"The possibility of reinfections could have significant implications for our understanding of Covid-19 immunity, especially in the absence of an effective vaccine," said Mark Pandori, for the Nevada State Public Health Laboratory and lead study author.
"We need more research to understand how long immunity may last for people exposed to SARS-CoV-2 and why some of these second infections, while rare, are presenting as more severe."
Vaccines work by triggering the body's natural immune response to a certain pathogen, arming it with antibodies to fight off future waves of infection.
But it is not at all clear how long Covid-19 antibodies last.
For some diseases, such as measles, infection confers lifelong immunity. For other pathogens, immunity may be fleeting at best.
The authors said the US patient could have been exposed to a very high dose of the virus the second time around, triggering a more acute reaction. Alternatively, it may have been a more virulent strain of the virus.
Another hypothesis is a mechanism known as antibody dependent enhancement — that is, when antibodies actually make subsequent infections worse, such as with dengue fever.
The researchers pointed out that reinfection of any kind remains rare, with only a handful of confirmed cases out of tens of millions of Covid-19 infections globally.