Three studies released Friday offered more evidence that COVID-19 vaccines are standing up to the Omicron variant, at least among people who received booster shots.

They are the first large U.S. studies to look at vaccine protection against Omicron, health officials said.

The papers echo previous research, including studies in Germany, South Africa and the UK, indicating available vaccines are less effective against Omicron than earlier versions of the coronavirus, but also that booster doses rev up virus-fighting antibodies to increase the chance of avoiding symptomatic infection.

The first study looked at hospitalizations and emergency room and urgent care center visits in 10 states in the U.S. from August 2021 to this month.

It found vaccine effectiveness was best after three doses of the Pfizer-BioNTech or Moderna vaccines in preventing COVID-19-associated emergency department and urgent care visits. Protection dropped from 94 percent during the Delta wave to 82 percent during the Omicron wave. Protection from just two doses was lower, especially if six months had passed since the second dose.

Officials have stressed the goal of preventing not just infection but severe disease. On that count, the study also found a third dose was at least 90 percent effective at preventing hospitalizations for COVID-19, both during the Delta and Omicron periods.

The second study focused on COVID-19 cases and death rates in 25 states in the U.S. from the beginning of April through Christmas, 2021. People who were boosted had the highest protection against coronavirus infection, both during the time Delta was dominant and also when Omicron was taking over.

Those two articles were published online by the U.S. Centers for Disease Control and Prevention (CDC).

Led by CDC researchers, the Journal of the American Medical Association published the third study that looked at people who tested positive for COVID-19 from December 10 to January 1 at more than 4,600 testing sites across the U.S.

It found that three shots of the Pfizer-BioNTech and Moderna vaccines were about 67 percent effective against Omicron-related symptomatic disease compared with unvaccinated people. Two doses, however, offered no significant protection against Omicron when measured several months after completion of the original series.

"It really shows the importance of getting a booster dose," said the CDC's Emma Accorsi, one of the study's authors.

As of now, millions of Americans who are eligible have not gotten a booster shot.

"If you are eligible for a booster and you haven't gotten it, you are not up to date and you need to get your booster," CDC Director Dr. Rochelle Walensky said during a White House briefing Friday. 

(With input from AP)