As the delta variant 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 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 and lower your risk for a breakthrough infection.

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?

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— 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 K. Jha, dean of the Brown University School of Public Health. “I don’t even think about it. I’m going to the office with a bunch of people, and they’re all vaccinated. I’m not worried about it.”

But once you start to venture into enclosed public spaces where the chances of your encountering unvaccinated people are 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 generated by your vaccine.

For that reason, the case rate and vaccination rate of your community are among 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. In some counties, overall vaccination rates are far lower.

“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.

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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 in his area of the country, infection rates are low and vaccination rates are high, 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.

Why is the delta variant prompting experts to rethink mask guidance?

When the U.S. Centers for Disease Control and Prevention announced that vaccinated people could stop wearing masks, case counts were dropping, vaccinations were on the rise and the highly-infectious delta variant had not yet taken hold. Since then, delta has spread rapidly and now accounts for more than 83% of cases in the United States.

People infected with the delta variant are known to shed much higher levels of virus for longer periods of time compared with earlier lineages of the coronavirus. One preliminary study estimated the viral load is 1,000 times greater in people with the delta variant. These high viral loads give the virus more opportunities to challenge your antibodies and break through your vaccine’s protection.

“This is twice as transmissible as the original lineage of COVID,” Hotez said. “The reproductive number of the virus is around 6,” he said, referring to the number of people a virus carrier is likely to infect. “That means 85% of the population needs to be vaccinated. Only a few areas of the country are reaching that.”

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Is it safe for vaccinated people to go to restaurants, museums, the movies, a wedding or other large gatherings?

The answer depends on your personal risk tolerance and the level of vaccinations and COVID-19 cases in your community. The more time you spend with unvaccinated people in enclosed spaces for long periods of time, the higher your risk of crossing paths with the delta variant, or any other variants that may crop up.

Large gatherings, by definition, offer more opportunities to get infected with coronavirus, even if you’re vaccinated. Scientists have documented breakthrough infections at a recent wedding in Oklahoma and July 4 celebrations in Provincetown, Massachusetts.

But even with the delta variant, full vaccination appears to be about 90% effective at preventing serious illness and hospitalization from COVID-19. If you are at very high risk for complications from COVID-19, however, you should consider avoiding risky situations and wearing a mask when the vaccination status of those around you is unknown.

Healthy vaccinated people who are at low risk of complications have to decide what level of personal risk they are willing to tolerate. Wearing a mask at larger indoor gatherings will lower their risk for infection. If you’re healthy and vaccinated but caring for an aging parent or spending time with others at high risk, you should consider their risk, too, when deciding whether to attend an event or wear a mask.

“If I go into a public area, I’ll generally wear a mask,” Hotez said. “Up until recently I took my son and his girlfriend out for dinner in a restaurant, and I wouldn’t wear a mask because transmission was way down. Now I’m not so sure. I may readjust my thinking about restaurants while delta is accelerating.”

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If breakthrough infections are rare, why do I keep hearing about them?

Breakthrough infections get a lot of attention because vaccinated people talk about them on social media. When clusters of breakthrough infections happen, they also are reported in science journals or the media.

But it’s important to remember that while breakthrough cases are relatively rare, they can still occur no matter what vaccine you get.

“No vaccines are 100% effective at preventing illness in vaccinated people,” the CDC states on its website. “There will be a small percentage of fully vaccinated people who still get sick, are hospitalized or die from COVID-19.”

A breakthrough case doesn’t mean your vaccine isn’t working. In fact, most cases of breakthrough infections result in no symptoms or only mild illness, which shows the vaccines are working well to prevent serious illness from COVID-19.

As of July 12, more than 159 million people in the United States had been fully vaccinated against COVID-19. Of those, just 5,492 had breakthrough cases that resulted in serious illness, including 1,063 who died. That’s less than 0.0007% of the vaccinated population. Meanwhile, 99% of deaths from COVID-19 are among the unvaccinated.

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Many infectious disease experts are frustrated that the CDC is only documenting cases in which a vaccinated person with COVID-19 is hospitalized or dies. But many breakthrough infections still are being detected in asymptomatic people who are being tested frequently, like baseball players and Olympic athletes. Many of those people are traveling or spending extended periods of time in close quarters with others.

“Sports figures are different,” Jha said. “Part of the problem is they are also encountering a lot of unvaccinated people, including in their own little circle.”

I’m vaccinated. How often should I be tested for COVID-19?

If you’re fully vaccinated and you know you’ve been exposed to someone with COVID-19, it’s a good idea to be tested, even if you don’t have symptoms.

And if you have cold symptoms or any other signs of infection, experts agree you should be tested. Many vaccinated people who aren’t wearing masks have picked up summer colds that cause runny noses, fever and coughing. But it’s impossible to tell the difference between a summer cold and COVID-19. Anyone with cough or cold symptoms should wear a mask to protect those around them and get tested to rule out COVID-19. It’s a good idea to keep a few home COVID tests on hand as well.

“If I woke up one morning and had cold symptoms, I would put on a mask at home, and I would get myself tested,” Jha said. “I don’t want to cause breakthrough infections for other members of my family, and I don’t want to give it to my 9-year-old kid.”

This article originally appeared in The New York Times.