The booster doses were 90 percent effective at keeping people out of hospital after they had become infected with the Omicron variant.
The doses were also 82 percent effective at preventing emergency department and urgent care visits, data indicated.
“It really shows the importance of getting a booster dose,” the CDC’s Emma Accorsi, one of the study’s authors, said on Friday.
“Americans should get boosters if at least five months have passed since they completed their Pfizer or Moderna series, but millions who are eligible have not gotten them.”
The research comprised the first large US studies to look at vaccine protection against Omicron, health officials said.