A booster dose of the Covid-19 vaccine significantly reduces a person's odds of hospitalization from the omicron variant, new research released Friday by the Centers for Disease Control and Prevention finds.
The three new studies from the agency are among the first to look at the vaccines' impact against omicron in the United States, which now accounts for more than 99 percent of new cases in the country.
Full coverage of the Covid-19 pandemic
The research underscores the importance of booster shots to protect against severe illness from the rapidly spreading variant. Though cases are beginning to decline in some hard hit areas, such as in the Northeast, much of the nation's hospitals remain overwhelmed with Covid patients.
"Protection against infection and hospitalization with the omicron variant is highest for those who are up to date with their vaccination, meaning those who are boosted when they are eligible," said CDC director Dr Rochelle Walensky during a White House Covid Task Force briefing Friday.
"There are still millions of people who are eligible for booster dose and have not yet received one," Walensky said.
An NBC News analysis of data from the Department of Health and Human Services finds that Covid-related hospitalizations are up 35 percent within the past two weeks, and admissions to intensive care units are up 28 percent in the same time frame.
The new studies find that a booster shot can provide robust protection against needing emergency medical care or hospitalization because of Covid-19.
In one analysis of 259 hospitals and 383 emergency departments from late August through early January, a third dose of either the Pfizer-BioNTech or Moderna vaccine was found to reduce the odds of a hospital or emergency room visit by 94 percent during the delta wave, and by 82 percent once omicron started spreading.
The data included adults who'd received either the Pfizer or Moderna vaccines. The study authors were able to tease out the effects of the boosters during both the delta and omicron waves of Covid.
When delta was surging, the two doses of the vaccine were 86 percent effective against visits to emergency departments and urgent care centers for Covid illnesses. That effectiveness fell to 76 percent after 6 months, but a booster raised the effectiveness to 94 percent.
Earlier this month, the CDC and Food and Drug Administration greenlit a third, or booster, shot for everyone ages 12 and over, five months after their second dose.
While the new research shows a drop in protection against omicron for people who haven't received a third dose of the Moderna or Pfizer-BioNTech shots, the CDC doesn't plan to change the definition of what it means to be fully vaccinated, Walensky said Friday.
Instead, Walensky said the agency is "pivoting our language" to make sure people are "up to date" on their Covid vaccinations.
"If you recently got your second dose, you're not eligible for a booster," she said during the briefing. "If you are eligible for a booster and you haven't gotten it, you're not up to date."
Less than half of the U.S. population eligible for a booster dose has received one, according to CDC data.
Download the NBC News app for full coverage of the Covid-19 pandemic
The new research suggests that a booster dose is even more important now that omicron accounts for virtually all Covid cases in the U.S.: Against omicron, the vaccines' effectiveness for keeping people out of ERs and urgent care centers fell dramatically, to 38 percent 6 months after the second dose. A booster shot raised that level of effectiveness to 82 percent.
When researchers looked specifically at patients who needed to be admitted to the hospital during the omicron surge, two shots were 81 percent effective. That effectiveness fell to 57 percent in people who were 6 months out from their second dose, but then rose to 90 percent after a third dose.
Two additional studies, also published Friday by the CDC, suggest that boosters offered "significant protection" against both infection and symptomatic Covid, though that protection was higher during the delta surge compared with omicron.
Dr. John Sanders, chief of infectious diseases at Atrium Health Wake Forest Baptist in North Carolina, said the findings should be "reassuring" to the public, and reflect much of what he and his colleagues have noted in clinics.
"We have these conversations all the time, at church, at football games, at basketball games: 'Do I need that booster? Is it really going to help me?'" Sanders said.
"Now I can cite three very large real-world studies that confirm what we are seeing all the time," he said. "People who have gotten their booster are less likely to have a breakthrough, and when they do have a breakthrough, it's much milder."
Follow NBC HEALTH on Twitter & Facebook.