New recommendations last month from the Centers for Disease Control and Prevention call for everyone, vaccinated or not, to wear a mask in public indoor spaces in areas with substantial or high transmission of covid-19.

As it turns out, that applies to more than 85 percent of U.S. counties — including many of the places travelers are heading this summer.

Of the top 25 destinations this summer for flight bookings on the Hopper app, most had high transmission and just five had substantial transmission category as of Aug. 6, according to CDC data. Officials recommend masking indoors for all 25, a list that contains popular cities across the country that are seeing cases rise rapidly as the delta variant spreads.

Just because the CDC says people should wear masks in a certain area doesn’t mean there will be a local mask mandate. But some destinations, including D.C. and Los Angeles County, have already announced indoor mask rules for all. Private companies can also put their own rules in place. All of that means it will be difficult for travelers to know exactly where masks will be required — especially as rules fluctuate.

The hyper-transmissible delta variant of the coronavirus has left would-be travelers uncertain. The Post spoke to an expert about how to safely make that call. (The Washington Post)

Driving the CDC’s recommendations is the transmissibility of the delta variant, which vaccinated people can spread in the case of a breakthrough infection, even if they are asymptomatic or only mildly ill. The CDC’s covid data tracking site has county-specific breakdowns that include vaccination rates, coronavirus case numbers, hospitalizations, deaths and the level of community transmission.

Keith Armitage, an infectious-disease expert and medical director of the University Hospitals Roe Green Center for Travel Medicine and Global Health, said in an email that the variant’s risk is low for a fully vaccinated traveler without underlying health conditions. Travelers should therefore consider any conditions they might have that raises their own risk, the risk of transmitting the virus to someone else and the level of community transmission wherever they’re going.

“Masking reduces the risk,” Armitage said. “The risk is greatest indoors, and to try to damp down transmission while there is an effort to get more folks vaccinated masking indoors can reduce, but not eliminate transmission. So travelers may need to adjust their activities to avoid being unmasked indoors.”

At the moment, according to the CDC, these 14 U.S. destinations require extra caution.

Las Vegas

The CDC recommends masks indoors in Sin City, where transmission is high — and the state requires them. Nevada Gov. Steve Sisolak (D) said last month that people in counties with substantial or high transmission, including Clark County, where Las Vegas is located, would be required to wear face coverings in public indoor areas.

Orlando

Orlando and the surrounding region are home to some of the world’s largest theme parks, including Walt Disney World and Universal Studios. The area is also the site of high levels of community transmission, according to the CDC, which means masks are recommended indoors.

Disney has already changed its rules to require all guests to cover their faces indoors. And while there are no statewide mask mandates, the mayor of Orange County — which includes Orlando — renewed a local state of emergency this week requiring masks in county buildings and urged residents and tourists to mask up indoors.

But it’s not just Orlando: The CDC recommends indoor masking at every tourism destination in Florida, including Miami, Fort Lauderdale and Tampa, which are also among Hopper’s most-booked summer getaways.

Los Angeles

Los Angeles County was ahead of the CDC’s updated guidance when officials reintroduced an indoor mask mandate, even for vaccinated people, earlier in July. The CDC says transmission is high in the county, which is home to around 10 million residents.

Atlanta

Mayor Keisha Lance Bottoms issued an executive order that requires everyone, vaccinated or not, to wear a face covering in public places indoors. According to the CDC, coronavirus transmission is high in Fulton County, which includes Atlanta.

Denver

With high levels of covid spread, Denver also falls under the CDC’s updated mask recommendations. The Denver Post reported however, that authorities there had no plan at the time to mandate mask-wearing.

Chicago

Cook County, which includes Chicago, now has substantial levels of coronavirus spread, according to the CDC. Chicago’s public health department is now recommending that everyone older than 2 wear masks in public indoor settings, with optional use outdoors.

Seattle

Because of a high level of community transmission, people in King County — where Seattle is located — should wear a mask in public indoor spaces, the CDC says. The city and county recommend that even vaccinated people wear masks indoors in places like grocery stores and theaters.

Phoenix

Transmission is high in Maricopa County, according to CDC data, meaning anyone in Phoenix, Mesa and their surrounding areas should mask up indoors. Phoenix said rules around face coverings and physical distancing would be reinstated for city buildings. Private businesses can set their own rules around masks, the city said.

Dallas

Transmission is high in Dallas County, according to the CDC, as well as in the counties that include Houston and Austin, which are also top summer destinations. While the federal agency recommends masks in those areas, Texas Gov. Greg Abbott (R) issued an executive order after the updated guidance to keep local governments from putting mask mandates into place.

New Orleans

Transmission in Orleans Parish — along with every other part of Louisiana — is high, the CDC says. New Orleans now has an indoor mask mandate in place regardless of vaccination status, meaning everyone is required to wear a mask indoors when they are with people not in their immediate household.

New York City

With high levels of transmission, New York City falls into the “everyone should mask” category under the CDC’s new guidelines. Mayor Bill de Blasio has “strongly” urged everyone, including vaccinated people, to cover their faces indoors, the Associated Press reported. The mayor also announced that the city will soon mandate proof of vaccination for activities like eating indoors or working out at gyms.

Nashville

The CDC reports high transmission in Davidson County, which includes Music City. But Nashville Mayor John Cooper (D) said , after the new guidelines were announced, that he had no plans to put a countywide mask mandate back into place, the Tennessean reported. Cooper said face masks would be required in city buildings, the paper said.

Honolulu

Honolulu is seeing high community transmission, the CDC says — but mask rules won’t need to change as a result. That’s because Hawaii is still requiring everyone, regardless of vaccination status, to wear a face covering indoors.

In an online Q&A , Gov. David Ige (D) wrote that mask-wearing is an effective public health measure that officials can take in circumstances where they don’t know who is or is not vaccinated. “Some states that rushed to drop their mask mandates have had to reinstate them because of the spike in cases,” Ige said. “I don’t want to have to step back once we move forward to loosen restrictions.”

San Francisco

Levels of community transmission are high in San Francisco, the CDC says. Earlier this week, San Francisco and six other counties in the Bay Area brought back indoor mask mandates for everyone, according to the Associated Press.