Delta appears to evade some antibodies, but there are other variants, particularly Beta, that are even better at dodging these defenses. For now, Delta is so infectious that it has managed to outcompete, and thus limit the spread of, these stealthier variants.

But as more people acquire antibodies against the virus, mutations that allow the virus to slip past these antibodies will become even more advantageous. “The landscape of selection has changed,” said Jessica Metcalf, an evolutionary biologist at Princeton University. “From the point of view of the virus, it’s no longer, ‘I just bop around, and there’s a free host.’”

The good news is that there are many different kinds of antibodies, and a variant with a few new mutations is unlikely to escape them all, experts said.

“The immune system has also evolved to have plenty of tricks up its sleeve to counteract the evolution of the virus,” Dr. Pepper said. “Knowing that there is this complex level of diversity in the immune system allows me to sleep better at night.”