Johnson & Johnson, the only major drug maker developing a single-dose vaccine for Covid, announced on Friday that its shot provided strong protection against Covid-19, potentially offering another powerful tool in a desperate race against a worldwide rise in virus mutations. But the results came with a significant cautionary note: The vaccine’s efficacy rate dropped from 72 percent in the United States to 57 percent in South Africa, where a highly contagious variant is driving most cases.

Studies suggest that this variant also blunts the effectiveness of Covid vaccines made by Pfizer-BioNTech, Moderna and Novavax. The variant has spread to at least 31 countries, including the United States, where two cases were documented this week. With these results, Johnson & Johnson became the fifth company supported by the U.S. government to develop an effective Covid vaccine in less than a year, and the only one that doesn’t need two doses — a big advantage when most countries are struggling to get shots in arms more quickly.

The Johnson & Johnson vaccine was extremely effective in preventing severe cases of Covid — including serious illness caused by the variant, the company said. Though less effective than the Moderna and Pfizer vaccines now authorized in the United States, Johnson & Johnson’s is still considered a strong vaccine by scientists. Annual flu vaccines, for example, are typically 40 to 60 percent effective.

“This is a really great result,” said Akiko Iwasaki, an immunologist at Yale University. “I hope this vaccine gets approved as soon as possible to reduce disease burden around the world.”