Three US studies offer more evidence that third dose of COVID vaccine is highly effective against Omicron.

Booster shots of the Pfizer-BioNTech and Moderna vaccines have proven highly effective at preventing Omicron-related hospitalisations, according to three new studies by the US Centers for Disease Control and Prevention.

The booster doses were 90 percent effective at keeping people out of hospital after they had become infected with the Omicron variant.

The doses were also 82 percent effective at preventing emergency department and urgent care visits, data indicated.

“It really shows the importance of getting a booster dose,” the CDC’s Emma Accorsi, one of the study’s authors, said on Friday.

“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.”

The research comprised the first large US studies to look at vaccine protection against Omicron, health officials said.

Previous research

The papers echo previous research – including studies in Germany, South Africa and the United Kingdom – 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 hospitalisations and emergency room and urgent care centre 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 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.

President Biden holds up his sleeve to receive his COVID-19 booster jab [File: Kevin Lamarque/Reuters]

The second study focused on COVID-19 case and death rates in 25 states from the beginning of April through the end of December.

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.

Third study

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.