As the number of coronavirus cases in US surpass 3.9 million, there are a growing number of reports of people getting COVID-19, recovering and then falling sick again.

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Such assertions if proved, could complicate efforts to make a long-lasting vaccine, or to achieve herd immunity where most of the population has become immune to the virus, reported The Washington Post. Doctors emphasize there is no evidence of widespread vulnerability to reinfection and that it is difficult to know what to make of these cases in the absence of detailed lab work, or medical studies documenting reinfections, the report said. It stated there is "still not enough evidence or sufficient time since the virus first struck to draw firm conclusions about how people develop immunity to COVID-19, how long it might last -- or what might make it less robust in some individuals than in others."

Daniel Griffin, an infectious diseases doctor and researcher at Columbia University Medical Center, was quoted as saying that with every virus -- including chickenpox, for which antibodies are supposed to last a lifetime -- there are cases of people who become sick again after recovering from the initial illness. On the basis of what is known about the novel coronavirus, physicians and public health officials believe reinfection is certainly a theoretical possibility. "No one is yet believing in reinfection since there is no good scientific report on it," Monica Gandhi, a professor of medicine and associate chief of infectious diseases at the University of California-San Francisco, said while adding that on the other hand, no one wants to dismiss the possibility.

Such a scenario would be prove challenging for vaccination for the disease which has claimed over six lakh lives worldwide. Robert Glatter, an assistant professor of emergency medicine at Lenox Hill Hospital and Northwell Health, the largest medical system in New York State, said if widespread reinfection is a possibility, people may have to be vaccinated over and over again, leading to logistical and supply challenges.