As horrific as the U.S. Covid-19 outbreak looks right now, it is almost certainly about to get worse. They’ve raced through South Africa, the United Kingdom, and, increasingly, elsewhere, and now, new, more infectious variants of the coronavirus have also gained toeholds in the United States. If they take off here — which, with their transmission advantages, they will, unless Americans rapidly put a brake on their spread — it will detonate something of a bomb in the already deep, deep hole the country must dig out of to end the crisis.