The data behind the CDC’s new mask guidance

This past Thursday, the CDC announced that, if you are fully vaccinated, the pandemic is basically over for you.

The announcement surprised everyone—from my parents to COVID-19 scientists—because it appeared to come out of nowhere. Before this week, the CDC’s most recent guidance revisions included an acknowledgment that surface transmission of the coronavirus is very rare and a recommendation that masks might not be necessary outside; both of these conclusions were already well-known in the scientific community. In fact, STAT published an article last week in which Nicholas Florko suggests that the CDC’s COVID-19 messaging has been “overly cautious” and perhaps even “irrelevant for most Americans.”

So, what led to the announcement on Thursday? The rest of this post will go over the CDC’s evidence for its guidance, taking the epidemiological perspective. Also, as two-thirds of Americans aren’t yet fully vaccinated, I’ll touch on another COVID-19 truism that has garnered some confusion lately: yes, you are significantly safer outside than you are inside.

CDC graphic on COVID-19 safety, updated with the new mask guidance on Thursday.

(I need to acknowledge, though, that a. there are certainly outside political and economic forces at play here, and b. the public health perspective on this guidance is far more complicated. For one perspective on the public health side, I recommend this Twitter thread by virologist Angela Rasmussen.)

Our first category of evidence: the mRNA vaccines work really well. It’s no surprise that the Pfizer and Moderna vaccines are both exceptionally capable of protecting people against the coronavirus, but a couple of recent studies really hammer this home:

  • In this recent study from Israel, the Pfizer vaccine demonstrated 97% effectiveness against symptomatic cases and 86% effectiveness against asymptomatic cases among about 6,700 healthcare workers who were regularly tested for COVID-19.
  • According to this MMWR report from the CDC, the Pfizer and Moderna vaccines were 94% effective in preventing COVID-19 hospitalization for fully vaccinated seniors (over age 65), demonstrating how well the vaccines protect against severe disease. Plus, the vaccines were 64% effective in preventing hospitalization for partially vaccinated seniors.
  • Another MMWR report, released this past Friday, demonstrates that the mRNA vaccines were 94% effective at preventing symptomatic COVID-19 in U.S. healthcare workers. A single shot of Pfizer or Moderna ws 82% effective at preventing symptomatic COVID-19.
  • A study from the Cleveland Clinic, a medical research center based in Cleveland, Ohio, studied COVID-19 cases among its caregivers after vaccines were made available; the Clinic found that a whopping 99.7% of those workers who tested positive for COVID-19 had not been fully vaccinated. Only 0.3% were breakthrough cases. Meanwhile, 99.75% of COVID-19 patients that the Clinic treated during the study’s time frame were not fully vaccinated.
  • According to the CDC’s breakthrough case data, out of about 122 million fully vaccinated Americans, less than 1,400 people have been hospitalized or have died due to COVID-19. That’s 0.001%. (The CDC’s breakthrough reporting focuses on severe outcomes rather than cases, as these cases may be difficult to systematically identify outside of studies.)

Second evidence category: the vaccines work against variants.

  • It’s important to note here that, when I say “vaccines work,” it’s not an all or nothing situation. A vaccine might protect you against severe COVID-19 disease or death (the primary goal), but not against an asymptomatic case that allows you to transmit the virus to someone else.
  • All the COVID-19 vaccines currently on the market protect us against severe COVID-19 disease and death—whether you’re infected with an older version of the coronavirus or with a variant. 
  • For a couple of the variants, that protection might not be quite as secure. Studies on B.1.351 (the variant first identified in South Africa), P.1 (identified in Brazil), and B.1.617 (identified in India) have all demonstrated lower vaccine effectiveness. But again, lower effectiveness here still means protection for the majority of people who get vaccinated.
  • And one big advantage of mRNA vaccine technology is, the Pfizer and Moderna vaccines may easily be adjusted to protect against particularly concerning variants. Moderna recently reported promising data for two booster shots designed to protect specifically against B.1.351 and P.1.
  • If you’d like a more detailed rundown of the vaccine v. variants battle, check out this article by STAT’s Andrew Joseph.

Third evidence category: U.S. cases are way down.

