But experts who reviewed the papers agreed that the findings raised two disturbing possibilities. People who had survived mild infections with the coronavirus may still be vulnerable to infection with a new variant; and more worryingly, 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 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 mildly or asymptomatically infected 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.