Vaccine boosters provide robust protection against severe disease from the omicron variant in the United States, according to three reports released Friday that use real-world data to show the shots are effective at keeping vaccinated people out of the hospital.

But the reports by scientists at the Centers for Disease Control and Prevention arrived late to the winter surge in coronavirus cases that have choked the corridors of hospitals across much of the country.

When omicron was first identified in late November and began spreading rapidly in the United States, millions of vaccinated people lined up for the extra shots. But that uptake has slowed significantly. Most people eligible for the booster shots, estimated at more than 86 million people, have not gotten them, according to the CDC.

Surveys by private polling firms suggest that uptake might be somewhat higher, and the CDC has acknowledged its data may be an underestimate. Still, despite a growing stack of scientific studies showing an extra dose jacks up antibodies to protect against severe disease and death, far fewer Americans have embraced booster shots than did the initial vaccine series.

The Biden administration continues to view vaccination uptake as critical to preventing severe disease and death in the wave of coronavirus infections driven by omicron. But even as that emphasis appeared vindicated by the new data, the policy received another blow Friday from a federal judge in Texas.

U.S. District Judge Jeffrey Vincent Brown, nominated to the federal bench by President Donald Trump, ruled the administration’s vaccine mandate for federal workers and contractors is an overreach of executive authority. The Justice Department filed an appeal to the ruling, and the case could be heard by the Supreme Court, although most federal workers are already vaccinated.

At a White House briefing Friday, top health officials continued to urge people to get boosters, citing the new data, and also highlighted a welcome trend – steep declines in daily cases in some parts of the country, including the Northeast, the Mid-Atlantic and parts of the Upper Midwest.

“The data here show the protection provided by vaccines and the importance of being up to date on your covid-19 vaccination, which for tens of millions of Americans means getting a booster dose,” said CDC Director Rochelle Walensky.

The number of hospitalized patients with confirmed or suspected covid-19 topped 160,000 earlier this week for the first time in the pandemic, however, though that number may be flattening. The seven-day average of covid-19-related deaths, meanwhile, topped 2,000 Friday for the first time since Sept. 27, when the delta variant was in ascent.

One of the new CDC reports analyzed data from hundreds of thousands of emergency room visits, urgent care visits and hospitalizations between August 2021 and Jan. 5, 2022. It showed that a third dose of either the Pfizer-BioNTech or Moderna shots reduces the chance of hospitalization by 90% compared to unvaccinated people, and reduces the chance of a trip to the emergency room by 82%. The data covered a period that includes three weeks when omicron was the predominant variant.

Since omicron became dominant, the protection from the two-dose vaccine regimen has declined, with the biggest drop among people six months past their second shot, said Mark G. Thompson, a CDC epidemiologist and lead author of the first report.

“That’s the bad news,” Thompson said. “The good news is that for people who received a third booster dose, the effectiveness of three doses is very high and protects against moderately severe and severe disease. This is among the first data that really shows the actual level of protection in the United States.”

Omicron has a gaggle of mutations that enhances its ability to elude the neutralizing effects of antibodies, the first layer of immune system protection produced by the shots, as well as by natural infections. Despite the protection against dire outcomes, boosters are not a guarantee against mild infections. Most omicron infections are asymptomatic or “mild” – meaning the person can recover at home.

Since September, federal health officials have urged people to get the third shots. CDC recommends booster shots for everyone ages 12 and older, five months after getting two doses of the vaccines made by Pfizer-BioNTech and Moderna, or two months after a single dose of the Johnson & Johnson vaccine. The messaging strategy changed in early January, when CDC officials said people should get three doses of Pfizer-BioNTech or Moderna’s vaccines to be considered up to date on their immunizations. They said that those who got a Johnson & Johnson vaccine should receive a second dose to be up to date, preferably a Moderna or Pfizer-BioNTech shot.

Health experts on Friday welcomed the new CDC data, saying they hoped it would encourage millions of vaccinated Americans to get booster shots.

“A booster is essential for preventing severe disease, hospitalization and deaths,” said Eric Topol, a molecular medicine professor at Scripps Research, referring to the findings. Public health officials need to communicate clearly that although the vaccines and booster shots are “not holding up against omicron infections, they are holding up the wall against severe disease . . . and that’s phenomenal,” he added.

