The “delta variant” has come to dominate headlines, having been discovered in India where it provoked an extreme surge in Covid-19 cases before spreading around the world. But now a mutation of that variant has emerged, called “delta plus,” which is starting to worry global experts. India has dubbed delta plus a “variant of concern,” and there are fears that it could potentially be more transmissible. In the U.K., Public Health England noted in its last summary that routine scanning of Covid cases in the country (where the delta variant is now responsible for the bulk of new infections) has...