The principal investigator who led the clinical trials of the meningococcal B vaccine says New Zealand was slow to act to stem the disease epidemic.

Professor Diana Lennon from Auckland University led the phase one and two trials of the vaccine and is conducting an independent study in Auckland on the its efficacy.

Because the introduction of the vaccine may have coincided with a waning of the epidemic Lennon says it may be hard to prove whether it worked.

She says a follow-up vaccination campaign may be needed in a few years for children who were very young when the first shots were given.

The Green Party has criticised the Ministry of Health for not conducting phase three trials of the drug before rolling it out to the population.

Deputy Director of Public Health Don Matheson answered questions about the vaccine before the health select committee on Wednesday.

Green's health spokesperson Sue Kedgley says she was disturbed to learn that the vaccine did not undergo phase three clinical trials.

Kedgley says such trials are usually considered essential to ensure a medicine's safety and efficacy and the Ministry of Health is introducing a new vaccine to more than a million children without the proper safety data.

Matheson said parents have been informed about the efficacy of the vaccine. He says very similar vaccines have been used worldwide and New Zealand consulted international experts before going ahead with it.

"To do phase three, you need large large numbers or a very very long period of time... internationally, it is well known that you do not need to do phase three, says Karen Poutasi of the Ministry of Health.