The unidentified patient from Nevada, who has no known immune disorders or underlying conditions, first tested positive for Covid-19 in April and suffered from mild symptoms of a sore throat, headache, nausea and diarrhoea.
A 25-year-old man has been confirmed as the first person in the US to have contracted coronavirus twice.
However, in June, the man tested positive for coronavirus again, but this time was hospitalised after suffering from more severe symptoms, according to a study published in the Lancet Infectious Diseases journal.
The patient received oxygen support as part of his treatment in June, after he suffered with muscle aches, shortness of breath and viral pneumonia, which can be deadly.
Researchers from the Nevada state public health laboratory and the University of Nevada, Reno school of medicine said that after the man recovered they analysed the genetic sequencing of the virus.
The results confirmed that he was infected twice by different strains of Sars-CoV-2, which is the name of the virus that causes the disease Covid-19.
He is the fifth person in the world to be reinfected with coronavirus, after patients in Belgium, the Netherlands, Hong Kong and Ecuador were previously confirmed to have had Covid-19 twice.
The lead author of the report, Mark Pandori, of the Nevada state public health laboratory, wrote in The Lancet that exposure to the virus might not guarantee future immunity to it.
He added: “It is important to note this is a singular finding and does not provide generalisability of this phenomenon.
“While more research is needed, the possibility of reinfections could have significant implications for our understanding of Covid-19 immunity, especially in the absence of an effective vaccine.”
The study was published just a few days after president Donald Trump claimed that he is now immune to the virus and “can’t get it and can’t give it” to anyone else, following treatment for coronavirus the week prior.
However, Mr Pandori said that the results from the unidentified patient in Nevada “strongly suggests that individuals who have tested positive for Sars-CoV-2 should continue to take serious precautions when it comes to the virus, including social distancing, wearing face masks, and handwashing.”
The authors of the report said that more research needs to be undertaken to understand why so few patients have been confirmed to have been reinfected with the virus, and to work out how long immunity from Covid-19 lasts.
They also suggested that the patient in Nevada might have come into contact with a stronger version of the virus the second time, which caused his more severe symptoms.
Mr Pandori added: “I think the study shows that whether you tested positive or not, we’re all in the same public health boat together.
“Be considerate that you might be able to get infected again because we cannot prove invulnerability.”
According to a tracking project hosted by Johns Hopkins University, there are now more than 7.8 million people who have tested positive for coronavirus in the US. The death toll has reached at least 215,355.
