Confused? You’re not alone. The guidelines got Linsey Marr, a professor at Virginia Tech and a leading expert on viral transmission, to remark that even she can’t remember all of this. “I would have to carry around a sheet of paper—a cheat sheet with all these different stipulations,” she said in an interview after the announcement.
And despite all the detail, social media was flooded with questions from people who couldn’t figure out what they should do in different settings. What happens if they live with someone who is not vaccinated or has medical issues? What counts as a crowd? How small is a “small, outdoor gathering”? Why are unvaccinated people “safest” at a small outdoor gathering but not at an outdoor restaurant? And why is a crowd a threat to the vaccinated? What does the color coding for unvaccinated people indoors mean exactly, since they are advised to wear masks at all times? The CDC should, at the very least, explain the scientific reasoning behind these rules. Not only would this empower people; it would inform the inevitable debate about the guidelines.
We wear masks for three reasons: to protect ourselves from people who might be infected, to protect others from our infections, and to set social standards and norms appropriate for a pandemic. The last one is the most important: A pandemic requires a collective response. As we learn more, we move from broader precautions to targeted mitigations. Early in the pandemic, the existing guidelines that suggested only the sick should wear masks and the objection that we didn’t know all we needed about the effectiveness of masks violated both the need for social norms, by stigmatizing the sick, and the precautionary principle, by letting remaining uncertainty stop us from protecting ourselves as best we could even with imperfect knowledge. So we changed the rules.
Now, a year later, both the sociology of outdoor masks and the precautionary principle operate in the opposite direction, because the science is in. We need to change the rules again, but also explain why.
Let’s start with the outdoors. Study after study finds extremely low rates of outdoor transmission. So far, I’m unaware of a single confirmed outdoor-only super-spreading event, even though at least thousands of confirmed super-spreading events took place indoors. (The Rose Garden party to celebrate Donald Trump’s nomination of Amy Coney Barrett to the Supreme Court and the multiday Sturgis motorcycle rally in South Dakota both had extensive indoor components.) When outdoor transmission does occur in small numbers, it’s not from fleeting encounters, but from prolonged contact at close distance, especially if it involves talking, yelling, or singing.
An increasing number of scientists believe that outdoor and indoor transmission differs so starkly because the coronavirus transmits through aerosols—essentially little floating particles that we emit, even if we are just breathing, but even more if we are talking, yelling, or singing. Unlike droplets, these aerosol particles do not immediately fall to the ground with the force of gravity within three to six feet, and they concentrate most around the person emitting them, so close contact remains risky. Crucially, they can disperse quickly if they are released in the great outdoors, or, conversely, they can keep accumulating in a poorly ventilated, enclosed environment and travel beyond the short distance in which droplets would fall.