Citing new information about the ability of the delta variant to spread among vaccinated people, the CDC also recommended indoor masks for all teachers, staff, students and visitors to schools, regardless of vaccination status.
The new guidance follows recent decisions in Los Angeles and St. Louis to revert to indoor mask mandates amid a spike in COVID-19 cases and hospitalizations that have been especially bad in the South. The country is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations.
RELATED: CDC reverses course on indoor masks in some parts of US
"In areas of substantial and high transmission, the CDC recommends fully vaccinated people wear masks in public indoor settings," said CDC Director Dr. Rochelle Walensky.
In the tri-state area, New Jersey falls into the "substantial" transmission category. Delaware and Pennsylvania remain in the "moderate" COVID-19 transmission category.
Dr. Darren Mareiniss, with Einstein Medical Center, said he is concerned about the delta variant's ability to spread.
"If you're fully vaccinated, you may not get really sick or hospitalized, but you could get a breakthrough infection and our concern is that you could pass that infection to young children who are not immunized, or compromised individuals," said Mareiniss.
In Philadelphia, masks are still a welcome accessory to some.
"They say COVID is on the rise recently, so I just started masking again recently," said Jake Capaldo of Center City.
In South Jersey, Burlington, Ocean and Atlantic counties are hot spots, according to the CDC.
"Governor Murphy and the New Jersey Department of Health will review the new CDC guidance on masking requirements in response to the spread of the delta variant. Governor Murphy continues to encourage all individuals ages 12 and up to receive the free and effective COVID-19 vaccination to reduce the spread of the virus," said a spokesperson for the governor.
Like many businesses, Ardit Cami managed to survive a year of restrictions.
"I don't think it will hurt our business because we went through it before. If we're going to go back to masks, then I think that will be the next step. That's how it went the first time," said Cami reacting to the CDC's new guidelines.
RELATED: Philadelphia again recommends masks for indoor public spaces, regardless of vaccination status
"I believe that there are always going to be people who are going to disagree and are not going to cooperate, but hopefully there will be more people who do cooperate, than not," said Joyce Shorts of Woodbury, New Jersey.
As of now, Pennsylvania is not considering another statewide mask mandate, but last week Philadelphia's health department strongly recommended residents wear masks in public places indoors.
"I don't think it's a bad idea and I think more people should get on board," said Allison Neff of Philadelphia.
.@CDCgov is listing New Jersey as a state with a “substantial” transmission. Out of our area Burlington, Ocean and Atlantic counties are considered hot spots as the Delta variant spreads. https://t.co/p26hb8IHBe pic.twitter.com/1GFsVr65Ae— Christie Ileto (@Christie_Ileto) July 28, 2021
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. 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, added Walensky.
The data emerged over the last couple of days from 100 samples. It is unpublished, and the CDC has not released it. But "it is concerning enough that we feel like we have to act," said the CDC director.
Vaccinated people "have the potential to spread that virus to others," she said.
The Associated Press contributed to this report.