• The CDC recommended Tuesday that fully vaccinated Americans start wearing masks indoors again in places with high Covid-19 transmission rates.
• The counties in the two groups for which the CDC recommends mask-wearing combined make up 225 million Americans, according to a CNBC analysis of the agency's data, or about two-thirds of the U.S. population.

The Centers for Disease Control and Prevention recommended Tuesday that fully vaccinated Americans start wearing masks indoors again in places with high Covid-19 transmission rates as nationwide infection levels are once again on the rise.

CDC Director Dr. Rochelle Walensky told reporters Tuesday that masks should be worn in public, indoor settings by everyone, including fully vaccinated people, in areas "with substantial and high transmission."

But what exactly is "high" or "substantial" transmission, and where are the areas the CDC is concerned about?

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

If a county has reported 50 to 100 cases per 100,000 residents over a seven-day period or has a positivity rate of 8% to 10%, it falls into the "substantial transmission" tier, while those reporting 100 cases or more per 100,000 or have a positivity rate of at least 10% are labeled as "high transmission." Those are the two groups for which the CDC recommends mask-wearing.

The CDC said 1,495 counties fall into the highest transmission grouping and another 548 counties in the "substantial" tier — the areas where masks should be worn inside restaurants, businesses, any public space. Those counties combined make up 225 million Americans, according to a CNBC analysis of the agency's data, or about two-thirds of the U.S. population.

The counties with moderate transmission, which aren't subject to the CDC's advisory, make up another 31% of the population while just more than 1% of Americans live in counties with low transmission rates, according to the CDC's criteria, which is current as of July 27.

Federal health officials still believe fully vaccinated individuals represent a very small amount of transmission. Still, the more contagious delta variant means some vaccinated people could be carrying higher levels of the virus than previously understood and potentially transmit it to others just as easily as unvaccinated individuals, Walensky said.

There are at least three states where every county falls within the CDC's mask advisory: Florida, Louisiana and Arkansas.

The delta Covid variant is one of the most infectious respiratory diseases ever seen by scientists, Walensky said last week. The variant is highly contagious, largely because people infected with the delta strain can carry up to 1,000 times more virus 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," Walensky told reporters at a briefing Thursday. "It is one of the most infectious respiratory viruses we know of, and that I have seen in my 20-year career."

The CDC's guidance is only a recommendation, and leaves it up to states and local officials to decide whether to reintroduce their mask rules for certain people. Some areas have started to reimpose mask mandates in recent weeks.

Dr. Natasha Bhuyan, a family physician with One Medical in Phoenix, said she is recommending mask-wearing to her patients since the delta strain is so much more contagious than other variants.

"We know that when you're vaccinated, you are significantly less likely to be hospitalized or die from Covid," she said. "But even if you're vaccinated, while rare you can still get Covid and you can still be contagious and pass Covid to other people."

Phoenix is in Maricopa County, which is in the highest community transmission category.

"Delta has changed our thinking about when people should wear masks," Bhuyan added. "It's not going to last forever. When we get the vaccination rate up and Covid case rate down, people will be able to take off their masks."

-- CNBC's Berkeley Lovelace Jr. contributed reporting.

Correction: This article was updated to remove Hawaii as one of the states where every county meets the CDC's mask advisory. Kalawao County, population 86, has low transmission rates.