The coronavirus pandemic has driven unprecedented innovation when it comes to vaccines.
Previous efforts to bring vaccines to market took years, if they were successful at all - but in less than a year after the SARS-CoV-2 virus was first brought to the World Health Organization's attention, several vaccines against it were developed.
"There's never been this level of global collaboration amongst scientists and governments in vaccine development," the New Zealand Ministry of Health website says. "This has improved the speed of its development and the launch of clinical trials around the world."
And even more surprisingly, both trial and real-world data suggests many of them might work even better than the vaccines we have for diseases that have been around for a long time.
So which are Kiwis getting? How do they work? And what's the evidence for and against each of them?
Pfizer/BioNTech
The first to get approval from Medsafe in New Zealand is Cominarty - also known BNT162b2 or simply the Pfizer vaccine.
Developed by US pharmaceutical giant Pfizer and German biotechnology company BioNTech, Cominarty is an mRNA vaccine - the first of its kind to ever get approval for use in humans outside of trials.
mRNA vaccines work by teaching the body's cells how to make the SARS-CoV-2 spike protein, which it uses to infect our cells. On seeing the spike protein, our immune system knows it shouldn't be there - so destroys it, but remembers what it looks like for next time. (An in-depth, yet easily understandable explanation of how it works written by microbiologist Siouxsie Wiles was published by The Spinoff in February - check it out here.)
The vaccine contains no trace of the original virus at all - the mRNA it contains, short for messenger RNA, is synthetic. The mRNA never enters the cell's nucleus, where our DNA resides; the instructions it contains are read and carried out in the cytoplasm, which surrounds the nucleus.
Cominarty had spectacular results in its clinical trials late last year, stunning the world with 95 percent efficacy - a performance that's been replicated in the real world, based on analysis done in Israel and the UK, where it's among the first vaccines being rolled out.
This vaccine is the one our border force and frontline health professionals have been receiving. We've ordered 1.5 million doses so far, according to the Ministry of Health website - enough for 750,000 people. While a single dose does appear to offer some protection, it was designed to be a two-dose vaccine.
mRNA vaccines, manufactured chemically rather than biologically like traditional vaccines, can be updated quickly if variants emerge that reduce the existing formula's efficacy. The jury is still out on whether the first version of Cominarty is as effective against mutations picked up in South Africa and the UK, for example, as the original strain - but the evidence to date is promising.
The difficulty with mRNA vaccines like this one is they need to be kept at extremely low temperatures or they lose their effectiveness.
The most common minor side effects are pain, headache, muscle aches, fatigue, chills, fever, joint pain and nausea - but serious reactions are "very rare", according to the Ministry of Health.
So what are the other vaccines we're getting?
Janssen Pharmaceutica
New Zealand has signed an agreement in principle to buy 5 million doses of Janssen Pharmaceutica's single-shot COVID-19 vaccine, known as JNJ-78436735, Ad26.COV2.S or simply the Johnson & Johnson vaccine, named after Janssen's better-known parent company.
The agreement was signed in November before the effectiveness of the vaccine was proven. Its initial trial data suggested 85 percent efficacy against severe disease, which although lower than that of Pfizer's, was still 100 percent effective at preventing death four weeks after being delivered. Its trial also was done after the emergence of more virulent and deadly strains of the virus, unlike Pfizer's vaccine, which only had to contend with the original strain of COVID-19.
The Johnson & Johnson shot is a human adenovirus viral vector vaccine. This type of shot uses an actual virus to deliver instructions on how to make the coronavirus spike protein - but it's been modified so it can't replicate itself or cause illness.
Adenovirus viral vector vaccines have been researched for decades, and Johnson & Johnson recently used the technology to make an Ebola vaccine.
The adenovirus latches itself onto our cells, which pull it in. It finds the nucleus and puts its DNA in. The nucleus reads the DNA and produces mRNA, which exits the nucleus. The cell then reads the mRNA and produces the coronavirus spike proteins, some of which end up on the outside of the cell - triggering the body's immune system.
The Johnson & Johnson vaccine doesn't need the kinds of low temperatures the Pfizer one relies on, making it easier to store and transport. It can spend three months in a normal fridge and still be used.
