Updated booster shots have bolstered Americans’ defenses against serious Covid, reducing the risk of hospitalization by roughly 50 percent compared with certain groups inoculated with the original vaccines, the Centers for Disease Control and Prevention reported in a pair of studies published on Friday.

The research represents the agency’s first look at how the reformulated boosters, tailored to protect against recent Omicron variants, are performing in the prevention of severe consequences of infection with the virus, including emergency department visits and hospitalizations.

Federal health officials are urging Americans to get the updated booster shots, hoping to revive a lagging vaccination campaign. So far, though, fewer than a fifth of American adults and only a third of people ages 65 and older have received updated shots, reflecting a retreat in many parts of the country from the more aggressive vaccination drives earlier in the pandemic.

New virus variants that are better able to dodge the immune system have gained traction, and Covid cases and hospitalizations have climbed in recent weeks. About 375 Americans are dying each day on average, an increase of 50 percent over the past two weeks. Older people have been hit especially hard.

The virus has exacerbated the difficulties facing a health care system already under strain from resurgences of the flu and respiratory syncytial virus after two years of reductions in those infections.

Even as federal health officials encourage testing and mask use in certain settings, precautions have become far less common in practice. Antiviral medication for Covid remains difficult to find for many who are infected.

“We probably won’t see waves of Covid like we have in the past, which is a good thing, but it doesn’t mean people aren’t still dying and that those lives couldn’t still be saved if we got more shots in arms,” said Dr. David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

One C.D.C. study released on Friday examined how the updated shots protected people from Covid-related emergency department visits and hospitalizations in seven health systems.

The study, which looked at about 15,000 hospitalizations, stretched from mid-September to mid-November, when Covid cases were largely being caused by the BA.5 Omicron variant — the target, in part, of the reformulated shots.

Since then, however, more evasive versions of Omicron known as BQ.1 and BQ.1.1 have become more common, and it’s not clear how relevant the conclusions are to the newer variants.

During the BA.5 period, people who had received the updated boosters had a 57 percent lower risk of hospitalization compared with unvaccinated people, a 38 percent lower risk compared with people who had recently received doses of the original vaccine, and a 45 percent lower risk compared with people whose last dose of the original vaccine was at least 11 months earlier.

But the C.D.C.’s study did not account for whether patients had previously been infected with the virus, potentially making the updated vaccines appear less effective than they are. And the research did not take into account whether certain groups were more likely to have received treatments like Paxlovid, which might have skewed the results.

A second study reported on the benefits of updated boosters for older Americans in 22 hospitals from early September to late November.

Among people ages 65 and older, the updated vaccines reduced the risk of Covid hospitalization by 84 percent compared with unvaccinated people, and by 73 percent compared with people who had received at least two doses of the original vaccines.

C.D.C. scientists said that the higher estimates of vaccine effectiveness in older age groups might reflect a variety of differences in the particular groups of patients being studied.