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The finalised groups of people eligible for a fourth dose of the Pfizer vaccine are expected to be announced by Director-general of health Dr Ashley Bloomfield on Thursday.
Without the law change people would have needed to access a GP to get a second booster.
The amendment to the Medicines Act enables voluntary booster doses to be administered without a prescription.
It comes after the Ministry of Health's Covid-19 Vaccine Technical Advisory Group recommended that people who were at high risk of getting very sick from a Covid-19 infection should have a second booster six months after the first.
Health Minister Andrew Little said the changes to the Medicines Act were a more enduring way to manage the administration of vaccine boosters from now on.
"The changes we have made today mean that the director-general of health now has the ability to make boosters available to those who need them, meaning people don't need an individual prescription to get one."
Advice from the Technical Advisory Group also proposed eligible groups should be people aged 65 years and over, Māori and Pacific peoples aged 50 years and over, residents of aged care and disability care facilities and severely immunocompromised people who have received a three-dose primary course and a first booster.
The proposal to reduce the age of eligibility for older Māori and Pacific peoples recognises that they have been disproportionately affected in the current Omicron outbreak and are at greater risk of hospitalisation and severe disease from Covid.
Little previously said the changes would apply to about 850,000 people at high risk of getting very sick from Covid-19. Majority of those would be eligible from July, about six months after receiving their first booster.
It would be particularly important heading further into the winter season, he said.