How does the vaccine build immunity? Why do we get two doses, and why has the Government reverted to recommending a three-week interval? Science reporter Jamie Morton explains.
Essentially, to let the vaccine do its thing.
The Pfizer-BioNTech shot is an mRNA vaccine that sends our body a set of instructions to teach it how to fight the virus.
It introduces an mRNA sequence, or the specific molecule that instructs our cells what to build, which is precoded for an antigen specific to the Sars-CoV-2 virus that causes Covid-19.
Once our cells produce and display the antigen - that being the "spike protein" which the virus uses attach itself to our cells - it attracts and activates T cells, the roving hunters and killers of our immune system, along with other immune cells.
This process generates an immune response that the body remembers if it ever comes into contact with the actual virus, ultimately making us into our own vaccine factory.
But it takes time.
The first dose allows us to develop the immune response - something that requires a minimum of 21 days, according to studies - while the second strengthens it.
Immunologists call this double-dosing "prime boosting" as the first shot primes or teaches the immune system what we want it to eliminate, before the second shot lets it put into practice what it's learned.
Because your body knows just what to do after you receive that second dose - quickly firing up an immune system response as soon as antibodies recognise the protein - it's more likely to come with some temporary side-effects.
That's not unusual - and is just a sign of our natural defence system doing its job.
What makes us feel poorly when we catch the flu, for instance, isn't the virus itself, but the inflammation that comes with our immune system swinging into action.
As well, the Pfizer vaccine can't and doesn't affect your DNA - RNA vaccines never enter the nucleus of the cell which is where our DNA is kept - nor does it contain any of the virus itself, so it can't give you Covid-19.
In the first trials of the vaccine, researchers looked at how much of the mRNA to include in each dose of the Pfizer vaccine, and how many doses should be given.
After the first dose, they found antibody levels were much lower compared to those seen after natural infection with Covid-19, but after the second, levels were much higher than those following both the first dose or infection.
Large clinical trials have shown protection against symptomatic Covid-19 to be around 52 per cent at about 12 days after the first dose of the Pfizer shot - but 95 per cent after the second dose.
More recently, "real-world" data from observations of front-line workers has put the level of protection at 62 to 91 per cent from two weeks after the first dose, and at between 68 and 97 per cent after the second.
However, it's still precisely not clear for long the first dose provides protection.
While leaving a longer gap between doses has been found to provide a better immune response overall, it also comes with a higher risk of catching the virus before full vaccination is achieved.
At the same time, taking your second dose too soon after the first can mean it won't produce as good an immune response, as your immune system hasn't had enough time to train.
And on the back of recent UK research, scientists suggested eight weeks could be the "sweet spot".
That was because that, while waiting 10 weeks for the second jab was shown to produce a much more potent response.
While both short and long intervals still afforded a strong response, the study found a three-week gap generated fewer antibodies, while a 10-week interval resulted in higher levels of a subset of T cells called "helper T-cells", which support immune memory.
In older people, waiting 12 weeks for the follow-up shot led to an antibody response 3.5 times greater.
Weighing up this extra effectiveness against risk of exposure, different countries have come to recommend different intervals, with Denmark and Norway advocating between six and 12 weeks, the US using three weeks but allowing for six, and Australia suggesting three to six weeks.
Just before Delta was detected in Auckland in August, the Government decided to extend the interval from three to six weeks, in line with international guidance.
But last week, amid a worsening outbreak, its official advice switched back to three weeks.
"When there is no Covid in the community, emerging evidence suggests vaccination at the standard six-week gap may give additional effectiveness," said Astrid Koornneef, the Ministry of Health's vaccination operations group manager.
"However, during an outbreak, vaccination at three weeks gives robust protection sooner and is recommended."
University of Auckland vaccinologist Associate Professor Helen Petousis-Harris described the longer interval as a "slight advantage" - and said people needn't read anything more into the change in guidance than necessity.
"We no longer have the luxury of taking our time."
Meanwhile, scientists are closely watching how mRNA vaccines like Pfizer hold up over the long term.
While they've generally been shown to keep up high protection, concerns about waning immunity over time have led officials to consider the possibility of booster shots in the future - something Israel and the US have already made calls on.
Koornneef said the ministry was advising Kiwis to consider a shorter gap.
"Reducing the gap between doses to three weeks or over means more people can be fully vaccinated sooner, increasing our community immunity," she said.
"This will provide greater protection for ourselves, our whānau and our communities in this outbreak.
"It also means that the sooner as many of us as possible are vaccinated, the sooner we can get back to doing more of the things that we enjoy, and the greater our possibilities for the future.
"This includes helping accelerate our economic recovery, reducing the risk of lockdowns, and safely allowing New Zealand's borders to begin re-opening."
For people making new bookings through Bookmyvaccine.nz and via the Covid Vaccine Healthline 0800 282 926, the gap between first and second doses would default back to three weeks.
"If you already have vaccinations booked around six weeks apart, you can keep your second appointment as it is, or choose to change it," she said.
"Either way the important thing is that you get two doses of the vaccine to be fully vaccinated.
"Walk-ins are also available throughout New Zealand and we have plenty of appointments. If you walk-in please remember to cancel your original appointment."
Where to get a vaccination in Auckland - without a booking
Kiwis were also being called on to take part in the "Super Saturday" push, in which vaccine clinics throughout the country will be open for all of this Saturday and into the evening.
"We will be asking all our political and civic leaders to contribute to a big collective effort to turn people out. There is cross-party support on supporting vaccination," she said.
"Those that want to offer incentives to the unvaccinated to get them in the door are encouraged to do so."
Koornneef said more than enough vaccines were available to support people bringing their vaccinations forward, with more than 700,000 doses of the vaccine expected to arrive this week.
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