A person wears a mask while walking in Grand Central Terminal on July 27, 2021 in New York City.

Spencer Platt | Getty Images

The Centers for Disease Control and Prevention on Tuesday recommended that fully vaccinated Americans return to wearing masks indoors in locations with high Covid-19 transmission rates as infection rates rise again across the country.

CDC director Dr. Rochelle Walensky told reporters Tuesday that in areas of “high and high transmission” everyone, including those who are fully vaccinated, should wear masks in public, indoors.

But what exactly is “high” or “significant” transmission, and where are the areas that the CDC cares about?

The agency uses two measures to divide U.S. counties into four levels of community transmission: the number of new cases per 100,000 residents and the percentage of positive Covid tests in the past week.

If a county has reported 50 to 100 cases per 100,000 population over a seven day period, or has a positivity rate of 8 to 10%, it falls into the “significant transmission” category, while those that report 100 cases or more per 100,000 or more have a positivity rate of at least 10% are referred to as “high transmission”. These are the two groups that the CDC recommends wearing masks.

According to the CDC, 1,495 counties fall into the highest broadcast tier and another 548 counties fall into the “significant” tier – the areas where masks should be worn in restaurants, businesses, and public spaces. These counties combined make up 225 million Americans, or about two-thirds of the US population, according to a CNBC analysis of CDC data.

The low-transmission counties that are not subject to the CDC’s recommendations make up an additional 31% of the population, while just over 1% of Americans live in low-transmission counties, according to the CDC’s criteria as of July 27 .

Federal health officials still believe that fully vaccinated individuals represent a very low level of transmission. The more contagious Delta variant, however, means that some vaccinated people may carry higher amounts of the virus than previously thought and may pass it on to others just as easily as unvaccinated people, Walensky said.

There are at least three states in which each county falls under the CDC’s mask recommendation: Florida, Louisiana, and Arkansas.

The Delta Covid variant is one of the most contagious respiratory diseases scientists have ever seen, Walensky said last week. The variant is highly contagious, mainly because people infected with the Delta strain can carry up to 1,000 times more viruses in their nasal passages than those infected with the original strain, according to new data.

“The Delta variant is more aggressive and much more transmissible than previously circulating strains,” said CDC Director Dr. Rochelle Walensky reporters at a briefing Thursday. “It’s one of the most contagious respiratory viruses we know and that I’ve seen in my 20-year career.”

The CDC’s guidelines are only a recommendation, leaving it up to state and local officials to decide whether to reintroduce their masking rules for specific individuals. Some areas have started to reintroduce mask requirements in the past few weeks.

Dr. Natasha Bhuyan, a GP at One Medical in Phoenix, Ariz., Said she recommends that her patients wear a mask because the Delta variant is so much more contagious than other variants.

“We know that you are much less likely to be hospitalized or die of Covid after a vaccination,” she said. “But even if you are vaccinated, you can rarely get Covid and you can still be contagious and pass Covid on to other people.”

Phoenix is ​​located in Maricopa County, which is in the highest category of community broadcasts.

“Delta has changed the way we think about when people should wear masks,” added Bhuyan. “It won’t take forever. If we increase the vaccination rate and the Covid case rate decrease, people can take their masks off.”

CNBC’s Berkeley Lovelace Jr. contributed to the coverage.

Correction: This article has been updated to remove Hawaii as one of the states where each county meets the CDC’s mask recommendations. Kalawao County has a population of 86 and has low transmission rates.