As of tomorrow, everyone in Aotearoa aged in their 30s or 40s is eligible for another booster shot – and it’s a new variety.

A good deal of the population will struggle to remember when they had their first Covid-19 booster (hint: if you were quick off the mark, it was probably more than a year ago). And with 11,258​ new cases of Covid-19 reported in New Zealand over the past week, the potential for winter surges looming, and our response to Covid-19 now pretty much relying solely on vaccines, many will be feeling like sitting ducks.

But from tomorrow, the eligibility criteria for booster vaccines expands, meaning more people who have had their two primary doses plus their first booster will be able to get a second one (and if you’ve already had more than one booster, you’ll be able to get an additional one). What’s more, anyone eligible for the booster will be given the newer bivalent vaccine, which replaced the existing booster at the beginning of March.

Here’s what you need to know.

What is a bivalent vaccine?

As Covid-19 has continued to spread, it’s evolved in a bunch of sneaky ways that allow it to transmit more easily and better evade pre-existing immunity given by vaccination and prior infection.

To keep up with this viral evolution, newer “bivalent” vaccines have been developed to target both the original strain of Covid-19 and the more advanced immunity-dodging variants. Bivalent vaccines work by combining two strains of a virus, which prompts the body to create antibodies against both strains, providing a greater level of protection.

Pfizer’s latest bivalent vaccine is one of these, including a component of the original strain of SARS-CoV-2 plus an omicron strain – in New Zealand’s case, Pfizer’s BA.4/5 vaccine will be used. Both BA.4 and BA.5 rose to prominence in 2022, overtaking previous subvariants like BA.1 and BA.2. 

The virus has evolved since 2022 though. How well does the bivalent vaccine protect against newer strains?

Yesterday, Stuff reported that the extremely transmissible XBB.1.5 strain of omicron – terrifyingly nicknamed “Kraken” – was now the most dominant variant in Aotearoa, having been detected first here in early January. Even though the bivalent vaccine was not designed to target newer strains of the virus like this one, it still provides some protection against them because of how close their sequences are to the earlier omicron variants.

In February, Covid modeller Michael Plank explained how the bivalent booster can still be effective. “Vaccines train our immune system, a bit like showing it a photo of the virus so it can recognise the real thing when it comes along. Unlike the original Pfizer vaccine, the bivalent booster contains an omicron-specific component. Although it’s not exactly the same as the currently circulating variants, it’s like a photo of the virus’s twin brother or sister rather than a distant cousin. This means our immune system will be better prepared to fight off current variants and future variants that are likely to be closely related.”

University of Auckland vaccinologist Helen Petousis-Harris was slightly more tepid in her assessment, saying, “procurement of a bivalent Covid-19 vaccine is a logical move at this stage of the pandemic but these vaccines are not game-changers”.

“The bivalent booster vaccination does appear to have an edge over the single-valent version, but not an awful lot,” said Petousis-Harris. “It has shown benefit in reducing infections and hospitalisations, and has greatest impact in those at increased risk, including elderly people and those with a medical risk condition.”

How long have bivalent vaccines been around for?

Globally, Covid-19 bivalent vaccines have been available for a while. In August 2022, the United States authorised a Pfizer bivalent vaccine for booster doses for anyone 12 years and older. In September 2022, the United Kingdom, Australia and the European Union registered versions of the bivalent vaccine as booster shots too.

In New Zealand, Pfizer submitted an application for provisional approval of the BA.4/5 bivalent version of its Covid-19 vaccine in October last year. In December, Medsafe granted provisional approval of the bivalent vaccine in Aotearoa.

The Pfizer BA.4/5 Covid-19 bivalent vaccine replaced the existing monovalent Pfizer booster on March 1, meaning that anyone eligible for their first booster (essentially, people 16+ who hadn’t yet had one) or second booster (people at increased risk of severe illness from Covid-19 – more on that below) since then will have received this vaccine.

So what else is changing?

Up until now the only people who have been eligible for more than one booster shot are those aged 50+, Māori and Pacific people aged 40+, residents of aged care and disability care facilities, severely immunocompromised people who have already received four doses, people aged 16+ with certain comorbidities, disabilities or medical conditions, and health, aged care and disability workers who are 30+.

From tomorrow, April 1, the eligibility will be expanded, meaning anyone aged 30 years or over (who has had their two primary shots plus their first booster) will be eligible to receive the bivalent booster vaccine as long as it has been at least six months since their last Covid-19 booster or positive Covid-19 test. People at higher risk of severe illness from Covid-19 will also be able to receive an additional booster, regardless of how many doses they have previously had.

Before the broadening of eligibility, 47.8% of people aged 50 years and over who were eligible for the second booster had received it.

So who still isn’t eligible for a second booster?

Anyone aged 16-29 who is not at increased risk of severe illness from Covid-19. If you are otherwise eligible, but have had a Covid-19 vaccine or positive Covid-19 test within the last six months, you’ll have to wait a little longer.

I’ve already had one booster – what’s the benefit of getting an additional one?

“Some viruses we remember forever, and some viruses we forget really quickly, whether it’s through vaccination or natural infection,” virologist Natalie Netzler told The Spinoff last year. The reason Covid boosters are useful is because “the Covid-19 virus is one of those viruses that our body would naturally forget quite quickly”. Infection offers some protection, but vaccination and boosters offer even better protection, she explained. The main reason for this is that you don’t have to get sick and expose yourself to the risks that infection comes with to gain protection. 

Are additional boosters for people 30+ available in other countries?

In some places, yes. In Australia, for example, over-30s have been eligible for a second booster since July 2022. When expanding eligibility, Australian health officials said those in the younger age bracket can choose “to reduce their risk of infection” but added that it would likely only have a “limited” overall impact on population-wide transmission. 

How do I book my booster?

You can make an appointment to get your booster through all the usual channels, including Book My Vaccine, and the Ministry of Health is encouraging people to book their flu vaccine alongside it.

To check when you had your last vaccine and see whether enough time has passed for you to get another one, head to My Covid Record.