Experts said the weaker performance of the vaccine in South Africa — where it was tested on about 6,500 people — almost certainly was a result of the predominance of the variant circulating widely there. Researchers believe that it is more contagious than other variants and that it has become more common in South Africa and elsewhere since the trial began in September.

Researchers said variants were also likely to blame for the subpar showing of the Johnson & Johnson vaccine in Latin America — where it was tested in more than 17,000 people in Argentina, Brazil, Chile, Colombia, Mexico and Peru.

The mutations of greatest concern are on the spike protein on the surface of the virus, because the current crop of vaccines train the immune system to recognize that protein. Mutations there increase the likelihood that the virus will slip by undetected.

Mina, the Harvard epidemiologist, likened the process to searching for a criminal by memorizing the appearance of only his nose and mouth. At first, this may prove sufficient. But if the criminal gets a nose job, investigators will wish they learned about his eyes, ears and hair as well.