At the same time, the coronavirus is mutating in predictable but alarming ways. As my colleague Sarah Zhang has written, the virus has developed more-infectious variants in several different places around the world. The variant that emerged in the United Kingdom may be more than 50 percent more transmissible than the coronavirus strain that dominates in the U.S. (British Prime Minister Boris Johnson claimed on Friday that the same strain may also be 30 percent more lethal.) So far, these variants seem to respond to the vaccine, but without widespread mitigation efforts, the risk increases that the virus will develop even more transmissible and lethal variants. The most immediate risk is that these new variants cause another surge of infection, and death, before mass vaccination can increase the number of Americans with protective immunity.

How likely is such a scenario? The highly contagious U.K. strain is already circulating in the U.S.: Epidemiologists have found it in at least 20 states, in many cases in Americans who have not recently traveled abroad. But that may be the least of our issues. The U.S. may have its own highly contagious strains, but we would not necessarily know about them. Infectious-disease researchers detect new variants of a virus by sequencing its genetic code as collected from hundreds of COVID-19 patients. And the CDC is only now beginning to surveil widely enough to identify new variants. One particular strain of the virus seems to have become prevalent in Southern California, but it is unclear whether this strain is more transmissible or just happened to infect more people in several unlucky super-spreading events.