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The CDC’s mask guidelines are scientifically sound — if you actually read them

Why the CDC is so confident that (most) fully vaccinated people can go maskless.

A person with a mask in their hand.
The science is sound. The science is confusing.
Jay L. Clendenin/Los Angeles Times via Getty Images
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

After a year of public health guidance to wear masks whenever we are out in public, taking them off after being vaccinated may feel ... weird. The trauma of the pandemic has etched a new set of social norms deep within many minds. And central to those norms is mask-wearing.

So it’s understandable that the Centers for Disease Control and Prevention’s surprise announcement last week that it had changed its masking guidelines for fully vaccinated people feels like whiplash. The CDC now says those who are fully vaccinated (or two weeks after their final dose of the vaccine) can “resume activities without wearing masks or physically distancing” in places that allow it. (The CDC also stresses extra caution, and a consultation with a physician, for those vaccinated who have weakened immune systems or are on immune-system-suppressing drugs.)

There are a lot of fair arguments circulating that these guidelines were issued prematurely and lacking some nuance. And it’s hard to know how unvaccinated people, and those who never intend to be vaccinated, will respond, particularly as local communities and businesses grow laxer when it comes to enforcing masking. The guidelines also neglect the reality that the absolute risk for Covid-19 infections among the vaccinated is dependent on the level of Covid-19 spread in a community. Plus, there’s still uncertainty about what to do about children under the age of 12, who currently cannot be vaccinated.

These arguments are worthy of discussion. But they also can distract us from something important at the core of the CDC’s guidelines: There’s good science to indicate that, in an ideal world where everyone does follow these guidelines, fully vaccinated people really can feel comfortable taking their masks off. These vaccines are really good. They prevent disease, infection, and transmission the majority of the time. And the best thing an individual can do to help end the pandemic is to get vaccinated.

“The return to normal activities is safe for vaccinated people, there is no debate about this fact,” Jeffrey Duchin, a physician with the Infectious Diseases Society of America, told reporters in a news briefing on Thursday. “However, the announcement led to widespread confusion and frustration because it was unexpected and needed context.”

Before we get to the confusion, let’s talk about why scientists and the CDC are so confident that most fully vaccinated people can go maskless.

The vaccines are really freaking good

There’s been a lot of news coverage on the very rare instances where vaccines fail, leading to sickness, hospitalization, or death. But these events are very rare. Even the news that eight players on the Yankees tested positive for Covid-19 after being vaccinated isn’t cause for alarm on closer inspection.

As of April 26, the CDC reported that of the around 95 million Americans who had been fully vaccinated, there were 9,245 reported cases of breakthrough infections — when people test positive after being fully vaccinated. That’s .01 percent of vaccinated people — or a rate of one in 10,000.

Of those, 835 led to hospitalizations and 132 led to deaths. Since the start of May, the CDC is only tracking breakthrough infections that lead to hospitalization and death, and they find that overall, just .001 percent of those vaccinated were hospitalized for Covid-19. That’s one hospitalization for every hundred thousand vaccinations. “When you start to talk about vaccinating millions of people, even things that occur relatively infrequently will start to pop up,” Natalie Dean, a biostatistician at the University of Florida, told Vox in an April interview.

The infrequency of these events is very encouraging, as is the fact that the vaccines approved for use in the US all appear to be effective against the most concerning Covid-19 variants. And more and more research is showing that vaccination doesn’t just mean you’re less likely to get sick, but you’re also less likely to be infected in the first place. Even if you are infected, you’re less likely to pass it on to others.

Here are some examples from studies:

  • A study from the CDC of 3,950 health care workers found that the mRNA vaccines (Moderna and Pfizer/BioNTech) were 90 percent effective at preventing any infection.
  • In Israel, a real-world study of the Pfizer/BioNTech vaccine found it was 94 percent effective at preventing any infection.
  • A UK study tracked 23,000 health care workers who underwent testing for SARS-CoV-2 every two weeks. Those who got the Pfizer/BioNTech vaccine were 85 percent less likely to be infected with the virus seven days after their second dose.

“Now there is pretty solid data that mRNA vaccines reduce the chance of any infection by 80, 90 percent,” A. Marm Kilpatrick, an infectious disease researcher at the University of California Santa Cruz, says.

