A subset of vaccinated people will still push pathogenic particles out into the world; the chances of spread are lower, not zero. But “pound for pound, you expect some of that virus to be less potent,” Alex Greninger, a virologist at the University of Washington, told me. Viruses that have been stewing in immunized airways can end up swathed in antibodies that render them less dangerous to others.

A lot of this is common knowledge by now among the experts who design our policies. And yet, American guidelines have not substantially shifted since two summers ago—when, in July of 2020, the CDC said most people should no longer be using tests to determine when to leave isolation. Instead, they could exit 10 days after the start of symptoms or the first positive test result, provided that they no longer felt sick. (People who are seriously ill or immunocompromised might shed the virus for much longer, though, and could require isolation of 20 days or more.) That change happened 17 months ago, at a time when vaccines weren’t authorized, tests were absurdly scarce or slow, and the best option was to estimate how long folks might shed, and tell them to hide away for about that length of time.

Things are very different now. Three super-effective vaccines have been greenlit for use in the United States, and hundreds of millions of Americans are doubly or triply dosed. Tests are faster and more available. And we know a lot more about how and when the virus spreads. The CDC has even been emboldened to modify its rules on quarantining after an exposure to a COVID case. The policy used to be 14 days of solitude for everyone. Now vaccinated people don’t have to quarantine at all. Even the unvaccinated can peace out at 10 days, or even seven, if they produce a negative test—a change that went into effect in December 2020.