One of New Zealand's top vaccine experts is urging people not to worry if they feel a "bit grotty" after getting the COVID-19 vaccine.
Helen Petousis-Harris, head of a new global effort to track the safety of vaccines, told The AM Show many people don't seem to know that it's common to experience side effects after getting a vaccine.
"This vaccine is immunogenic - gives you a really good immune response - and it is a little reactogenic - you can have reactions. Quite a lot of people feel grotty the next day.
"I think people need to know that - they feel a bit grotty, then they bounce back."
Common side effects of the Pfizer-BioNTech vaccine being rolled out in New Zealand include pain or swelling at the site of the injection, headaches, fatigue, muscle aches, chills, fever, joint pain and nausea.
A study in the UK looking at hundreds of thousands of recipients of the first dose of the Pfizer vaccine found close to 72 percent of them reported at least one local side effect (eg. pain at the site, swelling) and 13 percent had systemic side effects (headaches, chills, nausea, etc).
The second dose triggered local side effects in 68.5 percent of people and systemic side effects in 22 percent.
"You should be forewarned that you might feel a bit grotty the next day, that's okay and quite normal," said Dr Petousis-Harris, director of the Vaccine Datalink and Research, which is a part of the Global Vaccine Data Network.
"Generally people perk up after that. All of those symptoms that you feel are symptoms of your immune response kicking in."
She said many people, who might not have had a vaccine in several years, are caught by surprise.
"Some have been taken by surprise and they didn't realise that was actually potentially a normal response. Some people feel nothing at all. You might be one of those people who feel a bit off... Just know that that's normal and not to worry about."
The Pfizer vaccine has proven to be one of the safest rolled out to date, with the only life-threatening side effect detected to date incredibly rare cases of treatable anaphylaxis - about 2 to 5 cases per million doses.
"The vaccine we're getting and using now is incredible - it works really well," said Dr Petousis-Harris.
"The safety profile looks great. We have the technology to do this, so I think we should."
New Zealand ditched its initial plans to source vaccines from four different manufacturers after seeing the success of the Pfizer vaccine overseas. Two others we signed deals for - Johnson & Johnson and AstraZeneca - have been linked to rare cases of blood clots, while the fourth - Novavax - won't be ready until later in the year.
Getting the vaccines from a single supplier however has had its problems, with a brief delay pushing back the wider rollout until the end of July. Dr Petousis-Harris said there had been a "few little bumps", but there were always going to be "some messy bits" in the unprecedented vaccination drive.