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In a statement this morning, Covid-19 Response Minister Chris Hipkins confirmed people would be able to get their second booster dose six months after their first.
He said this fourth dose would cover several hundred thousand people in vulnerable groups, including elderly, aged care residents, and those in disability care facilities.
Severely immuno-compromised people who received a third dose as part of their primary course, as well as a first booster, would also be eligible for another, fifth, vaccination.
Hipkins said the legislation enabling all this - an amendment to the Medicines Act - was expected to be passed and take effect in mid-June, with eligibility criteria to be finalised over the next fortnight.
However, with the six-month gap, most would become eligible from July. People who had been infected with Covid-19 should also wait three months from the end of their infection before getting another dose.
The government had for months signalled it planned to make a fourth dose available.
Director-General of Health Dr Ashley Bloomfield had previously talked about the possibility of a second booster, and referred to advice that the timing of it may need to be managed to ensure peak immunity during the winter months when the threat of a second wave alongside other winter illnesses would be highest.
However, the government was waiting for advice from the Covid-19 Vaccine Technical Advisory Group. Hipkins said the group had now recommended the second booster dose, having found it could be beneficial for those most at risk of serious illness.
"Based on current advice, for those who are not at risk of severe illness from Covid-19, a two-dose primary course and a booster dose provides very good and lasting protection, which is why we can be more targeted in the rollout of the second booster," he said.
The advisory group would continue to review new information about Covid-19 and make further recommendations as needed, Hipkins said.
"And if you haven't had your first booster yet, it is not too late to get it in time for the winter season," he said.
Royal College of GPs medical director Bryan Betty said introducing a fourth dose was the right thing to do, based on all the evidence locally and internationally.
"We'll still be in winter. I think Covid will be around and looking at how at the antibody response or the response to the previous vaccine tends to drop a little bit over time [it's] certainly the right time to do it."
Betty said the ongoing protection of the country's most vulnerable was the key to fighting the Covid-19 pandemic.