Since New Zealand signed its agreement, the Johnson & Johnson vaccine has been approved for use in the US - the third behind Pfizer and rival mRNA vaccine manufacturer Moderna.
According to US regulators, side effects include "pain at the injection site, headache, fatigue, muscle aches and nausea", which lasted one to two days.
Novavax
The third vaccine New Zealand has bought is 10.7 million doses of NVX-CoV2373, produced by American company Novavax.
Like the Johnson & Johnson vaccine, NVX-CoV2373 can be stored in a refrigerator and takes two doses to reach its full effectiveness.
Novavax calls it a "recombinant nanoparticle vaccine". Unlike the others discussed above, it doesn't trick the body into making the coronavirus spike protein - instead it's "made up with proteins from the virus already attached to a carrier, and these trigger the immune system directly", according to University of East Anglia professor in medicine Paul Hunter.
The spike proteins are grown in moth cells using a modified baculovirus, harvested and placed onto nanoparticles which have been designed to mimic SARS-CoV-2, but don't cause COVID-19. From there, the immune system learns to recognise the spike protein.
Early data from the vaccine's phase III trial suggested it had 89 percent efficacy, though not quite as much against the mutant South African strain. It's yet to be approved by any country's health authorities yet, still in trials; but is expected to be ready for emergency use in the US by May.
Side effects include headaches, muscle pain, and fatigue.
AstraZeneca/University of Oxford
The fourth vaccine we've shelled out for is AZD1222 (also known as ChAdOx1 nCoV-19), developed by the University of Oxford in the UK and British-Swedish biopharmaceutical company AstraZeneca.
We've ordered 7.6 million doses - like the Novavax and Pfizer/BioNTech vaccines, a full course requires two doses.
Scientists modified a chimpanzee adenovirus to include the SARS-CoV-2 spike protein. It works in a similar way to Janssen Pharmaceutica's vaccine - putting DNA into the nucleus of a cell, which creates mRNA instructing the body to produce its own spike proteins. It can't replicate itself.
AZD1222 is relatively inexpensive to make and can be transported and stored at fridge temperatures, making it a favourite of the charitable COVAX scheme, which is acquiring vaccines for low-income countries.
But it's proved controversial in wealthier nations, with many rejecting it in favour of the Pfizer/BioNTech and Moderna vaccines - its case not helped when French President Emmanuel Macron claimed it didn't work in people over 65 (he later backtracked).
AZD1222 had a troubled beginning too, with mistakes made in the phase III trials overshadowing results that showed it was otherwise effective.
There have been concerns raised however it doesn't work as well against the South African variant.
Since being approved for use in a few countries, evidence is growing it's safe and effective for over-65s as the other vaccines - prompting hard-hit European nations desperate for any vaccines to start approving it for use in the elderly.
And recent data suggests even a single shot gives 80 percent protection against severe disease.
Side effects include tenderness, headache, muscle fatigue, tiredness, swelling of the glands and fever, according to the UK National Health Service.
Which vaccine should I get?
Scientists have sought to play down comparing each vaccine's effectiveness, as they were all trialled at different times, amongst different populations, against different strains of the virus.
COVID-19 Response Minister Chris Hipkins in February said Kiwis would not have a choice in which vaccines they were offered.
"It's based on science, it's based on us looking at each vaccine, working out which population group is this vaccine most effective with, and that's how we decide who gets what vaccine. It's not that you get to just rock on up and take your pick," he said at the 1pm briefing on February 4.
"We will actually make some decisions of which population groups we're going to use the supplies of vaccines we get for. And of course as the science begins to tell us more, then we have the ability to fine-tune and refine that programme as we go along."
The chief medical advisor to US President Joe Biden, Anthony Fauci, has said people should accept whichever approved vaccine they're offered.
"People should take the one that's most available to them," he said last week, talking about the Janssen Pharmaceutica, Pfizer/BionTech and Moderna vaccines (New Zealand has not signed a deal to get the latter). "People need to get vaccinated as quickly and as expeditiously as possible."
