The federal government will reduce the interval between people's second and booster COVID-19 vaccine to four months following advice from The Australian Technical Advisory Group on Immunisation (ATAGI).
Health Minister Greg Hunt said on Friday the interval, currently at five months, will be brought forward to four months from 4 January and to three months from 31 January.
"On the basis of advice of the Australian Technical Advisory Group on Immunisation, it's no surprise we will be bringing forward the eligibility for the booster dose to four months as of 4 January," he said.
"Currently that means that we will go from about 3.2 million people who are eligible today to approximately 7.5 million who will be eligible as of 4 January. That means that the cohort has expanded.
"It will be expanded again on 31 January to three months and that will take it out to 16 million Australians who will be eligible at that point in time."
ATAGI had recently advised reducing the interval from six months to five months, and has been undergoing constant review of international evidence, Mr Hunt said.
"These dates have been set out of an abundance of caution to give Australians early continued protection," he said.
He said on the advice of ATAGI, jurisdictions can bring forward the reduced booster eligibliity based on these dates if they have "the capacity" to do so.
Chief Medical Officer Professor Paul Kelly said changes in the booster program come down to protecting those at most risk of severe disease, and responding to emerging information about vaccine protection and the new Omicron variant.
"We know two key things. One is that there is a diminished [protection] but not completely - the protection does not completely disappear in relation to severe disease from the first two doses of the vaccine. It wanes over time, hence the time-based change in the boosters," he said.
"The positive effect of that is that we do know now that boosters do protect and increase that protection against severe disease once that third dose is given or a fourth dose in the case of immunocompromised people."
Professor Kelly said the second reason is because boosters at this stage can help stem transmission of the virus.
"It will be an important and is already becoming an important part of our control of the current Omicron wave in Australia," Professor Kelly said.
"It is not enough by itself, the public health and social measures that been introduced in many states in the last few days will assist with that as well."
States and territories are reintroducing restrictions and updating how they trace and isolate positive cases as infections rise across the country.
