A man who just recovered from COVID-19 leaves the John Garang Infectious Disease Unit in Juba, South Sudan | Credit | Xinhua/Gale Julius

A patient who was first diagnosed with Covid-19 four months ago has been re-infected, in what researchers are terming as the world’s first documented case of re-infection.

Researchers at the University of Hong Kong on Monday reported the confirmed case in a 33-year-old man who was first infected by Sars-CoV-2, the virus which causes the coronavirus disease, in late March.

Four-and-a-half months later, he seemingly contracted the virus again while traveling in Europe, casting doubt on whether patients actually develop immunity against the virus once they contract it.

“This is the world’s first documentation of a patient who recovered from Covid-19 but got another episode of Covid-19 afterwards,” the researchers said on Monday in a statement.

The news was also met with caution by other scientists, who questioned the extent to which the case pointed to broader concerns about reinfection.

The finding raises concerns as it not only suggests that immunity to Covid-19 may last only a few months in some people, but it also has implications for ongoing vaccine research.

Nine candidate vaccines are currently being supported by the Coalition for Epidemic Preparedness Innovations (CEPI) which is leading the Covid-19 Vaccine Global Access Facility (COVAX) vaccine research and development work.

Scientists have been trying to figure out whether you can get infected with Covid-19 more than once.

Sporadic anecdotal evidence, largely attributed to flaws in testing about reinfection, have sparked doubts about whether people can ever gain immunity against the pathogen.

Although current research suggests reinfection within a short time frame is unlikely, some researchers are hesitant to completely dismiss the idea.

Studies are increasingly finding that most people who recover from the illness mount a robust immune response involving both molecules that can block the virus from infecting cells again (antibodies) and which can help clear the virus (T cells), implying that people would be protected from another case for some amount of time.

But experts knew that immunity to Sars-CoV-2 would not last, based on what happens with other coronaviruses that showed that people generally became susceptible again.

“What we are learning about infection is that people do develop an immune response, and what is not completely clear yet is how strong that immune response is and for how long that immune response lasts,” Maria Van Kerkhove, World Health Organisation’s Covid-19 technical lead and an infectious disease epidemiologist, said at a briefing Monday, when asked about the Hong Kong report.