Researchers in Hong Kong are reporting the first confirmed case of reinfection with the coronavirus.

“An apparently young and healthy patient had a second case of Covid-19 infection which was diagnosed 4.5 months after the first episode,” University of Hong Kong researchers said Monday in a statement.

The report is of concern because it suggests that immunity to the coronavirus may last only a few months in some people. And it has implications for vaccines being developed for the virus.

The coronavirus crept into Heartland Health Care Center, a nursing home in Moline, Ill., on the last day of July, when a member of the nursing staff tested positive.

It was an ominous sign: The virus can spread through a nursing home in a flash. Older people — who are often sick and frail and need regular hands-on attention — are uniquely susceptible. Staff members who care for residents are at high risk of infection and of unintentionally spreading the virus.

Although nursing home residents make up just 1.2 percent of the United States population, they account for about 40 percent of Covid-19 deaths.

But this time, the nursing home was not defenseless. Heartland was the first facility to participate in a large clinical trial of a drug that might protect residents from the infection in nursing homes and assisted living facilities.