So far in New Zealand's vaccine rollout there have been 4521 reports of side effects - slightly more common than in the flu vaccine, but still very rare, according to one of the country's top experts.
Medsafe each week publishes data on the vaccine rollout, tracking side effects and deaths linked to the Pfizer-BioNTech vaccine, also known as Comirnaty. The latest report notes out of the 744,883 doses given up to June 5, there had been 4341 "non-serious" side effects recorded, and 180 serious.
"Many people are aware from having other vaccines you commonly get sore arms, you get aches and pains, you can get headaches, some people get fevers, some people can feel really tired and some need to take a day off work... this is normal, standard stuff," Nikki Turner, director of the Immunisation Advisory Centre, told The AM Show on Wednesday.
"It is more common with the second dose, and people recover. But there are very rare side effects, and I think the one people are aware of in particular is severe allergic reaction straight after the vaccine, which is why people are asked to wait for 20 to 30 minutes."
Of the 23 serious side effects reported in the week to June 5 - the latest data that's available - five were allergic reactions, including two cases of anaphylaxis.
"We have seen a few of these, and they are matching the international data," said Dr Turner. "Our frontline providers give adrenaline to prevent them getting worse and send them to hospital, and everybody's fully recovered - within a few minutes, once you get the adrenaline. It's very unpleasant though."
Others include stroke, atrial fibrillation, seizure, deep vein thrombosis, hypotension, ataxia (difficulty balancing and controlling muscles), Guillain-Barré syndrome, blood in urine, jaundice, Bell's palsy (facial paralysis) and inflammation. Medsafe notes they are "rare and may not necessarily be related to the vaccine" - these things happen.
"People are reporting a whole lot of severe things that happen straight after vaccines - they're not vaccine-related," said Dr Turner.
A new example Medsafe has uncovered is menstrual disorder. There was nothing in Pfizer's trial data that the vaccine could affect women's periods, but in April scientists said there was evidence it could be - again, in rare cases. But more data is needed.
"The number of reports received is low compared to how many females have been vaccinated and how common these types of disorders are generally," Medsafe said, noting the "current evidence does not suggest an increased risk of these disorders following vaccination with Comirnaty".
Rare side effects often aren't reported in formal trials simply because they're rare - not even 40,000 volunteers that phase 3 trials typically use is enough to pick them up.
Medsafe's data shows serious side effects have occurred after only 0.02 percent of doses. Mild side effects - aching and headaches, for example - has been reported following just 0.6 percent of doses.
"The thing we can feel very confident about is this vaccine is used now in hundreds of millions of doses around the world," said Dr Turner. "The countries with resources - American, Europe, England, Australia - have been looking actively at the safety surveillance. This vaccine has got more safety surveillance than any medical product, any food product, any other vaccine - we understand its profile, we really do."
The risk of ongoing illness in the case of a COVID-19 infection is far worse, experts say. Some studies have suggested as many as a third of people infected with the virus are still showing symptoms six months on, even if they only had a mild illness.
And while there have been eight deaths reported after a dose of the Pfizer-BioNTech vaccine in New Zealand, none have been definitively linked to the vaccine. Medsafe says four are "unlikely", two couldn't be assessed "due to insufficient information" and two remain under investigation.
In 2019, 94 people died on average every day in New Zealand. At that rate, nearly 10,000 Kiwis likely died between the start of the vaccine rollout and June 5.
"People and vaccinators are reporting anything that they're concerned about that happens after the vaccine," said Dr Turner. "They would have died of medical conditions - remembering we are vaccinating and will continue to vaccinate very elderly people, people with significant medical problems. We will see a range of medical conditions that happen coincidentally. We collect all of those and look at them very closely."
Was Pfizer-BioNTech the right choice?
New Zealand initially put in orders for four different vaccines, before switching to an all-Pfizer strategy after seeing the vaccine's success overseas and lack of serious side effects compared to the likes of the jabs made by AstraZeneca and Janssen. Dr Turner said while we could have rushed all the vaccines into the country, we didn't need to because of our success in eliminating the virus last year.
"I'd love to have seen it earlier, but remembering again we don't have rampant COVID," said Dr Turner. "It's a little unethical to rush and demand vaccines when other countries are more desperate than us... Yes, we could have run with other vaccines in a mix-and-match model, but the science and the data behind this vaccine is so good. I know we're a few months late, but we're in a good position."
It's also important to clamp down on outbreaks overseas, she said, to stop new variants that could bypass the existing vaccines. The more people the virus infects the more likelihood of variants emerging - and immunising a country like New Zealand, where there is no COVID, won't help in that regard.
"If we get on top of COVID in the world, you will get less new variants," said Dr Turner. "If that happens, they'll have to redesign a new version of it and we'll have to vaccinate again."