Many experts now predict that COVID-19, which so far has killed more than 5.5 million people worldwide, will remain endemic as new, infectious variants of SARS-CoV-2 emerge. These new variants could pose a greater risk of reinfection—infecting people who have already had COVID19—than previous ones. The rapid identification of reinfection cases and surges could improve public health responses and reveal variants that escape the protection offered by vaccination.