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.