New Zealand could have another locally approved vaccine available by August, with MedSafe expected to make a decision on Johnson & Johnson's shot within a fortnight.

But Prime Minister Jacinda Ardern said the Pfizer vaccine remained the predominant Covid-19 shot that Kiwis could expect to receive this year.

Director general of health Dr Ashley Bloomfield told a media conference today that an independent committee met this week to discuss the Janssen vaccine, and was seeking "further information" about it from the supplier.

"We're expecting that there will be a decision by MedSafe around approval within the next week or two," he said.

"And then that will be given to our vaccination technical advisory group to provide advice around the decision to use [it]," he said, adding that the final call was Cabinet's to make.

"And our intention is once that decision has been taken, we're expecting deliveries from August."

Janssen Pharmaceutica, a Belgium-based division of Johnson & Johnson, was one of the four vaccine providers with which New Zealand had signed in-principle purchase agreements – in this case covering up to 5 million vaccines.

Bloomfield said AstraZeneca was also in the approval process, namely because of issues around manufacturing, and Novovax was on a "slower timeframe".

Ardern reiterated that New Zealand had enough doses of its primary vaccine, Pfizer, coming into the country over coming months to offer to all of the eligible population.

She said agreements with each of the providers were made because, at the time, it wasn't clear which of them would be able to manufacture or complete approvals earliest

University of Auckland vaccinologist Professor Helen Petousis-Harris said the Janssen viral vector vaccine was widely used around the world, and had been approved by the US Food and Drug Administration (FDA).

Although all Covid-19 vaccines worked by training the body to recognise the new coronavirus - usually by spotting the spiky protein that coated it – they were made in different ways.

The Janssen vaccine, which was a "one-dose" shot rather than requiring two jabs, used a cold virus like a Trojan horse to carry the spike gene into the body, where cells made harmless copies of the protein to prime the immune system in case the real virus came along.

That was largely the same technology the company used in making an Ebola vaccine, and similar to Covid-19 vaccines made by AstraZeneca and China's CanSino Biologics.

The FDA said the Janssen vaccine offered strong protection against what matters most: serious illness, hospitalisations and death.

One dose was 85 per cent protective against the most severe Covid-19 illness in a massive study that spanned three continents — protection that remained strong even in countries such as South Africa, where the variants of most concern had spread.

There had been rare cases of a syndrome of blood clotting combined with low platelet counts – called thrombosis with thrombocytopenia syndrome – reported among people who'd been vaccinated, Petousis-Harris said the latest data showed the clotting risk was even lower than that of AstraZeneca's widely used shot.