But in this case, researchers at the University of Hong Kong sequenced the virus from the patient’s two infections and found that they did not match, indicating the second infection was not tied to the first. There was a difference of 24 nucleotides — the “letters” that make up the virus’ RNA — between the two infections.

“This is the world’s first documentation of a patient who recovered from Covid-19 but got another episode of Covid-19 afterwards,” the researchers said in a statement.

Experts cautioned that this patient’s case could be an outlier among the tens of millions of cases around the world and that immune protection may generally last longer than just a few months. They said that ongoing studies tracking patients who had recovered from Covid-19 would help reach more definitive conclusions.