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Monday, February 19, 2024

I’m Vaccinated. When Do I Need to Wear a Mask?

When the CDC announced in mid-May that fully vaccinated people no longer needed to wear masks in most indoor settings, it felt like a milestone to many Americans, indicating that we might be one step closer to the end of the pandemic. It’s understandable, then, that the agency’s decision several weeks ago to bring back indoor mask recommendations even for vaccinated people led to anger, frustration, or confusion for many.

“It is a little bit confusing that the public is told at one point that the vaccine is effective enough that they can pretty much ditch their mask anywhere, and now, this is clearly a different recommendation because of the very high transmissibility of the Delta variant,” Dan Diekema, an infectious disease epidemiologist at the University of Iowa College of Medicine, says. “I think the CDC did the right thing, but it definitely puts members of the public in kind of a difficult situation in terms of making day-to-day decisions.”

But now that the circumstances have shifted, so should our behavior to deal with them.

“The vaccines are highly effective, but we know they’re not perfect, so it’s important to use complementary measures like masking for those areas with higher levels of Covid in the community,” Saskia Popescu, an assistant professor and epidemiologist at George Mason Universit, says.

But that doesn’t mean everyone has to mask up everywhere outside their house, vaccinated or not. Let’s break down when to wear a mask and why, and when you can set it aside.

Why Vaccinated People Still Need to Mask

First, let’s briefly review why the CDC changed their guidance in May, and then again more recently. The clinical trials for the Covid-19 vaccines tested how well the vaccines prevented disease, and they did that remarkably well. Then data in spring revealed the vaccines also appeared to prevent most infections, as in the ability of the virus to begin replicating after getting into the body. That’s unusual—most vaccines don’t prime the body so well that it can dispatch a pathogen before it even begins hijacking cells—but it appeared to be true for the mRNA vaccines. Blocking infection also blocks transmission: If the virus never starts replicating, you never shed it, so you’re not infectious to anyone. Hence the CDC’s advice in May that vaccinated folks can ditch their masks.

Along comes Delta. Though not actually “as contagious as chickenpox,” despite the CDC’s leaked internal document, Delta is about twice as contagious as the original coronavirus strain that swept the nation in 2020. According to a recent study, it has a basic reproduction number (R0) of 5-9, which means a person infected with the Delta variant of SARS-CoV-2 will infect about 5 to 9 other people on average, instead of the 2 to 3 of the previous strains. (Since R0 is an average, some people will infect no one, and others may infect twice as many people.)

Based on the most recent data, it seems Delta is so contagious because it replicates very quickly, producing about 1,000 times more virus particles than the original strain did. That’s partly why people infected with Delta become contagious sooner than with past variants, and it’s possibly why Delta seems to cause more breakthrough infections—but a mask reduces that risk further.

“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.”

Outdoor concerts, for example, often have people tightly packed together for several hours and carry more risk, Popescu says. “Ventilation is important and there's great ventilation outside, but that can only do so much when you’re shoulder-to-shoulder with hundreds of people for hours, yelling and shouting,” she says.

But I’m Healthy and a Risk-Taker

Aside from the guidelines above, there are two basic factors to think about in deciding whether to wear a mask: protecting yourself and protecting others. Unless you’ve been completely away from all people and had a negative Covid test that morning, you have to assume there’s always a chance you’re infected, and therefore that you could infect others.

If you’ll be around people at high risk for infection or severe disease—such as immune-compromised people, unvaccinated children, older adults, or someone with a high-risk condition—it’s safest to wear a mask to protect them from you. In places like grocery stores and pharmacies, where nearly everyone needs to go even if they’re in a high-risk group, the considerate thing to do is always wear a mask, regardless of local transmission rates, because you don’t know if you’ll be around vulnerable folks.

In places like restaurants, bars, and home get-togethers where you’re not required to wear a mask, your decision depends more on your own level of risk tolerance. How much risk of exposure are you comfortable with in exchange for not wearing a mask?

“There’s a framework of interventions that are important to prevent you from getting Covid, and then there’s people's risk factors for a bad outcome,” says Anthony Harris, an epidemiologist at the University of Maryland School of Medicine. “If you combine those two hierarchies with someone's own subjective assessment of risk, that's how they should make decisions. There’s no black or white.”

Aside from local infection rates, you have several factors to consider:

  • How likely are you to develop severe disease if you have a breakthrough infection? If you’re in your 70s or immune-compromised, you may fare worse with a breakthrough infection even if it doesn’t kill you.
  • Are others around you vaccinated or unvaccinated? (If you don’t know, assume some are unvaccinated.)
  • How close are you to others?
  • How long are you there or how long are you close to others?
  • How big is the area?
  • How well ventilated is the area?

Mask Quality Matters Too

Also keep in mind that not all masks are created equally.

“I encourage people to focus on utilizing a quality mask, which is about fit and filtration,” Popscu says. “Ensuring the quality of masks becomes that much more important indoors.”

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, has advocated for months that Americans wear better protection than cloth masks, whose quality and effectiveness vary greatly. In the first half of 2020, public health experts discouraged the general public from wearing N95, KN95, and similar respirator masks so that healthcare workers had enough. But now, Osterholm says, healthcare workers and other professions who need respirators have plenty. There’s no reason the general public shouldn’t regularly wear these much more effective masks too. If nothing else, pair a cloth mask with a medical mask.

Ventilation, Ventilation, Ventilation

Of the factors above, that last one is among the most important, says Alex Huffman, an atmospheric chemist who studies bioaerosols at the University of Denver. The reason “outdoors is dramatically safer than indoors” is that virus particles exhaled by others don’t have a chance to build up. They just float up and away. Indoors is a different story.

The virus-containing aerosols that an infected person exhales “get mixed into the room and can build up in concentration if the ventilation is insufficient,” Huffman says. In a stagnant room where the aerosols build up, it takes fewer breaths to inhale enough virus to become infected. Room volume matters too, he says.

“The bigger, the better,” Huffman says. “If you breathe out aerosols into a bigger volume, it dilutes more quickly.” His research has found that a room’s height is one of the most influential factors in how much aerosols can build up.

Will Masking Ever End?

It’s easy to feel discouraged, maybe even a bit hopeless, with Delta surging and masks recommended for everyone again. But things will get better again, Harris says.

“It's been a very long haul for everyone,” he says, but he points out how much better protected vaccinated people are now than a year ago. That’s not nothing. “I view this back-to-masks as a temporary setback, and it's not going to be the new norm.”

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