How much protection will NZ's four Covid-19 vaccines give us? Will regular shots be needed? Will they work differently in Maori and Pacific people? And does our virus-free environment have implications for immunity? Jamie Morton explains.
How do the vaccines actually give us immunity from the coronavirus?
Similarly, yet differently.
New Zealand has pre-purchased four vaccines - from Pfizer and BioNTech; Janssen Pharmaceutica; Novavax; and AstraZeneca - and they're based on various technologies.
But they're all the same in that they're based on the Sars-CoV-2 virus' spike protein.
That's the pointy part of the virus particle which enables the virus to enter cells in our body and cause infection.
"In the vaccines, the spike is presented on its own," University of Auckland immunologist Associate Professor Nikki Moreland explained.
"That means our bodies see the critical part of the virus, the 'key' to the door if you like, without the rest of the virus particle.
"This makes the vaccines very safe with zero chance that the vaccine itself can cause infection."
The flu shots we typically get each year are "inactivated" vaccines - or those that don't contain any live viruses.
Instead, a virus that has been rendered inactive by special treatment is introduced into the body, allowing the immune system to learn from its antigens how to fight live versions of it in the future.
The Oxford-AstraZeneca "viral vector" vaccine works slightly differently, by using a virus that has been genetically engineered so it can't cause disease - but carries the instructions for our body to produce coronavirus proteins to safely generate an immune response.
The Novovax vaccine is a "protein" vaccine and uses only harmless fragments of the virus shells that mimic the Covid-19 virus, to safely generate an immune response.
The Pfizer-BioNTech vaccine is more sophisticated, and part of a new generation of shots called mRNA vaccines.
These teach our cells how to make a protein, or even just a piece of a protein, that triggers an immune response inside our body.
That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our body.
Covid-19 mRNA vaccines give instructions for our cells to make a harmless piece of the spike protein.
The Janssen vaccine, meanwhile, is a "recombinant vector vaccine" that uses a human adenovirus to express the virus spike protein in cells.
How much immunity will we actually get?
Companies which make the vaccines say they are between 90 to 95 per cent effective.
We don't know exactly how long that immune protection from the vaccines will last, as the first large-scale vaccine trials only took place in the second half of last year.
"This data is being gathered in real time by following those who have been vaccinated to see how long their antibodies, immune memory and protected from infection lasts," Moreland said.
Two doses are required for some vaccines to boost the immune system and strengthen that memory response.
"When some of the vaccines were first tested, a moderate immune response was seen after one dose, but the response was much stronger after the second dose."
There have been suggestions we might need regular shots, but that's far from clear, too.
"It will be informed by ongoing studies to track those vaccinated last year and see how long they remain protected," Moreland said.
University of Auckland vaccinologist Associate Professor Helen Petousis-Harris doesn't expect Covid-19 shots to become an annual requirement, like flu jabs.
"But we should be open to the possibility for needing at least another," she said.
"Over time, I think the chances of future vaccines goes down as we have fewer cases."
What about herd immunity?
Director general of health Dr Ashley Bloomfield has indicated a vaccination target of 70 per cent of the population, which experts say would be at the lower end of the threshold to achieve herd immunity against the virus.
Petousis-Harris said this still depended on other virus-beating measures.
"I've heard a lot of numbers bandied around - like the 70 per cent figure - but it's all based on that magical R0 number."
Simply put, R0 - also known as the basic reproduction number - represents the average number of people infected by one infectious person.
"When we put masks on, and when we do social distancing, for instance, the R0 number goes down. When you take these away, it goes up. So the estimates are all over the place."
Another factor to consider was new viral variants, such as the B.1.1.7 or UK strain, that was at the centre of Auckland's Valentine's Day cluster.
While there was no evidence yet to suggest the four vaccines that New Zealand has pre-purchased won't work on new variants, it was still possible further shots or vaccines might be required to combat them.
Initial studies indicate the Pfizer-BioNTech vaccine - the first to be rolled out in New Zealand, among high-priority groups - and the AstraZeneca shot should still work on the UK variant.
Like the other shots, the Novavax and Janssen vaccines may have a slightly lower - but still relatively high - efficacy against the variant.
One early analysis of the AstraZeneca vaccine has, however, been shown to offer reduced protection against low to moderate infections of the South African variant - prompting South Africa to suspend its roll-out.
And some cases of the South Africa variant have been shown to share the same mutation - called E484K - with the UK variant.
Are there any factors that make New Zealand different?
New Zealand is obviously unique in that the virus hasn't been swirling among our population for the past year.
That was top-of-mind for Moreland and her colleagues when they carried out a recent study into 112 previously-infected New Zealand patients, the bulk of whom suffered mild symptoms.
Their results showed the antibodies within them persisted for up to eight months after infection.
While this had been shown in overseas studies, this was the first time the long-lingering effect had been proven in an environment where Covid-19 had been wiped out, which clearly boded well for the vaccine roll-out.
Moreland pointed to another important fact.
"In clinical trials, the vaccines have produced levels of antibodies and immune responses that are generally in line or even higher to those seen after a Covid-19 infection," she said.
"So there is no reason to think that our levels of immunity might be affected by a lack of circulating virus."
Petousis-Harris said there were still individual-level factors to consider.
"There are some vaccines where you can see more variability between individuals, and in some people, they don't always respond well," she said.
"But what I'm seeing with the RNA vaccines, specifically, is just about everyone who gets them, gets a really good immune response."
Does ethnicity matter?
That also remains to be seen, but past research on other vaccines by Petousis-Harris' group suggests it might.
"For instance, we've done some work looking at injection site reactions to one vaccine - not adverse effects, just factors like the size of swelling - and it differs by ethnicity quite significantly," Petousis-Harris said.
"We found that there was a bigger reaction in New Zealand European and Māori, and less in Pacific people.
"But we've done other research that shows the impact of vaccines, and found a greater reduction in disease among Pacific and Māori.
"I've also seen data showing a better antibody response for one particular vaccine among Māori and Pacific people, compared to New Zealand European.
"So difference doesn't necessarily mean it's negative - it can actually be positive."