If viruses know no borders, they
 have an equal disdain for human holidays. On Thanksgiving 2021, Dr. Rochelle Walensky shut off her phone for about 45 minutes and placed the turkey on the dining table to enjoy the holiday meal with her husband and three sons. When she turned the phone back on, she found, among the messages that had piled up, warnings that SARS-CoV-2 had morphed into a potentially dangerous new variant, identified in South Africa. Health officials there predicted that what would become known as the Omicron variant was much better at spreading than previous variants, and Walensky knew it was only a matter of time before Omicron cases would appear in the U.S.

As director of the U.S. Centers for Disease Control and Prevention (CDC), Walensky’s job is to p…