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".