A new subvariant of the novel-coronavirus called XBB dramatically announced itself earlier this week, in Singapore. New COVID-19 cases more than doubled in a day, from 4,700 on Monday to 11,700 on Tuesday—and XBB is almost certainly why. The same subvariant just appeared in Hong Kong, too.A highly mutated descendant of the Omicron variant of the SARS-CoV-2 virus that drove a record wave of infections starting around a year ago, XBB is in many ways the worst form of the virus so far. It’s more contagious than any previous variant or subvariant. It also evades the antibodies from monoclonal therapies,...