The Centers for Disease Control and Prevention reported in a pair of studies on Friday that updated booster shots have strengthened Americans' defenses against serious Covid, reducing the risk of hospitalization by roughly 50 percent.
The research shows how the reformulated boosters, tailored to protect against recent Omicron variants, are performing in the prevention of hospitalizations and emergency department visits.
The updated booster shots are being urged by the federal health officials. Less than a fifth of American adults and only a third of people over the age of 65 have received updated shots so far.
Recent weeks have seen a rise in Covid cases and hospitalizations due to new viruses that are better able to dodge the immune system. Over the last two weeks, the number of Americans dying has increased by 50 percent. Older people have been hit harder than anyone else.
The health care system is already under strain from the resurgence of the flu and respiratory syncytial viruses after two years of reductions in those infections.
As federal health officials encourage testing and mask use, precautions have become less common. It's hard to find anti-viral medication for Covid.
"We probably won't see waves of Covid like we have in the past, 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."
The study looked at how the updated shots protected people from Covid-related emergency department visits and hospitalizations.
The study spanned from mid-September to mid-November, when Covid cases were mostly caused by the BA.5 Omicron variant.
Since then, more evasive versions of Omicron known as BQ.1 and BQ. 1.1 have become more common, and it is not clear how relevant the conclusions are to the newer versions.
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 the original vaccine, and a 45 percent lower risk compared with people who had not received the original vaccine.
The C.D.C.'s study didn't take into account whether patients had previously been exposed to the virus, which could make the updated vaccines appear less effective than they are. The research didn't take into account whether certain groups were more likely to have received Paxlovid.
The benefits of updated boosters for older Americans were reported in another study.
The updated vaccines reduced the risk of Covid hospitalization by 84 percent compared with unvaccinated people and 73 percent compared with people who had received at least two doses of the original vaccine.
C.D.C. scientists said that the higher estimates of vaccine effectiveness in older age groups might be related to different groups of patients being studied.
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