The first confirmed US case of coronavirus reinfection has been reported by researchers.
A 25-year-old man with no known immune disorders or underlying conditions was infected with COVID-19 on two separate occasions, according to a study published in the Lancet Infectious Diseases journal.
While the authors said further research was required, they added the findings indicate previous exposure to the virus may not guarantee total immunity.
It is the fifth confirmation of reinfection worldwide, researchers said, with at least four other cases confirmed in Belgium, the Netherlands, Hong Kong, and Ecuador.
The second infection of the patient, who lives in Nevada, was more severe than the first and resulted in hospitalisation with oxygen support.
Researchers from the Nevada State Public Health Laboratory and the University of Nevada said he tested positive for the virus in April this year, and later tested negative on two separate occasions.
Experiencing COVID-19 symptoms in June, including fever, headache, dizziness, cough, nausea and diarrhoea, he was admitted to hospital and tested positive for a second time.
Genetic sequencing of the virus showed he was infected twice by different strains of Sars-CoV-2, according to researchers.
Lead author Mark Pandori of the Nevada State Public Health Laboratory, said it was important to note the single finding should not be generalised.
"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," he said.
"It also 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."
He added that more research was needed to understand how long immunity may last for those exposed to the virus, and why second infections, while rare, present as more severe.
The authors gave several explanations on the severity of the second infection, including the patient encountering a very high dose of the virus when he was reinfected.
He may have also come into contact with a more virulent version of the virus, according to researchers.
They added that confirmed reinfections had occurred among patients who showed symptoms, meaning that reinfections could occur among those who are asymptomatic and therefore go undetected.
Pandori said there was a lack of "comprehensive genomic sequencing of positive COVID-19 cases" worldwide, as well as a lack of screening and testing, which limited researchers.
Australian Associated Press