Real-world studies show updated Covid-19 boosters offer important protection against urgent care visits and hospitalizations

"Both studies show that there's quite an important benefit from the bivalent booster that it's adding quite a lot of extra protection against hospitalization due to Covid-19," said Ruth Link-Gelles, an epidemiologist at the US Centers for Disease Control and Prevention who co-authored one of the studies. The studies published on Friday in CDC's Morbidity and Mortality Weekly Report.

The bivalent boosters made by Moderna and Pfizer/BioNTech target the original coronavirus strain and the Omicron BA.4 and BA.5 subvariants. The shots became available in early September.

The bivalent boosters appear to provide strong protection -- up to 84% -- against hospitalizations for seniors. When the analysis included younger adults, they provided moderate protection against needing to go to the emergency room or hospital.

In one study, which included adults ages 65 and older who didn't have compromised immune function and had been hospitalized for Covid-19, researchers estimated the bivalent boosters were 84% effective against hospitalization compared with being unvaccinated, and 73% effective at preventing hospitalizations compared with two or more doses of the original Covid-19 vaccines.

The study drew its data from 798 patients who were admitted to 22 hospitals in 18 states between September 8 and November 30.

Adults over age 65 have always been among the most vulnerable to Covid-19, and current hospitalization data shows that seniors are again being hit the hardest during the latest wave of Covid-19 cases. This wave is being dominated by BQ.1 and BQ.1.1, which are descendants of BA.5. While hospitalizations have begun to rise for all age groups, they are rising faster for seniors.

While 94% of seniors have completed at least their primary series of Covid-19 vaccinations, CDC data shows that only about 36% of this age group have received an updated booster.

A second study released on Friday included younger adults and looked at how effective the updated boosters were at preventing urgent care and emergency room visits, as well as hospitalizations at a network of hospitals in nine states. The study included more than 78,000 urgent care and emergency department visits and more than 15,500 hospitalizations. People who came to the hospital for treatment were eligible to be include in the study if they were over age 18, had tested positive for Covid-19, and didn't have compromised immune function.

Compared with those who were unvaccinated, those who had gotten an updated booster were 56% less likely to seek care in the emergency room or urgent care, and 57% less likely to be hospitalized.

Compared with those who'd been vaccinated but gotten their most recent dose 11 months or more prior, the updated boosters lowered the risk of being seen in the ER or urgent care by 50% and lowered the risk of hospitalization by 45%.

Compared with those who'd been recently boosted with the original vaccines -- two to four months prior -- those who got updated boosters were 31% less likely to need to go to the ER or urgent care. The study didn't provide an estimate on hospitalizations among people who were recently boosted with the original vaccine.

These are early studies. At the time they were done, only about 5% of adults in each study had received a bivalent booster.

That means the vaccine effectiveness of the booster is likely to be underestimated, the researchers said in an interview with CNN. Normally, the people who are quickest to get a booster are those who know they really need them because of an underlying condition or something else they feel puts them at higher risk.

These people are also most likely to need care in the clinic or hospital if they get Covid-19, Link-Gelles said.

"What happens in kind of these early evaluations is that we do sometimes see this where the vaccine effectiveness may be a little lower than we would have expected against more severe ends of the spectrum," she said.

"I think it's likely we'll see the the numbers, the vaccine effectiveness estimates for younger adults, creep up a bit."

Experts who were not involved in the studies say they underscore the need for all adults to get their updated boosters, but also for people to use additional tools, including masks, ventilation and testing, for even better protection.

"This is a reminder of what we've been saying now for a number of months, that we have to stop this idea of saying two immunizations is full vaccination," said Dr. Peter Hotez, dean for the National School of Tropical Medicine at the Baylor College of Medicine in Houston. "You need your booster, and you need your booster every four to six months because the protection even against hospitalization starts to go down."

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