“People with Delta are shedding more virus, and if you’re around them for a long period of time, you’re going to be exposed to the minimum infectious dose in a shorter period of time than you would have otherwise,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization-International Vaccine Centre in Saskatchewan, Canada, says. “Wear a mask, reduce the numbers. It’s that simple.”

Since it replicates so quickly—often before vaccinated people’s immune systems can obliterate it—there’s a higher chance that a vaccinated person exposed to Delta might shed some virus during the few days it takes their body to clear the virus. Their body might successfully destroy the virus before the person ever knows they were infected and without causing symptoms, but if their viral load—the total virus particles in their body—was nearly as high as what’s seen in unvaccinated people in the first few days after exposure (as research suggests), they still could have transmitted the virus to others.

The good news is that vaccinated people aren’t as contagious as unvaccinated people, and they’re infectious for a shorter period of time, maybe a few days if they never have symptoms. But that might be all it takes to pass along the virus to someone more medically fragile than they are. Even if you’re healthy and vaccinated and can handle a couple days of feeling crappy while your body beats back a breakthrough infection, those you potentially infect may not be as lucky, especially if they’re immune-compromised or too young to be vaccinated.

Who Should Always Mask

Instead of a blanket recommendation that all vaccinated people should mask everywhere, the CDC identifies a few populations who should always mask up indoors, vaccinated or not, and then what other vaccinated folks should do. If you’re immune-compromised, follow the advice that Dorry Segev, a Johns Hopkins researcher and transplant surgeon, gives his patients: “Get vaccinated, act unvaccinated.”

That means wearing a mask in any indoor setting where you’re around people from outside your household and in any outdoor situations where it’s especially crowded or you’re in very close contact with people for an extended period (15 minutes or more.)

The CDC also advises all students, faculty, staff, and visitors in all schools to wear masks during the school day, a recommendation echoed by the American Academy of Pediatrics. Many students are too young to be vaccinated or otherwise unvaccinated (less than half of eligible teens have gotten the vaccine,) and others may have conditions making them high risk for infection and severe disease. Spending many hours of day together in the same room means it’s safest if everyone wears a mask to lower their likelihood of spewing virus around the classroom.

For Everyone Else

For people with properly functioning immune systems who aren’t in a school, the CDC advises fully vaccinated people to wear masks indoors if they’re around people from outside their household and in an area deemed by the CDC to havewith “substantial” or “high” transmission. Right now, though, that’s nearly the entire U.S. If you’re someone who obsessively checks local transmission levels each day and it drops below “substantial” transmission, there’s probably little enough virus circulating in your area that you’re not risking much by leaving your mask home. You can check your county’s transmission rates here. But with a virus as contagious as Delta, an area can flip from “low” transmission to “substantial” overnight. If you’re not checking local rates daily, it’s wise to assume everywhere in the US has high transmission until the Delta surge passes.

What About Outside?

Fortunately, outdoor transmission appears rare. The CDC doesn’t recommend masking outdoors unless the area has high transmission and it’s particularly crowded or you’re in close contact with others.

“You can think about it like peeing in the pool versus peeing in the ocean,” Rasmussen says. “If you’re peeing in a pool, more people around you are going to feel warmth because it’s a confined space. Obviously, if you pee in the ocean, people probably aren’t going to know. But if you’re in the middle of a huge group of people, like on a raft or something, and you piss, yeah, the people around you will probably be able to detect that.”