Adults who received the updated COVID-19 booster shots are better protected against severe disease than those who haven't, cutting their risk of having to visit an emergency room or being hospitalized with the coronavirus by 50 percent or more, according to new federal data.

Two reports released Friday by the Centers for Disease Control and Prevention give the first detailed look at how well the updated boosters from Pfizer and Moderna protect against serious illness. But uptake of the "bivalent" boosters rolled out in September has been low among vaccine-weary Americans, with only about 14 percent of those eligible - ages 5 and up - having received an updated shot.

Administration officials are renewing a push for more Americans to get the latest shots in anticipation of another COVID-19 winter surge coming on top of an early and aggressive influenza season and high levels of respiratory syncytial virus (RSV). The triple whammy of viruses is straining hospitals, keeping families sick for weeks, and forcing parents to miss work in record numbers.

COVID-19 cases are averaging above 66,000 a day for the first time since mid-September, and deaths are averaging about 400 a day for the past seven days, according to the CDC. More than 40,000 patients are now hospitalized with covid, with more than 9,000 patients admitted with COVID each day, according to federal health data.

At this time last year, there were twice as many COVID-19 cases and about 70,000 people hospitalized, with deaths averaging about 1,300 a day. (Case data is a much less reliable indicator now because at-home test results go unreported.)

Jeanne Marrazzo, director of infectious diseases at University of Alabama in Birmingham, said the new CDC data on booster effectiveness is encouraging.

"Pretty amazing that both studies could show a significant (and in older adults, quite substantial!) effect so quickly, given that the vaccine wasn't even available till Sept. 1," she wrote in an email.

Marrazzo's takeaway: "In this winter of respiratory viruses run amok, the best thing people can do (especially older adults) is to get the bivalent booster and influenza vaccine (data indicate it's pretty well matched with circulating strains this year)!"

CDC data last month showed that Americans who had received the updated boosters had better protection against symptomatic COVID infection than those who had not. The latest reports provided more granular detail and show the boosters offer "an even bigger benefit," especially for adults who received their last vaccine shot almost a year ago, said Ruth Link-Gelles, leader of the CDC's coronavirus vaccine effectiveness team.

For people 65 and older who have received the updated booster, "there is significant added protection against severe disease and hospitalization," Link-Gelles said. "You're really getting in the sort of range of 70 to 80 percent extra protection on top of your monovalent or your older vaccine doses."

But about 150 million Americans who are eligible for an updated booster have yet to receive one. That includes more than 28 million who are 65 years and older, the population at highest risk for severe COVID-19, who have not had a shot in more than 11 months, she said.

The vast majority of Americans have not had a COVID-19 dose this year, putting them at risk of hospitalization due to waning protection of their previous vaccine doses, Link-Gelles said. Heading into the holiday season, when people will be traveling and attending indoor holiday parties, their risk of infection will rise because of exposure to new people.

"There is really no time like the present to get vaccinated," Link-Gelles said.

As part of the Biden administration's renewed strategy for controlling COVID-19 cases this winter, officials are also encouraging people to make use of tests and treatments and wear masks if they are in high-risk groups, on public transportation, and in public indoor spaces, especially communities with high rates of COVID-19. The administration is making free rapid tests available by mail again for a limited time. All U.S. households can order a total of four at-home coronavirus tests from covidtests.gov starting the week of Dec. 19.

Officials say the new shots will help broaden immunity because their formulation includes a recipe against highly contagious versions of the virus - BA.4 and BA.5. When the shots were rolled out in September, BA.5 accounted for nearly 90 percent of cases in the United States.

Officials assessed the effectiveness of the updated booster by comparing people who had received two, three or four doses of the older vaccine as well as the updated booster to people who had only received the older vaccine and no new booster. The studies did not compare the effectiveness between the updated Moderna and Pfizer boosters.

One large study looked at data on 78,303 emergency room and urgent care visits and 15,527 hospitalizations from people in nine states. The second analysis looked at hospitalization data in about 800 people 65 and older in 20 states.

The two latest reports looked at patient data from September to late November, when the BA.5 variant was still dominant, Link-Gelles said. By mid-October, new variants emerged, making up between 5 to 15 percent of circulating virus. Still, all the circulating viruses are considered descendants of BA.5, and because they are so closely related, Link-Gelles said researchers were reassured that "vaccines are continuing to protect even against the new variants."

Going forward, experts worry the virus will continue to outrun the pace of new vaccine development.

Still, officials say the effectiveness of the updated boosters may be even higher than what the data show. The studies were not able to take into account someone's previous infection with COVID. The comparison group - people who haven't had a vaccine dose since last year - may have had several COVID-19 infections that gave them natural immunity, boosting their protection with additional virus-fighting antibodies.

It's still too early to know how long the protection from the bivalent booster will last because the updated shots were only rolled out in September. But officials expect COVID case numbers to rise by January and February.

"Unfortunately, we are heading into this COVID surge," Link-Gelles said, "and we will see more cases over winter break and as people come back from travel."

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Jacqueline Dupree contributed to this report.