I can’t read her mind, but if I were Walensky, I’d be scared because those who are not protected through vaccination or past infection are still at grave risk, a fact that may be overshadowed by all the good news. Even as our vaccines continue to work very well against it, the particular variant we’re facing in this surge is both more transmissible and more deadly for the unvaccinated.
Throughout this pandemic, Americans have become used to asking one another to pull together and enact mitigations for everyone’s benefit. One of the slogans for mask wearing was “My mask protects you, and your mask protects me.” Although we were always polarized, and the effects were always unequal—our mitigations helped those who could work from home more than the essential workers who made that possible—at least theoretically, we were all in it together, even if some of us did not act like it.
You see this appeal to the collective good in the many discussions around achieving herd immunity, too: a goal that will protect us all. That’s still true to some degree, for the future, but it was always an oversimplification. Now, with uneven but increasing rates of vaccination, understanding how those divisions work is even more important, starting with herd immunity.
Herd immunity is sometimes treated as a binary threshold: We’re all safe once we cross it, and all unsafe before that. In reality, herd immunity isn’t a switch that provides individual protection, just a dynamic that makes it hard for epidemics to sustain themselves in a population over the long term. Even if 75 percent of the country has some level of immunity because of vaccination or past infection, the remaining 25 percent remains just as susceptible, individually, to getting infected. And while herd levels of immunity will eventually significantly drive down the number of infections, this may not happen without the epidemic greatly “overshooting”—infecting people beyond the levels required for achieving herd immunity, somewhat like a fire burning at full force even though it is just about to run out of fuel.
Worse, people’s infection risks are not distributed evenly: Some people have lots of contacts, while others have a few. People are also embedded in different social networks: Some may have a lot of friends and family members who are immune, others not so much. Some work in jobs that increase their risk, others not so much. So it’s perfectly possible for a country as a whole to have herd immunity against a pathogen, but for outbreaks to happen among communities that have a lot of unvaccinated people among them. That’s happened in California, Michigan, and New York for measles among vaccine-resistant communities. In addition, this coronavirus is highly overdispersed. Infections occur in clumps. A single event can result in dozens or even hundreds of people being infected all at once in a super-spreader event.