Health Minister Greg Hunt confirmed 25 million doses had been ordered from the biotech giant, but the way these vaccines will be used signals a new front in Australia's vaccine strategy.

The Moderna vaccine is an mRNA vaccine, using similar technology to the Pfizer vaccine. Manufactured in the United States, it is also a two-dose vaccine but with a four-week interval between injections.

Because of the technology Moderna uses for the jab, it is more easily adaptable to be tweaked to protect against more variants of the COVID-19 virus, which is a key reason why the Australian government made the decision to sign up.

This is the first vaccine the Australian government has ordered with the purpose of using it as a booster for other shots.

Of the 25 million doses ordered, 10 million are set to be delivered at the end of this year, while another 15 million are set to be delivered next year as booster shots.

Health Minister Greg Hunt said the Moderna vaccine doses would serve two purposes.

"Firstly, as a reserve supply for this year if other elements of the supply chain were to run into any challenges. Secondly, they are our foundation of a booster and variant strategy," he said.

"Moderna is, on the advice that we have, the most advanced of the vaccine products with relation to the capacity to adapt to booster or variant requirements."

While the government hasn't said it had put away the chequebook when it came to buying vaccines, we haven't had an announcement like this in a while. And that's not because this vaccine is new, or has only recently been proven.

Moderna has been in use in other countries for months, and has shown good efficacy levels against COVID-19.

But the government didn't chase a deal with the company originally, wanting to sign up only for the Pfizer mRNA vaccine. Until now, mRNA technology had shown promise, but hadn't successfully been used in a vaccine.

So why was Pfizer chosen over Moderna? Pfizer was a more well-established company and seen as a better bet in the early days when deals were being made before particular vaccines were proven.

Yes, Australia now has more than 170 million vaccine doses on order, and even though three of the vaccine types on order need two doses, that is significantly more than the 26 million people in Australia.

The government is trying to avoid having all its vaccine eggs in one basket - so far we have seen the University of Queensland's vaccine candidate be shelved, and the blood clot problems for the AstraZeneca vaccine have limited the ways in which it can be used in people under the age of 50.

Of the millions of vaccine doses on order, 53 million are also the Novavax vaccine, which is yet to be approved in Australia, or any country in the world. Despite promising results from clinical trials, the company this week warned the process was delayed and doses would be more likely to be available towards the end of this year.

The government needs to order more doses than there are people to be prepared for supply issues, changes in medical advice and the possibility Australians will need booster shots.

"Whilst we know we have more than enough vaccine already ordered to cover our primary vaccination program this year, this provides some additional redundancy, which we've always sought," Health Secretary Brendan Murphy said.

Yes it does. While there is good news on the efficacy of the two main vaccine types in use in Australia so far, we don't yet know how long they are effective and if we will need booster shots. The announcement this week makes it seem like the Australian government is getting prepared for the vaccine rollout to stretch into 2022 with third shots needed.

Professor Murphy said the government is closely watching the results of trials overseas where vaccine types are "mixed and matched" to see whether that produces a better immune response.

"The trial data is not out yet. There's some trials being done in the UK, looking at AstraZeneca first dose and Pfizer or Moderna second dose," he said.

"There's no good scientific reason why you can't boost with one and then have another vaccine later. In fact, there may be some benefit in doing that because you might get some, particularly for the adenovirus vaccines, you might get adenovirus antibodies that might reduce efficacy for a booster dose."

Professor Murphy said the investment in the Moderna vaccines was assuming it would be possible for a person to have more than one type of vaccine.

"We'll be looking at those studies when they come out and we have sufficient redundancy of supply to adjust our strategy according to what those data show," he said.

The advantage of ordering 15 million doses in 2022 also means that the vaccine that arrives may be updated to deal with more variants of the virus. It means Australians who are already vaccinated, or will be soon, may benefit from the Moderna deal.