Lead researcher Mark Pandori, from the Nevada State Public Health Laboratory in the U.S., said: "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".
Scientists say that a 25-year-old Nevada man has contracted COVID-19 twice, on two separate occasions.
Despite those factors, the unnamed man - who has since recovered from both bouts - required hospital treatment following his second infection.
An 89-year-old Dutch woman with a rare type of bone marrow cancer died after she became infected with COVID-19 for the second time, researchers have reported. The patient tested negative for the virus after testing positive for SARS-CoV-2 in April 2020, the researchers said. She tested positive for coronavirus and remained hospitalized for 5 days, after which her symptoms subsided completely, except for persisting fatigue.
There have only been 23 cases of reinfection worldwide so far according to the researchers, and in all previous cases the patients have made a full recovery.
While experts have been quick to stress that reinfections now appear to be relatively rare, this case is especially unusual as the patient was hit harder by the virus during the second infection.
"There are still many unknowns about Covid infections and the immune system's response, but our findings signal that a previous Covid infection may not necessarily protect against future infection", said Mark Pandori of the Nevada State Public Health Laboratory, located at the University of Nevada, Reno School of Medicine and lead author of the study. Nevertheless, everyone, whether they previously had the illness or not, should continue taking precautions to avoid infection.
Another hypothesis is a mechanism known as antibody dependent enhancement - that is, when antibodies actually make subsequent infections worse, such as with dengue fever. "They may have been exposed to a lot higher levels of the virus the second time around", she said. "Right now, we can only speculate about the cause of reinfection", he added.
Alternatively, it may have been a more virulent strain of the virus.
US President Donald Trump tosses a face mask to the crowd as he takes the stage for a campaign rally, his first since being treated for the coronavirus disease, at Orlando Sanford International Airport in Sanford, Florida, US [Jonathan Ernst/Reuters] The report was published just hours after US President Donald Trump, who was infected with COVID-19 and hospitalised earlier this month, said he believes he now has immunity and felt "so powerful".