As the coronavirus sweeps across the globe in relentless waves, a succession of more-infectious strains has washed up. So far delta, the variant first reported in India in October, is the most worrisome. About twice as transmissible as the original virus that emerged in late 2019, delta combines a multitude of genetic features that have enabled it to pierce public-health defenses to stoke severe epidemics and trigger fresh rounds of movement restrictions or delay planned easings of them.
