A man in the United States caught Covid-19 twice and the second infection was far more severe than the first, doctors have said.
The 25-year-old man, with no known immune disorders or underlying conditions was infected with coronavirus 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 that the findings indicate previous exposure to the virus may not guarantee total immunity, and that all individuals should comply with control measures.
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 Washoe County, Nevada, was more dangerous than the first and resulted in hospitalisation with oxygen support.
Researchers from the Nevada State Public Health Laboratory and the University of Nevada, Reno School of Medicine 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 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.
“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.