CDC recommends some vaccinated people wear masks indoors. Here’s what you should know about vaccines and COVID-19 variants.

The U.S. Centers for Disease Control and Prevention revised its masking guidelines Tuesday to recommend that even vaccinated people wear a mask indoors in parts of the country where the virus is surging. That doesn’t now include the Philadelphia region, but the CDC’s second recommendation -- that masks be worn in all schools regardless of vaccine status -- does.

The update comes weeks after the CDC loosened its guidance to say fully vaccinated people no longer needed to wear masks indoors — even in the company of unvaccinated individuals.

But a more contagious strain of the virus, called the delta variant, is raising concern amid an uptick in COVID-19 cases, particularly in pockets of the country with low vaccination rates.

On July 22, Philadelphia issued new guidance “strongly recommending” that everyone wear masks indoors, in part because of the variants. Bucks, Chester, Delaware and Montgomery Counties, as well as New Jersey, have said they do not plan to change their mask guidance.

“We’re both afraid of it and blasé about it,” said Thomas Fekete, an infectious disease physician at Temple University Hospital, of the public’s approach to the pandemic.

» READ MORE: CDC expected to backpedal on some masking guidelines

Making sense of conflicting guidance can be challenging because in many cases there is no right or wrong answer, said Jennifer Khelil, chief medical officer at Virtua Health. Whether you wear a mask, attend indoor events, allow unvaccinated guests to visit unvaccinated children, or take that long-awaited vacation will depend on your personal tolerance for risk, she said.

“Part of the difficulty in answering that question is everyone has a different circumstance,” Khelil said. People who’ve gotten vaccinated have “done the most significant thing you can to protect yourself,” she said, while those with children may want to exercise more caution in the coming weeks.

Khelil, Fekete and some of their colleagues in the Philadelphia area weighed in on questions fully vaccinated people may consider as they decide how to navigate the next phase of the pandemic.

So far, vaccines appear to be effective against the variants, including delta. The CDC says that just 5,914 of the 161 million vaccinated Americans have been hospitalized or have died of COVID-19. In other words, roughly 0.004% of vaccinated people have gotten serious COVID-19. Most infections in fully vaccinated individuals — known as breakthrough cases — are mild.

More research is needed, but a British study found that the two doses of the Pfizer-BioNTech vaccine were 88% effective against symptomatic disease from delta and 96% against hospitalization. Moderna reported that its unpublished study found only a small reduction in laboratory signs of effectiveness against the delta variant.

Evidence so far is mixed on the one-dose Johnson & Johnson vaccine. The company says the vaccine works against delta. A new study from New York University that is not yet peer-reviewed found the vaccine was likely less effective against against symptomatic disease caused by delta. The authors said people who got the vaccine should get boosters.

» READ MORE: Do I need a COVID-19 vaccine booster to protect from delta and other variants?

Studies have shown that vaccinated people who contract COVID-19 have much lower viral loads, which suggests they are less likely to spread the infection to others. But more research is needed to understand the likelihood of transmission of the virus, especially its more contagious variants.

Yes. Knowing you are infected could prevent you from spreading the virus to others and might help you get early access to medicines, like monoclonal antibodies, said John Zurlo, an infectious diseases doctor at Jefferson Health. Stay home if you feel sick. Regardless of whether you have COVID-19 or one of the cold-like viruses that have been circulating as people have ditched their masks, you can avoid infecting others by keeping your distance. If you have to leave home when you aren’t feeling well, wear a mask.

The indoor masking recommendation establishes “a public threshold for risk” that individuals can use as a benchmark when deciding how much risk they can tolerate, said Fekete, of Temple.

“We have means of preventing illness that are not costly, not out of reach for most individuals,” said Khelil, of Virtua. “Why would you not want to implement those measures?”

Masking is a way to protect the most vulnerable with minimal effort, said Zurlo, who is vaccinated and still wears a mask in public, indoor spaces.

The city recommendation, he said, gives the most cautious people “license” to keep wearing masks. Plus, the city is seeing only a “modest” uptick in cases now. This is a good time to stop the trend before it gets worse.

On the other hand, Zurlo said, if there suddenly were no masks available, “I think vaccinated people by and large would do just fine.”

» READ MORE: What you need to know about Philly’s new mask recommendations and rising cases

You may want to wear a mask, depending on whether your destination is experiencing a surge in cases, the local vaccination rate and your personal risk tolerance. The delta variant is driving a larger surge of cases in some parts of the country, especially where vaccination rates are lower.

People who are more concerned about the virus may reconsider summer travel plans, or adjust their itineraries to exclude crowded, indoor activities, where the likelihood of contracting the virus is higher.

“It’s a moving target. You should modify your behavior based on the current spread in your community,” said Darren Mareiniss, an emergency medicine physician at Einstein Medical Center.

Mareiniss isn’t taking his family on a summer vacation and recommends his patients skip unnecessary travel.

It depends. If everyone in your household is young, healthy and vaccinated, hosting unvaccinated friends or family is “probably OK,” Zurlo said, because the rates of vaccinated individuals developing COVID-19 are very low.

People with young children who are not eligible to be vaccinated may want to be more cautious, Fekete said.

Turning away guests because of their vaccination status can be a sticky situation. Fekete recommends establishing your own safety guidelines to apply to any potential gathering.

“If someone says, ‘I don’t want to get vaccinated,’ that’s their choice. You might not judge that person as a human being, but you may judge they’re not fit to come visit,” he said.

Parents should be vigilant when they are in public, especially indoors, and carry a mask in case they need it, said Debra Powell, chief of the division of infectious diseases at Tower Health.

“I think it’s reasonable for people to wear a mask if they’re uncomfortable,” even if it is not mandated, she said.

U.S. Surgeon General Vivek Murthy also encouraged parents to exercise caution. “I actually think that, in a moment like this when we’re seeing COVID spreading so widely because of the delta variant, that it’s prudent to err on the side of caution,” he told NPR on Thursday.

Unvaccinated children over age 2 should continue to wear masks in public.

There isn’t enough research to say whether a mild breakthrough case is equivalent to getting a COVID-19 booster shot, though mild cases of other childhood illnesses have been shown to boost immunity, said Powell, of Tower Health.

Stephen Gluckman, an infectious diseases doctor at Penn Medicine, said exposure to almost all viruses boosts pre-existing immunity, so that would likely be true for the coronavirus as well. However, he said, vaccines are still working quite well. “I wouldn’t deliberately expose myself as a booster shot,” he said. He added that vaccines have been shown to provide better immunity than infection with the virus.

This is unknown, but doctors who treat patients with COVID-19 symptoms that linger for months recently told NBC that they rarely see vaccinated people.

Gluckman said “there is an association between the severity of disease and the likelihood of getting long COVID. … The breakthrough cases tend to be mild.”

Early testing of the vaccines showed they worked well in people of all ages, but older people who are frail and have chronic diseases may not develop as much immunity as younger family members. Anyone who lives in a multi-generational home should be extra careful.

Older adults may be among the first candidates for booster shots. Though not currently recommended in the U.S., boosters are begin given to at risk individuals, including older adults, in Israel.

Our immune systems don’t work as well as we age. People in nursing homes, who are often in poor health, have proven especially vulnerable to viral infections. A University of Pittsburgh study in April found that residents of long-term care facilities developed measurable antibodies after COVID-19 vaccination, but it was not known whether that was enough to prevent infection or how long it would last.

People with certain cancers and those who have had transplants are at risk for poor response to COVID-19 vaccines.

» READ MORE: The COVID-19 vaccine doesn’t fully protect people like me who are immunocompromised. Science is working on it.