The pandemic grew so painfully out of control in part because people could pass on the virus before they realized they were sick. Being fully vaccinated means your chance of passing on the virus is greatly reduced.

“If you go into a place where there’s unvaccinated people, you’re protecting them because you’re vaccinated,” says Monica Gandhi, a physician and medical professor at the University of California San Francisco.

This protection has been seen in real-world studies, too: A CDC study of nursing homes in Chicago found no transmission among vaccinated residents, despite there being a few breakthrough infections among staff and residents (most of these breakthrough infections were asymptomatic).

One criticism of the CDC’s change in guidelines is that it could have waited longer for more real-world data on the Johnson & Johnson/Janssen vaccine. It’s true that there’s less real-world data on the one-dose Johnson & Johnson vaccine. But the CDC writes that “early evidence suggests that the J&J/Janssen vaccine might provide protection against asymptomatic infection.”

“We don’t know as much about the issue of prevention of transmission in asymptomatic cases in the J&J vaccine,” Duchin says. But “everything we know [indicates that it] does provide a very high degree of protection not only against infection, but against severe infection leading to hospitalization and death. I think we should be reassured that regardless of which vaccine, you’re getting a very high level of protection.”

Even if you do get infected after being vaccinated, the vaccines will likely make you less contagious than you would have been.

Recently, a 5,000-patient study out of Israel compared cases of breakthrough infections (after vaccination) with infections that occurred among the unvaccinated. Simply put: The study found that those who had breakthrough infections also had smaller quantities of virus than people who weren’t vaccinated. This study did not include information on symptom severity. But lower viral loads have been correlated with lower disease severity; they also lower the risk of transmission to other people.

In debates over the CDC’s guidelines, Gandhi says these study results should continue to be highlighted. “It’s pretty amazing,” she says.

The guidelines make sense in an idealized world. Do they make sense in the real world?

The CDC’s new guidelines are based in good science, showing the vaccines work to protect the vaccinated, and also everyone else around them. If everyone were to follow the guidelines to the letter — with unvaccinated people continuing to wear masks and vaccinated people relaxing masking if they choose to — “then the transmission of the virus is going to be just astronomically reduced,” Kilpatrick says, and the US could quickly end its epidemic.

But in the real world, there are unvaccinated people who choose not to wear masks. And because there’s generally no official way to verify a vaccinated person’s status, an unvaccinated person could get away with being unmasked.

It is possible that these unmasked, unvaccinated people could still seed some breakthrough infections among the vaccinated, and among themselves.

“As with any declaration, how people interpret this for their own individual circumstances will be hard to predict and hard to control,” Abraar Karan, a physician at Brigham and Women’s Hospital/Harvard Medical School, writes in an email. “The CDC could have been clearer that while individuals could theoretically stop masking, there are reasons why community masking is still important.”

One reason is that the risk of breakthrough infections depends on how much virus is circulating in a community. “If we’re in India, right now, there’s no way I take off the mask if I were fully vaccinated, because the case rates are so high,” Gandhi says.

The more cases, the more chances there are for a vaccinated person to be exposed and get sick themselves. Gandhi says she wishes the CDC guidelines came with goals for community transmission and vaccination rates. Instead of saying everyone who is vaccinated can go maskless in places that allow it, the agency could have said they can go maskless when their community hits certain case-rate and vaccination-rate milestones.

Gandhi also thinks the CDC should recognize that the Johnson & Johnson vaccine may be most effective after four weeks. So perhaps people who received that vaccine should think of themselves as “fully vaccinated” after four weeks, not two.

Kilpatrick adds that it’s still going to be the case that a small, poorly ventilated room crowded with people singing poses more risk for the maskless than a place with more breathing room. It’s still the case that some people, due to their individual health and demographic characteristics, might want to remain more cautious.

There are a lot of people for whom the new guidelines may not apply — including the immunocompromised — who may, after consultation with their health care providers, decide to keep wearing masks in public. “Millions of people fit this bill,” Jeanne Marrazzo, a physician with the Infectious Diseases Society of America, told reporters Thursday.

The CDC’s new guidelines are highly simplified. But they are also not wrong. And again, they were issued because “these vaccines are astounding,” Gandhi says. They’re the most powerful pathway to return life to normal.

Correction: This post originally misstated the percentages of breakthrough infections and breakthrough hospitalizations.