  • As I noted in today’s National Numbers post: cases have dropped by 50% in the last month. And beyond that, cases have dropped from a peak of 250,000 new cases per day in January to under 40,000 new cases per day now.
  • During this time frame, most states did not impose lockdowns or other restrictions on the level of what we saw in spring 2020. So, these drops can primarily be attributed to the vaccines.
  • We still do not have much evidence showing how well the vaccines protect against asymptomatic transmission and infection, but the evidence we have so far is promising, as Apoorva Mandavilli explains in the New York Times. (One major study investigating this question in college students is currently underway.)
  • Still, the massive case drops—occurring even as B.1.1.7 and other more contagious variants spread through the country—indicate that the vaccines must be doing some work to stall coronavirus spread from one person to another. This supports the CDC’s argument that vaccinated Americans can take off their masks in public without worrying about spreading a latent coronavirus to someone else.

Fourth evidence category: outdoor transmission is incredibly low.

  • Earlier this week, the New York Times’ David Leonhardt provided a compelling argument for why, though the CDC said “less than 10%” of COVID-19 transmission occurs outside, the true number is actually much lower. In fact, fewer than 1% (and possibly even fewer than 0.1%) of COVID-19 cases happen due to someone getting infected outside.
  • As I’ve previously reported, there is not a single recorded superspreading event that took place solely outside. This includes the large Black Lives Matter protests last summer. (A few superspreading events have both outdoor and indoor components.)
  • A new study from researchers at Drexel University specifically examined COVID-19 transmission in parks, and found no correlation between the number of people using a park and the number of COVID-19 cases in the surrounding ZIP code. The research suggests that you should feel safe at your local park, even if it seems a bit more crowded and less mask-adherent than it did a few months ago. I spoke to Franco Montalto and Bita Alizadehtazi, two authors on this study, who emphasized that “it’s important to get outside,” take advantage of the green infrastructure in your neighborhood, and feel safe while doing so.

This evidence brings us to what The Atlantic’s Drew Thompson calls the “Two Commandments of COVID-19”:

1. COVID-19 is an indoor aerosol disease.

2. Vaccination protects you; more vaccinations protect everyone.

Speaking just for myself: I am fully vaccinated, but I fully intend to keep wearing a mask in stores, on the subway, and even outside when I’m in a large crowd of people. This is partially because my state still has a mask mandate in place, but also because there are still a lot of people in my community who aren’t yet vaccinated—and I don’t want to pose a risk to them, no matter how small that risk may be. (In Brooklyn, where I live, 41% of the population has had at least one dose and 33% are fully vaccinated, according to city data.)

Suffice it to say, the CDC makes recommendations about COVID-19 safety. It doesn’t issue requirements. I made a personal masking decision for myself, based on the community where I live; I hope this article helped you understand the science behind the guidance change so that you can do the same. And if you have questions—my inbox is always open.

More vaccine coverage

  • U.S. moves to approve booster shots despite minimal evidence
    This week, the federal government announced that the U.S. intends to provide third vaccine doses to all Americans who received the Pfizer or Moderna vaccines. This booster shot distribution will start in September, with adults becoming eligible once they hit eight months after their second shot. Many epidemiologists, vaccine experts, global health experts, and other scientists have criticized the decision.
  • Three more COVID-19 data points, August 15
    A couple of additional items from this week’s COVID-19 headlines: children hospitalized with COVID-19, immunocompromised Americans now eligible for a third dose, and low cases linked to Lollapalooza.
  • The case for a moratorium on booster shots
    This week, the World Health Organization (WHO) called for wealthy nations to stop giving out booster shots in a push towards global vaccine equity. These nations should stall any booster shots until at least September, said WHO Director-General Tedros Adhanom Ghebreyesus at a press conference on Wednesday.
  • A dispatch from Provincetown, Mass.
    This week, I had the opportunity to talk to Mike, a Bear Week attendee from Pittsburgh, who caught COVID-19 in Provincetown. He told me about his experience attending parties, getting sick, and learning about the scale of the outbreak.
  • Vaccine requirements are the next big strategy
    After vaccine incentives largely failed to drive up vaccination numbers, government agencies and corporations alike are now opting for requirements. Hundreds of thousands of Americans learned this week that, in order to keep their jobs, they need to get their shots—or go through a more arduous process like weekly COVID-19 testing.

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