Delta is far from the last variant. But what shape the virus takes next depends on us.

By Katherine J. Wu Originally published on August 24, 2021

If evolution is a numbers game, the coronavirus is especially good at playing it. Over the past year and a half, it’s copied itself quickly and sloppily in hundreds of millions of hosts, and hit upon a glut of genetic jackpots that further facilitate its spread. Delta, the hyper-contagious variant that has swept the globe in recent months, is undoubtedly one of the virus’s most daring moves to date. This variant is the product of unfettered transmission, and will thrive further on it; if allowed to, Delta could morph into something even more formidable. “Delta is already a really strong competitor,” Michal Tal, an immunologist at Stanford University, told me. “It could get significantly worse.”

We can’t precisely predict what worse will look like. There is no playbook for evolution. Delta could continue to ratchet up its rate of spread, or it could be ousted by another super-infectious variant. But the speed that has powered Delta’s transmission for months probably can’t sustain SARS-CoV-2 forever, at least not on its own. Humanity’s collective immunity to the virus is growing, which means the next variants we encounter might be better off taking a tack that relies a lot more on stealth. “There’s some sort of tipping point where immune evasion becomes a bigger fitness advantage than transmission,” Stephen Goldstein, an evolutionary virologist at the University of Utah, told me. No one yet knows exactly where that tipping point is—just that we will probably, eventually, collide with it.

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