The United States will begin moving away from describing people as "fully vaccinated" against Covid-19 in favour of new terminology to reflect the need for ongoing booster doses.
Many countries, including Australia, have either changed or left the door open to changing the definition of fully vaccinated from two doses to include a third or even fourth dose of a vaccine, as data reveals waning efficacy particularly after six months.
White House chief medical adviser Anthony Fauci said on Tuesday health officials were moving away from the term "fully vaccinated", but the Centres for Disease Control and Prevention (CDC) has reiterated the definition is not changing for the purposes of travel requirements or other federal guidelines.
"We're using the terminology now 'keeping your vaccinations up to date', rather than what 'fully vaccinated' means," Fauci said during a National Institutes of Health lecture on Tuesday, Bloomberg reported.
"Right now, optimal protection is with a third shot of an mRNA or a second shot of a J&J."
Fauci made the remarks in response to a question on whether the US would follow Israel and start to offer fourth doses.
"We need to find out what the durability of protection of the third shot is before we start thinking about the fourth shot," he said.
Last month, the federal government's vaccine advisory group, the Australian Technical Advisory Group on Immunisation (ATAGI), recommended Australians who refuse to get a third jab be stripped of their "fully vaccinated" status.
Australia this week shortened the interval for booster doses to four months. It was previously five months and before that, six months.
NSW Premier Dominic Perrottet on Friday announced that workers in sectors covered by mandatory vaccination policies such as healthcare would be required to receive a booster shot when they become eligible.
On January 31, the booster eligibility will again be changed to three months after the second dose.
ATAGI advised the government to bring forward the eligibility for booster doses to increase protection against the Omicron variant.
Pfizer claims a third dose of its vaccine produces a 25-fold increase in neutralising antibodies against the new strain, while Moderna claims its third shot produces a 37-fold increase.
In the US, the CDC is pushing for the booster dose interval to be shortened from six months to five months.
But CDC director Rochelle Walensky said the definition of fully vaccinated remained two weeks after the primary dose or doses.
"Individuals are considered fully vaccinated against Covid-19 if they've received their primary series," she said at a White House press briefing on Wednesday. "That definition is not changing."
However, Walensky said the CDC had updated its guidelines to include language on staying up to date with vaccines.
"Consistent with how public health has historically viewed or even talked about how we recommend vaccines, we are now recommending that individuals stay up to date with additional doses that they are eligible for," she said.