In a study of about 4,000 health-care personnel, police, firefighters and other essential workers, the Centers for Disease Control and Prevention found that the vaccines reduced the risk of infection by 80 percent after one shot. Protection increased to 90 percent following the second dose. The findings are consistent with clinical trial results and studies showing strong effectiveness in Israel and the United Kingdom, and in initial studies of health-care workers at the University of Texas and in Southern California.

The CDC report is significant, experts said, because it analyzed how well the vaccines worked among a diverse group of front-line working-age adults whose jobs make them more likely to be exposed to the virus and to spread it.