Unvaccinated people in every age group are at higher risk of infection, hospitalization and death than those who have been immunized, according to the C.D.C.’s data — a persistent trend ever since vaccines were introduced.

As of Dec. 25, the rate of hospitalization among unvaccinated adults older than age 65 was 246 per 100,000 people. That rate dropped to 27.4 per 100,000 among people who were vaccinated without a booster dose, and to 4.9 among those who were vaccinated and received a booster.

There were roughly 44 deaths per 100,000 unvaccinated adults 65 and older. Vaccinations dropped that number to about 3.6 deaths per 100,000, one-twelfth as much. Booster shots reduced the rate further, to about 0.5 deaths per 100,000, a figure 90 times as small.

But such risk comparisons were less useful in younger people, for whom the rate of severe outcomes was already low.

Among adults 50 to 64, 73 unvaccinated adults per 100,000 were hospitalized, compared with nine per 100,000 among those who were vaccinated and just two per 100,000 among those who had also received a booster shot.

Boosters made less of a difference in the number of Covid deaths in this age group. Vaccinations decreased the rate to 0.4 deaths per 100,000 from 8.26 per 100,000. With boosters, that number fell to 0.1 deaths per 100,000 people.

“This is the difference between a relative risk reduction and an absolute risk reduction,” Dr. Gounder said. “If you’re starting off with a relatively low risk, and you further reduce that risk, in the big picture that may not be such a big impact.”