It’s not clear why booster uptake has been relatively modest. Omicron is generally described as a milder variant, despite its extraordinary transmissibility. That fact, combined with general exhaustion with the pandemic, may sap the motivation of some people to take additional steps to protect themselves. As whole nations shift their strategy to one of “living with the virus,” many people may discount the need to follow health officials’ advice.

The booster issue has also been dogged by controversy within the scientific and regulatory community and among global health leaders. The World Health Organization has repeatedly called for prioritizing vaccinations in countries where shots are hard to come by, rather than giving boosters in wealthy countries that are already largely vaccinated. And last year, some scientists in the United States, including at the Food and Drug Administration and CDC vaccine advisers, were skeptical that boosters were necessary, since the initial doses continued to show good protection against severe disease and death.

Misinformation is also rampant in the United States, including the idea the vaccines don’t work at all. Former president Donald Trump was booed last year at a rally when he revealed he had gotten a booster shot.

The Biden administration made its booster recommendation last summer at a time when there were fewer studies and greater uncertainty.

“We have lacked this level of data. That’s one of the reasons why there was so much discussion of booster recommendations back in the summer,” said Joshua Sharfstein, vice dean at the Johns Hopkins Bloomberg School of Public Health and a former top official at the Food and Drug Administration.

“There’s uncertainty as decisions are made, and these recent papers are filling in the blanks,” he said. “The evidence is coming in, and it’s very strong.”

Sharfstein, who regularly volunteers at a Baltimore city vaccination clinic, said many people getting their first shots say they never got around to it until their job required it. Others, including those getting boosters, are scared of omicron because they know people who are sick, he said.

In January, with the omicron wave slamming the country, about 682,000 booster doses have been administered daily, according to the latest CDC data. That pushed up the share of vaccinated people who received a booster to 39%, up 6 percentage points. Just under half of the boosters went to people between 18 and 50. Some people are not yet eligible for the shots, however, because not enough time has elapsed since their most recent vaccine shot.

For vaccinated people older than 65, the age group most vulnerable to severe disease, more than one-third have not received a booster. Only 4% of that group received boosters so far this month.

Administration officials pushing boosters should refine their message, and target the large cohort of older people who haven’t had the extra shot, argues Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security.

“I think that the booster discussion has been sidetracked by a desire for a one-size-fits-all-strategy,” Adalja wrote in an email.

“When I work in the hospital, it is not people that lack boosters that are occupying ICU beds – it is those that lack first and second doses. If everyone eligible to be boosted was boosted, hospitals would still have capacity problems because of the lack of first doses in high-risk individuals,” he wrote.

Some experts say the messaging around the number of shots needed for vaccination has been confusing. When vaccines were first rolled out in December 2020, officials made a “communications blunder” when they said vaccinations would be “two shots, and done,” Andrew Noymer, an epidemiologist at the University of California at Irvine, wrote in an email.

This fall, many were confused as a result of complicated booster guidelines. It was not until Nov. 19 that all American adults became eligible for coronavirus vaccine boosters.

Many ordinary people may regard the push for boosters as a case of “moving the goal posts, and are wary of getting on the train of never-ending boosters,” Noymer wrote.

A second CDC report that looked at cases and deaths among unvaccinated and vaccinated adults from April to Dec. 25, 2021, found that when delta was the dominant variant, and in the initial days of omicron, people who were fully vaccinated and boosted had the highest protection against infection.

A study published in JAMA by CDC scientists also found that a third dose of an mRNA vaccine provides significant protection against symptomatic disease for both the delta and omicron variants, with greater protection afforded against delta infections.

The winter wave of coronavirus cases appears to have peaked nationally on Jan. 14, when the seven-day average of new daily cases topped 800,000. That average dropped below 722,000 on Friday, with the decline occurring in the areas that were earliest hit by omicron. As some experts predicted, omicron’s profile is a sharp spike and rapid decline. New York and New Jersey have seen cases drop 45% in one week.

States across the south are in varied stages of their own surges. Florida, Louisiana and Georgia are showing a drop in daily new cases compared to one week ago. Alabama, Tennessee, Kentucky and the Carolinas are either continuing to climb or their waves appear to be flattening. A few Midwestern states and all states west of the Mississippi have a higher seven-day average of new cases than two weeks ago.