The Government has set aside nearly half a billion dollars for purchasing and rolling out another dose of the Covid-19 vaccine, and other Covid-19 treatments next year.

The money has been set aside for a possible fourth dose, if one is required, however Covid-19 Minister Chris Hipkins' office confirmed on Monday that a decision to roll out a fourth dose had not yet been made.

Budget documents published on Thursday include an estimate of $284.3 million for implementing the Covid-19 immunisation strategy, and a further $189.2m for the purchase of "additional Covid-19 vaccines from mRNA suppliers" in 2022 and 2023.

A fourth dose would be paid for from that pool of funding - however the Herald has been told the Government would also deploy surplus vaccine stock it already has on hand. This funding would take the Government's vaccine programme up to the end of the next fiscal year, in June 2023.

Some countries have begun rolling out a fourth dose of Covid vaccine. Currently, Israel and Germany offer a fourth dose to all adults. The UK rolled out a fourth dose to over-75s and other vulnerable groups in April.

A trial in the UK found fourth-dose Covid booster vaccines increase protection against Covid-19, particularly in people aged over 70. However, the same study found any short-term protection against infection was likely to fall away fast.

In late March, director-general of Health Dr Ashley Bloomfield said a decision on whether to offer a fourth dose would be made shortly.

He said the decision would consider whether there was "a good reason not to offer a fourth dose if there is evidence of waning immunity?"

"And there is some evidence of that," Bloomfield said, adding he would be looking overseas for evidence.

Officials are extending the time to look into whether New Zealand should roll out a fourth dose of the Pfizer Covid-19 vaccine for vulnerable groups and frontline health workers ahead of winter.

National's Covid-19 spokesman Chris Bishop said National supported spending on a fourth vaccination.

"We don't have any objections to the spending of the money on the fourth vaccine. Many other countries are rolling it out," Bishop said.

"My understanding is its likely to be 65 plus or with underlying health conditions, it's likely to be a voluntary," he said.

"It's a sensible thing for the Government to be doing. They should probably get on with it and give people some clarity," he said.

Earlier this month, Hipkins said he would have more to say about a fourth dose "in the next week or two".