A vaccine expert says Covid-19 "booster" shots are not needed right now, but may well be in the future.
A booster shot is being considered in other countries. The US is intending to roll them out, planning to offer a top-up shot from around eight months after a second dose to keep people's immunity high.
Immunisation Advisory Centre director and GP Dr Nikki Turner, who advises the government on vaccines, said a booster shot is fairly common for a number of different types of vaccines.
"Over time our immune response wanes. If it wanes low enough, you're at risk of getting the disease again. So we give booster shots for many diseases to stop losing all of your immunity, and to keep protecting people long-term," Turner said.
Booster shots tend to be the same vaccine again. In New Zealand's case it would likely be a third dose of the Pfizer/BioNTech vaccine.
"The other possibility is if the virus did mutate further, so that the original immune response wasn't that effective, then they would change the formulation to make it different. That's a little bit like what we do with flu vaccines every year where they're actually slightly different formulations," Turner said.
A Oxford University study published this month found Covid vaccines do not protect people as well even after three months.
It found the Pfizer vaccine was 75 percent effective in preventing infection after 90 days, down from 85 percent two weeks after the second shot. The study is not yet peer-reviewed.
Associate Professor James Ussher, a microbiologist at the University of Otago, is on the government's Covid-19 vaccine technical advisory group.
He said the Pfizer vaccine is highly effective at preventing serious disease, hospitalisation and death.
Whether or not we'll need a booster is not yet clear, he said.
"I think that remains an open question at the moment. There are certain groups where a booster shot may be recommended, immunocompromised groups. But a broader recommendation for a booster shot - there's not evidence there to support the requirement for that at the moment," Ussher said.
Academics and the government would be keeping a close eye on emerging data, he said.
The vaccines continued to perform well, including against the Delta variant, and serious cases worldwide are generally only occurring in the unvaccinated.
"Recent CDC data out of the US suggested that there was a 25-fold reduction in hospitalisation and death in those who are vaccinated versus those who are not, and an eight-fold reduction in symptomatic infection."
Dr Turner said fully vaccinated New Zealanders do not need to worry about a booster at this stage.
"Right now there is no need to offer boosters to the New Zealand population. This is an issue in the long-term, looking forward."
Covid-19 Response Minister Chris Hipkins said New Zealand will have had 10 million Pfizer doses delivered by the end of the year. Around 8m would be needed for everyone to have two shots, so approximately 2m would be left over that could - if the science recommended it - be used as boosters.
The World Health Organisation has called for rich countries to delay buying excess supplies, to ensure poorer countries get a first and second shot before others get thirds.