Scientists believe that people who recover from COVID-19 have some immunity to the virus, but no one knows how strong, how long, and how varied that protection may be.

On Monday, University of Hong Kong researchers added another wrinkle, reporting the first confirmed case of reinfection. The 33-year-old man first tested positive in late March in Hong Kong and got reinfected about four months later while traveling in Spain. DNA sequencing revealed two slightly different strains of the virus, indicating the second infection was not just a lingering remnant of the first.

Experts differed about the significance of the case. But questions about the nature of immunity underlie vaccine development, hopes for “herd immunity,” and the therapeutic use of blood plasma donated by recovered COVID-19 patients. President Donald Trump on Sunday hailed the emergency approval of convalescent plasma as a “historic breakthrough,” despite regulators’ uncertainty about its effectiveness.

Akiko Iwasaki, an immunobiologist at Yale University, tweeted that the Hong Kong case “is no cause for alarm — this is a textbook example of how immunity should work.”

In her view, it’s encouraging that the man had mild symptoms when he was first infected in March and no symptoms after reinfection: “While immunity was not enough to block reinfection, it protected the person from disease.”

She also speculated that since reinfection can occur, herd immunity — which prevents spread of the virus because most of the population, or herd, is not susceptible — is unlikely to happen through natural infection. “The only safe and effective way to achieve herd immunity is through vaccination,” she tweeted.

Luna Bhatta Sharma, a virologist at Osaka City University Medical School in Japan, had a dimmer view. If the disease-fighting antibodies generated by the first infection could not fight off the slightly different strain, that is “Not good news at all!” she tweeted.

The evidence of reinfection comes as scientists try to figure out what mix and amount of immune markers signal protection against the coronavirus.

There are two waves of immune activation. The first involves antibodies that kill viruses outside of cells before they can break in. The second wave involves immune cells that destroy infected cells so the virus can’t keep replicating inside them.

Vaccine development is racing ahead without a perfect understanding of COVID-19 immunity. Researchers can extrapolate from the body’s reaction to other viruses, including other disease-causing coronaviruses. There is even preliminary evidence that some people have preexisting immunity to the new virus because of exposure to coronaviruses that cause the common cold. If so, that could help explain why the severity of COVID-19 ranges from harmless to deadly.

Antibodies are concentrated in the fluid, or plasma, part of the blood. Using the plasma of patients after they recover is a low-risk approach that has been used to treat infectious diseases for more than a century, such as in a 1934 measles outbreak at the Hill School in Pottstown.

Recent studies suggest plasma may modestly reduce the chance of death in severe COVID-19 patients, but more rigorous, definitive clinical trials are still underway. That’s why the U.S Food and Drug Administration held off granting emergency authorization for the treatment this month.

The agency’s reversal on Sunday came after the president suggested, without proof, that the delay was a political ploy to hinder his reelection chances on Nov. 3.

Mount Sinai Hospital in New York City, one of the first in the nation to use COVID-19 convalescent plasma, is now collecting donations with the goal of making a fortified version, said media director Elizabeth Dowling. The hospital is sending plasma donations to a company called Emergent BioSolutions, where the fluid will be pooled and purified to make a “hyper immune globulin” product.

Although the hospital envisions the product as a bridge to a vaccine, “we will be initiating a clinical trial to study its efficacy for [preventing infection] in at-risk health-care workers,” Dowling said.