A 25-year-old Nevada man has been confirmed to be the first American confirmed to have caught COVID-19 twice—and doctors have noted that the unnamed man’s second infection was notably worse than his first. The man recovered both times, but the apparent reinfection raises questions on how long people are protected from COVID-19 after their initial infection. It could also have ramifications for the effectiveness of a future vaccine. “It’s a yellow caution light,” said William Schaffner, an infectious-disease expert at the Vanderbilt University School of Medicine in Nashville, who was not involved in the case. There have been at least 22 documented cases of reinfection around the globe, but more research and bigger studies are needed to know how prevalent it is.