As the delta variant of the coronavirus spreads among the unvaccinated, many fully vaccinated people are also beginning to worry. Is it time to mask up again?

While there's no one-size-fits-all answer to the question, most experts agree that masks remain a wise precaution in certain settings for both the vaccinated and unvaccinated. How often you use a mask will depend on your personal health tolerance and risk, the infection and vaccination rates in your community, and who you're spending time with.

The bottom line is this: While being fully vaccinated protects against serious illness and hospitalization from COVID-19, no vaccine offers 100% protection. As long as large numbers of people remain unvaccinated and continue to spread the coronavirus, vaccinated people will be exposed to the delta variant, and a small percentage of them will develop so-called breakthrough infections. Here are answers to common questions about how you can protect yourself.

When should a vaccinated person wear a mask?

To decide whether a mask is needed, first ask yourself these questions:

• Are the people I'm with also vaccinated?

• What's the case rate and vaccination rate in my community?

• Will I be in a poorly ventilated indoor space, or outside? Will the increased risk of exposure last for a few minutes or for hours?

• What's my personal risk (or the risk for those around me) for complications from COVID-19?

Experts agree that if everyone you're with is vaccinated and symptom-free, you don't need to wear a mask.

"I don't wear a mask hanging out with other vaccinated people," said Dr. Ashish Jha, dean of the Brown University School of Public Health. "I don't even think about it."

But once you start to venture into enclosed public spaces where the chances of your encountering unvaccinated people is greater, a mask is probably a good idea. Being fully vaccinated remains the strongest protection against COVID-19, but risk is cumulative. The more opportunities you give the virus to challenge the antibodies you've built up from your vaccine, the higher your risk of coming into contact with a large enough exposure that the virus will break through the protective barrier provided by your immune system.

For that reason, the case rate and vaccination rate of your community is one of the most important factors influencing the need for masks. In Vermont, Massachusetts, Connecticut and Rhode Island, for instance, more than 70% of adults are fully vaccinated. In Alabama, Mississippi and Arkansas, fewer than 45% of adults are vaccinated.

"We're two COVID nations right now," said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and co-director of the Center for Vaccine Development at Texas Children's Hospital. In Harris County, Texas, where Hotez lives, case counts are rising, up by 114% in the past two weeks, and only 44% of the community is fully vaccinated. "I'm wearing a mask indoors most of the time," Hotez said.

Finally, masking is more important in poorly ventilated indoor spaces than outdoors, where risk of infection is extremely low. Jha notes that he recently dashed into a coffee shop, unmasked, because vaccination rates are high in his area, and he was only there for a few minutes.

Your personal risk matters, too. If you are older or immune compromised, your antibody response to the vaccine may not be as strong as the response in a young person. Avoiding crowded spaces and wearing a mask when you're indoors and don't know the vaccination status of those around you is a good idea.