has not released it.
"It is concerning enough that we feel like we have to act," Walensky said.
For much of the pandemic, the CDC advised Americans to wear masks outdoors if they were within 6 feet of one another. Then, in April, as vaccination rates rose sharply, the agency eased its guidelines on the wearing of masks outdoors, saying that fully vaccinated Americans no longer needed to cover their faces unless they were in a big crowd of strangers.
In May, the guidance was eased further for fully vaccinated people, allowing them to stop wearing masks outdoors in crowds and in most indoor settings.
The guidance still called for wearing masks in crowded indoor settings, like buses, planes, hospitals, prisons and homeless shelters, but it cleared the way for reopening workplaces and other venues.
Subsequent CDC guidance said fully vaccinated people no longer needed to wear masks at summer camps or at schools, either.
For months COVID cases, deaths and hospitalizations were falling steadily, but those trends began to change at the beginning of the summer as the Delta variant, a mutated and more transmissible version of the virus, began to spread widely, especially in areas with lower vaccination rates.
Some public health experts said they thought the earlier CDC decision was based on good science, which indicated that the risk of vaccinated people spreading the virus was relatively low and that the risk of them catching the virus and becoming extremely ill was even lower.
But those experts were also critical, noting that there was no call for Americans to document their vaccination status, which created an honor system. Unvaccinated people who did not want to wear masks in the first place saw it as an opportunity to do what they wanted, they said.
"If all the unvaccinated people were responsible and wore mask indoors, we would not be seeing this surge," said Dr. Ali Khan, a former CDC disease investigator who now is dean of the University of Nebraska's College of Public Health.
Lawrence Gostin, a public health law professor at Georgetown University, drew a similar conclusion.
"It was completely foreseeable that when they (the CDC) made their announcement, masking would no longer be the norm, and that's exactly what's happened," Gostin said.
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The CDC may be seen as "flip-flopping," he said, because there's been no widely recognized change in the science, he said. Furthermore, it's not likely to change the behavior of the people who most need to wear masks.
"I don't think you can effectively walk that back," he said.
(The Associated Press contributed this report)
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