The trials included 32,449 adult participants in all age groups, most of them in the United States. The volunteers received either two standard doses of the Oxford-AstraZeneca vaccine or a placebo, at a four-week interval, the researchers said.

The scientists said the data show the vaccine is 79 percent effective against symptomatic covid-19, the disease caused by the coronavirus, and 100 percent effective against severe illness. There were five cases of severe illness in the trial, all of them in the group that received a placebo.

“The data look good. The numbers don’t lie,” said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, who said he was briefed on the data on Sunday.