New data from the Centers for Disease Control and Prevention (CDC) show the bivalent COVID-19 booster shot was effective at reducing the risk of hospitalization and emergency room visits by at least 50 percent.

Two separate reports released by the CDC Friday offer some of the first evidence of the booster’s effectiveness against hospitalizations and medical encounters. 

The reports come as infections are increasing and the Biden administration readies for an expected surge this winter.

One study found a bivalent COVID-19 booster dose reduced the risk of hospitalization by 57 percent in adults over the age of 18 compared with being unvaccinated, and by 45 percent compared with being unboosted. 

Previous data from the CDC suggested that bivalent boosters provide a modest degree of protection against symptomatic infection among adults compared with receipt of two, three or four doses of monovalent vaccines only.

Bivalent COVID-19 booster vaccines were developed to improve protection against circulating omicron subvariants and waning immunity from monovalent vaccine-conferred protection over time.

“With co-circulation of multiple respiratory viruses, including SARS-CoV-2, influenza, and respiratory syncytial virus (RSV), vaccination against respiratory diseases for which vaccines are available is especially important to prevent illnesses resulting in health care encounters and to reduce strain on the health care system,” the authors wrote.  

The boosters were especially effective in adults over the age of 65, who are at highest risk for severe COVID-19–associated illness. 

A second report showed the bivalent shot was 84 percent effective at preventing hospitalizations compared to people who were unvaccinated, and 74 percent effective compared with people who received at least two doses of the monovalent vaccines.

Everyone, especially those 65 and older “who are at highest risk for severe COVID-19 illness, should receive a bivalent booster dose as soon as they are eligible and to consider masking in indoor public spaces to maximize protection against COVID-19 hospitalization this winter season,” the authors of the second report wrote.

Both analyses were conducted between September and November, during which time multiple omicron subvariants, including BA.5 and the combination of BQ.1 and BQ.1.1, have been the dominant strains.

Administration officials are pushing for more Americans to get the latest shots in an effort to stem a new surge of infections, which would come on top of an early and particularly hard-hitting flu season and high levels of RSV. 

But uptake is still extremely low, even among the most vulnerable. Only about 16 percent of adults over age 18 and 36 percent of people over age 65 have received an updated shot.