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USA TODAY

An otherwise healthy 25-year-old Nevada man is the first American confirmed to have caught COVID-19 twice, with the second infection worse than the first.

He has recovered, but his case raises questions about how long people are protected after being infected with the coronavirus that causes the disease, and potentially how protective a vaccine might be.

“It’s a yellow caution light,” said Dr. William Schaffner, an infectious disease expert at the Vanderbilt University School of Medicine in Nashville, Tennessee, who was not involved in the research.

Respiratory infections like COVID-19 don’t provide lifelong immunity like a measles infection. So, Dr. Paul Offit, an infectious disease expert at Children’s Hospital of Philadelphia, said he’s not at all surprised people could get infected twice with the coronavirus, SARS-CoV-2. 

It’s too soon to know whether the man from Washoe County, Nevada, who had no known health problems other than his double infection, was highly unusual or if many people could easily get infected more than once with SARS-CoV-2, Schaffner said.

“There’s hardly an infectious disease doctor in the country who hasn’t encountered a patient who thinks they’ve had a second infection,” he said. “Whether that’s true or not, we don’t know. There are lots of respiratory infections out there.”

How rare is he?

There have been at least 22 documented cases of reinfection worldwide since the start of the pandemic, but it’s unclear how many cases there have actually been, and how common it may be among people who don’t even know they’re infected.

“It could be a one in a million event, we don’t know,” said Akiko Iwasaki, an immunologist at Yale University and an investigator with the Howard Hughes Medical Institute, who wrote a commentary with the study.

With millions of people infected, it’s hard to know if case studies like the new one represent very rare events or the tip of an iceberg, she said. “It’s possible that the vast majority of people are completely protected from reinfection, but we’re not measuring them, because they’re not coming to the hospital.” 

Also, many people don’t know they are infected the first time, so it’s hard to say whether they’re getting re-infected.

In one of the recent cases, a Hong Kong man only knew he was re-infected because it was caught during a routine screening when he returned from outside the country, months after he had cleared an infection and tested negative. 

One reason there may not be more documented cases of reinfection: It’s tough to prove, said Mark Pandori, a pathologist at the University of Nevada, Reno School of Medicine, and senior author on the new study. 

His team coordinated early in the pandemic with members of the Washoe County Health District to look for repeat infections. They had the benefit of sequencing equipment on campus, as well as