The U.S. is struggling to monitor Covid-19 variants, a key part of watching for the emergence of dangerous mutations that might spread quickly, evade vaccines or kill more infected people. The country ranks 32nd in the world for the number of tests it’s done to detect mutations per 1,000 Covid cases. Kristen V. Brown reports that other countries, like the U.K., have established robust, nationwide surveillance programs to identify new Covid genomes and track the spread of existing ones.