A leading New Zealand vaccinologist says the MMR vaccine, used to protect against measles, mumps and rubella "is extremely safe" with "a long track record of safety".
Auckland University vaccinologist Dr Helen Petousis-Harris spoke out today after two babies died in Samoa last Friday, after being injected with what was thought to be the MMR vaccine.
The first baby, a 1-year-old girl, died within three minutes of being injected by the vaccine by a nurse at Safotu Hospital in Savaii.
Two hours later, another family brought their baby, a 1-year-old boy, to the hospital for his vaccination.
According to TV1 Samoa, when the family heard about the first death they refused to let their son be vaccinated but allegedly the nurse did it without the parents' consent.
He died within a minute of being injected.
Dr Petousis-Harris said the deaths were highly unusual.
"There has never been a death associated with the administration of this vaccine in New Zealand," Dr Petousis-Harris told the Science Media Centre.
"Rarely a tragic event such as this occurs. There are two main reasons why something like this might happen."
She said that was either medical error - when the vaccine was prepared for injection incorrectly and the wrong substance was injected - or through contamination, leaving the vaccine at room temperature for too long.
The Samoan Government has now seized supplies of the MMR vaccine and is testing them. All hospital staff involved have been stood down.
Note: The MMR vaccine is recommended for all children in New Zealand, and the Ministry of Health says it has an "excellent safety record".