Experts welcome the idea of a shorter gap between both doses of the Pfizer vaccine, particularly for older adults, but also urge people not to feel rushed.

The Government has updated its vaccine advice. It is now asking people to consider a three-week gap between doses, instead of six weeks, in a bid to get "more people fully vaccinated sooner, increasing our community immunity".

It said the standard gap between doses is now three weeks or more.

It's a change from advise issued on August 12 by director general of health Dr Ashley Bloomfield, which told people to extend the gap between doses from three weeks to six weeks.

"This allowed us to give one dose (partial protection) to a larger number of people faster," the Unite against Covid-19 website states.

"Since then, the Delta outbreak has increased the risk of contracting Covid-19 for everyone in New Zealand, no matter where you are in the country."

All New Zealanders are now advised to consider a shorter gap.

Medical adviser at the Immunisation Advisory Centre, Professor Peter McIntyre, said it was a particularly good idea for older adults because their immune system was not as "robust" as younger adults.

But people under the age of 30 shouldn't feel rushed to get the second dose, he said.

"If you're a younger person, you'd be better off sticking with that six-week recommendation.

"Particularly in that younger teenage group... you have relatively more benefit in just waiting a bit because, in many cases, you would have a robust immune response already.

"You'll be well protected even after one dose."

McIntyre said the Government has altered its advice in order to get "everyone's immunity as high as possible, as quickly as possible", but this is "much less of an issue for the younger people, particularly if they had a strong response to the first dose, than it is for the older adults".

In terms of the effectiveness of the vaccine with two doses three weeks apart, compared with six weeks apart, McIntyre said information is limited.

"We don't have a lot of information about what six weeks can look like compared to three weeks.

"What we do have is that more direct information about what your immune response looks like."

University of Auckland vaccinologist Associate Professor Helen Petousis-Harris said levels of antibodies had been a reason to wait six weeks between doses, but the importance of that was disputed.

"If you're measuring antibodies, for example, you see slightly higher levels after a longer gap, but there is also an argument about what that means, and how meaningful that is, because what you get after three weeks is excellent anyway."

Petousis-Harris said her current advice, with the Delta outbreak ongoing, is to bring the gap between doses forward from six weeks to three.

"I'm suspecting our freedom and easing of these restrictions is going to be very dependent on how many people are vaccinated, both with one dose and with two.

"The more [people] who have had two doses is going to mean lower risk of the community in terms of the virus."

But she also said no one should feel rushed to book their second dose.

"If they've got one dose and they're booked [for a second], and they would like to bring it forward then that's absolutely fine.

"If you'd rather wait, if you feel better about leaving it, then you should do so."

She said by bringing the second dose forward, more people will be fully vaccinated quicker.

"Three weeks incurs the advantage of you being fully vaccinated as soon as possible.

"The six-week [gap] might incur a slight advantage in terms of immunity, but I don't think there's really enough to spend a lot of time losing sleep over it."

More than 50 per cent of New Zealanders eligible for the vaccine have received their first dose, and 80 per cent have received their second.

New bookings made through the online system Book My Vaccine or via the vaccine Healthline will default back to three weeks between doses.