At this point, the CDC has recorded that less than a quarter of adults who are fully vaccinated under the existing definition have gotten a third shot. That leaves about 150 million people who are vaccinated but unboosted. Given that the people in this group are less protected against infection, they’re at greater risk of passing on the disease to unvaccinated or partially vaccinated kids, as well as to unvaccinated or immunologically vulnerable adults. They will also pass the coronavirus more readily among themselves. Settings that might have previously seemed safe for vaccinated folks—say, a restaurant or performance venue that strictly checks vaccination status—could become fertile ground for transmission, because the people inside them are more likely to catch and spread the virus. Indeed, anecdotal reports already suggest that large indoor gatherings of fully vaccinated people can become super-spreader events in the age of Omicron.

Population-level immunity could suffer in another way too, Omer said: People who were previously protected because of a prior infection could now be quite vulnerable to getting reinfected and passing on the disease. In fact, it’s possible the only parts of the country where community transmission might be blunted are those that faced devastating early waves of the virus and subsequently had strong vaccination rates—mostly a handful of areas in the Northeast. “It’s really very, very challenging to consider how those differences might play out,” Joshua Schiffer, a disease-modeling expert at the Fred Hutchinson Cancer Research Center, told me.

Here’s the upshot: Each fully vaccinated person might still be at minimal risk of getting seriously ill or dying from COVID this winter, but the vestiges of normalcy around them could start to buckle or even break.