Data from the U.K. government found that reinfections were 10 times higher during the recent Omicron outbreak compared with the earlier Delta outbreak between May and December last year. People who were unvaccinated, younger and lived in areas the study described as more deprived were more likely to be reinfected between July 2020 and March 2022.

Reinfections usually appear to be mild in otherwise healthy people, doctors say. But some reinfections are serious, and it’s not clear what the risk of long Covid might be. People who had a milder first infection with a lower viral load were at higher risk of being reinfected, the U.K. data indicates.

“On average at a population level, the people who get reinfected have milder symptoms,” says Francois Balloux, an infectious disease epidemiologist and director of the UCL Genetics Institute in London. “That doesn’t mean that some people might not have a worse infection the second or even third time.”

Currently in the U.K. about 12% of Covid-19 infections are reinfections, Dr. Balloux says. He expects that number to increase over time and says that it’s likely that the viral dose or number of particles a person is exposed to will influence the symptom severity.

Scientists are still learning about the frequency and severity of reinfections; much of the research is preliminary so far. The U.S. doesn’t track national reinfection rates broadly.

Doctors and health agencies typically say most people are protected from a Covid reinfection for at least 90 days. Your body’s immune defenses typically strengthen after an infection, then wane.

However, there is evidence that people can get reinfected within shorter periods.

An April Centers for Disease Control and Prevention report identified 10 people who were reinfected with Covid-19 less than 90 days later. In at least one case, reinfection occurred 23 days later.

Researchers in Spain recently presented preliminary results of a study documenting the case of a 31-year-old woman who caught Covid-19 twice within three weeks. The woman, a healthcare worker, was fully vaccinated and received a booster shot 12 days before testing positive for the Delta variant on Dec. 20. She didn’t develop any symptoms the first time she was infected. Twenty days later, she developed a cough and fever and tested positive for the Omicron variant.

There is also preliminary evidence that reinfections with the same variant can occur.  A Danish preprint, a study that hasn’t been peer-reviewed, found that reinfection of different subvariants of Omicron are rare but do take place. Among 187 reinfection cases, 47 were people infected with BA.1 and then BA.2 20 to 60 days later, researchers found. The majority of people were young, unvaccinated and had mild symptoms.  There was no difference in severity between their first and second infection.

A medical worker administers a test at a Covid-19 testing site in Brooklyn, N.Y.PHOTO: SPENCER PLATT/GETTY IMAGES

Some reinfections can be serious

A March preprint looked at more than 300,000 Covid-19 patients in the U.S. VA system. About 9,200 of them tested positive again at least 90 days after their first positive test; 17% of the reinfected patients were hospitalized. Another 189 cases had a third positive test at least 90 days after their second infection and about 26% of them were hospitalized.

“This data suggests to me that it is reasonable to be cautious about reinfection, ” says Theodore Iwashyna, an intensive care unit physician at the VA in Ann Arbor, Mich., who is first author of the preliminary study.

“There’s a lot of reinfection going on in less than 90 days,” says Mark Pandori, director of the Nevada State Public Health Laboratory and an associate professor of pathology at the University of Nevada, Reno. Dr. Pandori and colleagues have documented two cases that were significantly more severe the second time around.

Risk of long Covid after reinfection is unclear

Scientists don’t yet know what the risk of long Covid is after a reinfection. Amesh Adalja, an infectious-disease physician and a senior scholar at Johns Hopkins University Center for Health Security, speculates that you’re less likely to get long Covid after a reinfection–just as vaccines reduce the likelihood of long Covid– because the immune system activates faster the second time around.

“The first infection is the one most likely to lead to complications based on what we know with many other endemic respiratory infectious diseases,” he says.

But Amy Duckro, an infectious-disease specialist with Kaiser Permanente, says many questions about long Covid remain unanswered.

“I don’t think we should make any bets on someone who has a reinfection having a less likelihood of having long Covid,” says Dr. Duckro.

A research group that grew out of the Body Politic patient support group is launching a reinfection study with part of a $3 million grant it recently received to study the impact of reinfections on long Covid patients.

Gina Assaf, a co-founder and co-leader of the research group, says Body Politic has heard from people who got long Covid after a reinfection. “We don’t know to what degree it’s happening, but it is happening,” she says.

Write to Sumathi Reddy at Sumathi.Reddy@wsj.com