There’s no reason to assume Delta represents any sort of ceiling in infectiousness. “I wouldn’t be incredibly surprised if something else came along that’s even more transmissible,” says Vail. Such a super-spreading virus might burn through the unvaccinated, non-previously infected population so fast that hospitals couldn’t come close to coping.

Making that possibility more likely is the fact that sheer transmissibility, more than any other characteristic a virus might acquire through mutation, confers the greatest advantage on a variant when it comes to outcompeting other versions. “If a mutation comes up anywhere that’s more transmissible, it will be selected out to propagate,” says Green. That means a single ultra-transmissible mutation popping up anywhere in the world in a single infected person could be enough to unleash a fresh round of heightened global misery…

COVID-19 may well continue to evolve into new, widely spreading variants, but there’s reason to think that none of them are likely to routinely blow past the immune defenses conferred by vaccine, and even the lesser natural-immunity defenses. One reason, notes Vail, is that the vast majority of COVID-19 virus in circulation is in unvaccinated people who weren’t previously infected, and mutations that can avoid immunity have no real advantage in that environment. An immune-evading variant would be more likely to thrive in a population of vaccinated or recovered people, where such a mutation would allow it to outcompete non-mutated viruses—but there just isn’t enough virus circulating in that population to allow for rapid mutation.

Green points out a second reason being immune-evasive will be a huge challenge to COVID-19: The human immune system, once it’s activated by vaccination or infection, is more resilient and effective than even most studies indicate.