After the first known reinfection in the United States, scientists warn COVID-19 immunity may not be guaranteed. A 25-year-old Nevada man tested positive for the coronavirus six weeks after he originally contracted the disease, scientists said Monday in The Lancet medical journal.

In the report, researchers studied the viruses that caused the man’s infections and determined they had different genetic makeups. The findings suggest he was infected two separate times.

That means exposure to the virus “might not guarantee total immunity in all cases,” according to the study.

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