Evidence is mounting that getting infected and surviving the coronavirus disease 2019 (Covid-19) may not protect against getting infected again with some of the new variants of the virus.

People also can get second infections with earlier versions of the Covid-19 if they mounted a weak defense the first time, new research suggests.

How long immunity lasts from natural infection is one of the big questions in the pandemic. Scientists still think reinfections are fairly rare and usually less serious than initial ones, but recent developments around the world have raised concerns.

In South Africa, a vaccine study found new infections with a variant in 2 percent of people who previously had an earlier version of the virus.

In Brazil, several similar cases were documented with a new variant there. Researchers are exploring whether reinfections help explain a recent surge in the city of Manaus, where three-fourths of residents were thought to have been previously infected.

In the United States, a study found that 10 percent of Marine recruits who had evidence of prior infection and repeatedly tested negative before starting basic training were later infected again. That work was done before the new variants began to spread, said one study leader, Dr. Stuart Sealfon of the Icahn School of Medicine at Mount Sinai in New York.

“Previous infection does not give you a free pass,” he said. “A substantial risk of reinfection remains.”

Reinfections pose a public health concern, not just a personal one. Even in cases where reinfection causes no symptoms or just mild ones, people might still spread the virus. That’s why health officials are urging vaccination as a longer-term solution and encouraging people to wear masks, keep physical distance and wash their hands frequently.

“It’s an incentive to do what we have been saying all along: to vaccinate as many people as we can and to do so as quickly as we can,” said Dr. Anthony Fauci, the US government’s top infectious disease expert.

“My looking at the data suggests … and I want to underline suggests … the protection induced by a vaccine may even be a little better” than natural infection, Fauci said.

Doctors in South Africa began to worry when they saw a surge of cases late last year in areas where blood tests suggested many people had already had the virus.

Until recently, all indications were “that previous infection confers protection for at least nine months,” so a second wave should have been “relatively subdued,” said Dr. Shabir Madhi of the University of the Witwatersrand in Johannesburg.