Johnson & Johnson says its vaccine protects against Delta, adding to the arsenal against the variant.

Johnson & Johnson said its Covid vaccine was effective against the highly contagious Delta variant, adding to the growing body of evidence that the most widely available Covid shots offer protection against its most dangerous variants.

Even eight months after inoculation, the single-shot J.&J. vaccine is proving to be highly effective against Delta, the company reported on Thursday, a reassuring finding for the 11 million Americans who have gotten the shot and for countries around the world betting on receiving the vaccine. In the United States, the variant, first identified in India, now accounts for an estimated one in four new cases, and the C.D.C. has listed it in 23 states.

Johnson & Johnson said its vaccine showed a small drop in potency against the Delta variant, compared with its effectiveness against the original virus, and a larger drop against the Beta variant first identified in South Africa. That is the same pattern seen with the mRNA vaccines made by Pfizer-BioNTech and Moderna.

The intense discourse about Delta’s threat has left some people who are vaccinated feeling anxious about whether they are protected. The variant’s global spread has prompted new restrictions from Ireland to Malaysia.

Frustration had been building about the lack of clarity around the Johnson & Johnson vaccine’s efficacy against Delta. And reports of a cluster of cases among players on the Yankees baseball team who had received the Johnson & Johnson shot, though all asymptomatic or mild, did nothing to assuage fears.

Studies have shown that the Delta and Beta variants slightly lower the efficacy of the Pfizer and Moderna vaccines. For Pfizer, studies show that two doses offer 88 percent protection against the Delta variant, just below the 93 percent protection against Alpha. The Moderna vaccine has performed similarly to Pfizer’s in earlier studies.

Johnson & Johnson has collected less data than its peers on the vaccines, and the study released on Thursday was small and has not yet been published in a scientific journal. Updates on the efficacy of the Johnson & Johnson vaccine have been slow because it was rolled out later than the Pfizer and Moderna vaccines in the United States. The vaccine offered about 72 percent protection against early versions of the virus.