Doctors can make the medical decisions for the four-month-old baby, a judge at the High Court in Auckland ruled.

New Zealand's health authority, Te Whatu Ora, went to court seeking temporary guardianship of the boy in order to authorise the use of vaccinated blood during the operation.

The boy, who is in hospital, has a congenital heart defect and needs life-saving surgery.

His parents said they were unwilling to go through with the surgery unless there was a guarantee the blood would come from donors who had not been vaccinated for COVID-19.

They said they were concerned about the possible health impacts of their child receiving blood from vaccinated donors.

Otago University immunologist James Ussher told news website Stuff the blood posed "no safety concerns" and said while there would be antibodies in the blood it was "exceedingly unlikely" any vaccine would be present.

The boy's parents said they had other willing donors lined up who had not been vaccinated against COVID-19.

However, the boy is likely to need plasma products which are formed from pooling blood from various donors. Doctors said it was "simply impractical" to get this from directed donors, news website Stuff reported.

New Zealand's blood service argued that allowing the parents to handpick the donors would set a dangerous precedent for patients demanding to choose where their blood came from.

The court granted Te Whatu Ora's application, meaning the surgery can go ahead using blood from vaccinated people.

Guardianship was given to the court and two medical specialists were made agents of the court so they can make decisions about the boy's surgery. The boy's parents will retain guardianship in all other matters.

The guardianship will last until the boy has recovered from his surgery, likely in January.