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The rise of the Delta variant of COVID-19 is causing concern and confusion about how best to avoid getting sick and spreading the disease. The confusion worsened with seemingly contradictory guidance on wearing masks indoors if you are fully vaccinated. The World Health Organization said vaccinated people should continue wearing masks indoors, as did the Los Angeles Health Department. The Centers for Disease Control and Prevention maintains that fully vaccinated people need not wear masks indoors.

Confusing. So who’s right and how are Americans to navigate this?

First, let’s get the science clear. If you are fully vaccinated and encounter the Delta variant, you are about 90 percent less likely to be infected than if you had not been vaccinated. And if you do get infected, you are extremely unlikely to get very sick. These facts underlie the CDC’s guidance that vaccinated people don’t need to wear masks indoors.

But here’s another way to see it: 90 percent is not 100 percent. And if a vaccinated person encounters the virus repeatedly or in high enough concentrations, the chances of a breakthrough infection gets more substantial.

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That’s the context for recommendations from WHO, whose remit is the entire planet, much of which remains unvaccinated. With the Delta variant spreading quickly and large outbreaks happening in many places, WHO recommends vaccinated people stay masked because breakthrough infections happen. Further, when indoor masking is lifted for the vaccinated, most places struggle to keep it for the unvaccinated, driving infections up.

Los Angeles, meanwhile, has vaccinated half its residents — a great start, but not enough to prevent the spread of the highly infectious Delta variant. With cases climbing, the LA Health Department decided to ask all residents, vaccinated or not, to wear masks in indoor public spaces — stores, movie theaters, public transit. Reasonable people can disagree whether it’s necessary, but ultimately it’s a local decision.

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And this explains the CDC’s reluctance to change national guidance. Because no single policy or recommendation could possibly make sense for every region and every state. Massachusetts has fully vaccinated more than twice as much of its eligible population as Mississippi. Infection rates in Mississippi are about six times higher than they are in Massachusetts. The coronavirus pandemic is playing out differently state by state, community by community. Guidance needs to acknowledge that reality.

Here’s another way to think about it. Being fully vaccinated is like getting a great hockey goalie (think of Boston Bruin legend Gerry Cheevers) who blocks around 90 percent of attempts. In Massachusetts, there isn’t much virus around to even put a shot on target, and therefore little reason to wear a mask indoors. In Mississippi, there are simply many more shots on goal, and even a terrific goalie will occasionally let a shot in.

But how much should we care about breakthrough infections — infections that occur among vaccinated people? Here there are some things we can say with certainty, and others much less so.

Breakthrough infections are usually mild but occasionally can be miserable, with high fevers, headaches, and even loss of smell and taste. They rarely lead to the kind of COVID pneumonia that lands people in hospitals or kills them. That’s good. We don’t know if breakthrough infections are associated with long COVID — the dreaded long-term side effects that many fear. Most experts believe that the risk of long COVID in fully vaccinated people who get infected is much less likely. But we don’t know.

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Finally, there is the issue of practicality. If a community is experiencing rising infection rates, it should ask unvaccinated people to mask up but could leave fully vaccinated people alone. The reality is that, without vaccine verification systems, that’s just not possible to enforce. So local public health officials have a simple choice: Ask everyone to mask up or acknowledge that few will. Rely on an honor system for masking, or ask everyone to mask.

As we wind our way through this pandemic, we are now entering a phase where guidance will — and should — be local and will change as the realities on the ground change. The Delta variant represents one such changing reality. And the guidance from public health agencies will need to become more nuanced.

The evidence right now says if you live in a highly vaccinated community with low infection rates, fully vaccinated people can skip the mask if they are willing to tolerate a minuscule risk of a breakthrough infection. For people who live in low vaccination, high infection communities, the choice is different: Mask up indoors or be willing to accept a far more meaningful risk of getting the virus.

Dr. Ashish K. Jha is dean of Brown University School of Public Health and professor of Health Services, Policy, and Practice.

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