The Journal of the American Medical Association published the third study, also led by CDC researchers.
It looked at people who tested positive for COVID-19 from December 10 to January 1 at more than 4,600 testing sites across the US.
Three shots of the Pfizer 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, researchers found.
“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 on Friday.
