Viruses mutate all the time, including the one, SARS-CoV-2, that’s caused the Covid-19 pandemic. Although most genetic changes are innocuous, some can make the mutant more adept at infecting cells, for example, or evading antibodies. Such “fitter” variants can outcompete other strains, so that they become the predominant source of infections. A succession of more-transmissible variants has emerged over the past year, each harboring a constellation of mutations. The most worrisome so far is the so-called delta variant. It’s become dominant in much of the world since its discovery in India in October, leading to surges in cases and hospitalizations, especially in places where less than half the adult population has been fully immunized.