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 US 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 boosters 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, from August to this month.
It found vaccine effectiveness was best after three doses of the Pfizer or Moderna vaccines in preventing Covid-19-associated emergency department and urgent care visits. Protection dropped from 94% during the delta wave to 82% 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, some good news: A third dose was at least 90% effective at preventing hospitalizations for Covid-19, both during the delta and omicron periods, the study also found.
The second study focused on Covid-19 case and death rates in 25 states from the beginning of April through Christmas. 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 Centers for Disease Control and Prevention.
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 Dec. 10 to Jan. 1 at more than 4,600 testing sites across the US
Three shots of the Pfizer and Moderna vaccines were about 67% 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, the researchers found.
"It really shows the importance of getting a booster dose," said the CDC's Emma Accorsi, one of the study's authors.
Americans should get boosters if at least five months have passed since they completed their Pfizer or Moderna series, but millions who are eligible have not gotten them.
"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.