Explained: The Novavax Covid-19 vaccination NZ has just approved as a booster

Regulator MedSafe has granted expanded provisional approval for Novavax's protein-based shot, Nuvaxovid, as a first or second booster in New Zealand. Photo / Novavax

New Zealand's regulator has approved the Novavax vaccine for use as a booster – while AstraZeneca's vaccine is now being phased out here. Science reporter Jamie Morton explains.

How many Kiwis have now been boosted?

According to the Ministry of Health, a total 2,716,998 first boosters have been administered, along with 459,874 second boosters that've been given to those eligible.

But just under half of Kiwis older than 65 have received that second top-up – and uptake among eligible people aged between 50 and 64 is even poorer, at only 18 per cent.

What's Novavax's booster?

Regulator MedSafe has formally given expanded provisional approval for Novavax's protein-based shot, Nuvaxovid, as a first or second booster for adults in New Zealand.

Novavax has been bookable as a booster for some time - and the vaccine itself was first given provisional approval back in February.

University of Auckland associate professor Helen Petousis-Harris, a vaccinologist, explained this vaccine works differently than Pfizer's.

That is essentially by directly providing the protein to combat the virus, rather than training our bodies to become their own "vaccine factories", as Pfizer's does.

MRNA-based shots like Pfizer's pack the genetic code for a crucial part of the virus called the spike protein – or small projections on its surface which it uses to infect cells.

On receiving an mRNA vaccine, our bodies read this genetic code and make copies of the spike protein, so it learns how to recognise and attack the actual virus upon infection.

Importantly, this genetic code becomes broken down, then is quickly and easily removed from our bodies – and all without modifying our DNA or creating any new genetically modified organism within us.

Novavax's protein-based vaccine also does not - and cannot - affect someone's DNA.

Instead of supplying the genetic code for the virus spike protein, it contains non-infectious proteins within it – and thus teaches our immune system to recognise it as a threat and begin building an immune response against it.

It also has an ingredient called the Matrix-M adjuvant, which helps create a stronger immune response to the vaccine.

Common side effects after receiving the Novavax vaccine – such as injection site pain and tenderness, muscle and joint aches, and tiredness - are similar to those of Pfizer's, and are mostly mild and go away within days.

Pfizer's mRNA shot has been New Zealand's Covid-19 vaccine of choice. Photo / Sylvie Whinray

While some Covid-19 vaccines have been linked with rare side effects of myocarditis and pericarditis, such information about Nuvaxovid is limited by comparison.

But Petousis-Harris - who is leading a world-first research initiative analysing global vaccine safety surveillance data - says there so far isn't anything about Nuvaxovid to cause serious concern.

How effective is this booster?

For those who've received three doses of the Pfizer vaccine, US data indicates this would have offered a high degree of protection against being hospitalised (82 per cent) or dying (90 per cent) with an Omicron infection.

However, the estimated level of protection against infection was much lower: at 31 to 67 per cent according to one study in England. Another major paper put the effectiveness of mRNA boosters against Omicron infection at around 50 per cent.

The next generation of mRNA boosters will target Omicron: Including the BA.4 and BA.5 subvariants that now account for more than 90 per cent of sequenced cases here in New Zealand.

Millions of Kiwis who've been vaccinated but also infected with Omicron also would've developed "hybrid immunity" - something Covid-19 modellers say likely helped blunt the impact of this year's winter wave.

While Novavax's own vaccine was found to be 90 per cent effective against mild, moderate, and severe disease in a phase three trial involving 30,000 adults, that was against the original Covid-19 strain (which Pfizer's vaccine worked much better against).

Lab tests suggest Nuvaxovid does have efficacy against Omicron, but real-world data from people vaccinated with it is lacking.

"We're really in a new place with Omicron – and the question is, how does this vaccine stand up to these variants?" Petousis-Harris said.

Why do we have more than one booster option?

Despite the Government going "all in" on Pfizer's Comirnaty vaccine for the national roll-out, Kiwis have had some options to pick from.

AstraZeneca's Covid-19 vaccine, which could also be used as a booster dose, worked slightly different to the Pfizer shot.

That was by delivering a short section of DNA containing the "viral vector" for the gene producing the spike protein to cells, where mRNA was then made and converted into spike proteins for the immune system to recognise.

While shown to be similarly effective against Omicron as Pfizer's vaccine, it appears relatively few Kiwis have opted for it.

TVNZ reported that just eight per cent of the country's 120,000-dose, $1.2 million supply has been used – and with stock about to expire, the vaccine will only be available until September 4.

Petousis-Harris said that, at the time the Government was trying to secure supplies last year, it made sense to have a "broad portfolio" of vaccines.

Novovax's shot also offered the public a "non-mRNA" choice – even though mRNA vaccines are held to the same rigorous safety and effectiveness standards as other vaccines, and have often proven superior.

The Immunisation Advisory Centre (IMAC) said Nuvaxovid would be an effective booster for people who'd experienced severe adverse events to their second dose of the Pfizer vaccine, "although it appears less immunogenic than Comirnaty as a booster dose".