New coronavirus variants could challenge vaccine efforts.

Confirmed coronavirus cases from new variants found first in Britain, then in South Africa, Brazil and the United States have people worried about whether vaccines can protect against altered versions of the virus. Experts said in interviews that so far vaccines are capable of providing that protection.

But two small new studies, posted online Tuesday night, suggest that some variants may pose unexpected challenges to the immune system, even in those who have been vaccinated — a development that most scientists had not anticipated seeing for months, even years.

The findings result from laboratory experiments with blood samples from groups of patients, not observations of the virus spreading in the real world. The studies have not yet been peer-reviewed.

But experts who reviewed the papers agreed that the findings raised two possibilities. People who had survived mild cases may still be vulnerable to infection from a new variant; and the vaccines may be less effective against the variants.

Existing vaccines will still prevent serious illness, and people should continue getting them, said Dr. Michel C. Nussenzweig, an immunologist at Rockefeller University in New York, who led one of the studies: “If your goal is to keep people out of the hospital, then this is going to work just fine.”

But the vaccines may not prevent people from becoming mild or asymptomatic infections with the variants, he said. “They may not even know that they were infected,” Dr. Nussenzweig added. If the infected can still transmit the virus to others who are not immunized, it will continue to claim lives.

The studies published Tuesday night show that the variant identified in South Africa is less susceptible to antibodies created by natural infection and by vaccines made by Pfizer-BioNTech and Moderna.

Neither the South African variant nor a similar mutant virus in Brazil has yet been detected in the United States. The more contagious variant that has blazed through Britain does not contain these mutations and seems to be susceptible to vaccines.