In January, with the omicron wave slamming the country, about 682,000 booster doses were administered daily, according to the latest CDC data. That pushed up the share of vaccinated people who received a booster to 39 percent, up 6 percentage points. Just under half of the boosters went to people between 18 and 50. Some people are not yet eligible for the shots, however, because not enough time has elapsed since their most recent vaccine shot.