At least five in every six Kiwis will need to be vaccinated against COVID-19 to stop it spreading in the community, new research has found.
But that might be too low to protect us against the more contagious Delta variant, scientists say.
"Until we get close to that threshold we are still at risk of a significant health impact from an outbreak that would include overwhelming our healthcare capacity," said Shaun Hendy, researcher at Te Pūnaha Matatini, whose modelling of disease outbreaks has helped inform the Government's zero-tolerance response.
The vaccine rollout is ahead of the Government's schedule, but well behind other countries because we shifted strategy earlier this year, rejecting cheaper but potentially less effective vaccines like those made by AstraZeneca in favour of the high-performing jab made by Pfizer/BioNTech.
"While the rollout is still underway, the elimination strategy gives us the best options for controlling any outbreaks and protecting people who haven't yet been vaccinated," said Dr Hendy.
The scientists looked at demographic data - particularly age, since COVID-19 outcomes are typically much worse for the elderly than the young - and how much contact they have with other age groups. They also took into account vaccine efficacy - assuming 85 percent - and the virus' reproduction number (R0 number) - how many people the average person will go on to infect - which they assumed to be 4.5.
Herd immunity is when enough people are vaccinated that the virus struggles to find new hosts, preventing widespread outbreaks and protecting people who can't be vaccinated (for health or age reasons, for example).
The simulations found the absolute minimum vaccine takeup that would result in herd immunity against the original strain of the SARS-CoV-2 virus was 83 percent.
"We’re not going to one day magically hit a population immunity threshold where we can open the borders and everything goes completely back to normal," said Te Pūnaha Matatini biomathematician Michael Plank.
"It will be more of a gradual relaxation of border measures alongside continued testing and contact tracing measures. If we relax border restrictions, we will see COVID-19 cases and it’s quite likely that we’ll see outbreaks. The way to protect against those outbreaks is to get vaccinated."
But the Delta variant is more infectious. Assuming it has an R0 number of six, herd immunity might not be reached until 97 percent of us are protected.
"Because the Pfizer vaccine is not approved for children under 15, that's 19 percent of the population - that means it will be very difficult to get enough coverage to protect us from the Delta variant," said Mick Roberts, a professor of mathematical biology at Massey University, who was not involved in the study.
He says in addition to vaccines, there's another easy way we can prevent the need for a lockdown if the Delta variant gets into the community here.
"I would like to see people more conscientious about scanning the QR code," he told Newshub. "I see, if I go to shops, people walking past me if I'm scanning the code. If there is an outbreak, the tracing of contacts is the best way of stopping the outbreak from spreading too far. You can only do that if you know who the contacts are."
So far just 7.9 percent of Kiwis have been fully vaccinated against COVID-19. Israel leads the way at 60 percent.
The R0 number for the SARS-CoV-2 virus is difficult to pin down, as it seems to be highly dependent on superspreader events. It's estimated around 80 percent of transmission comes from just 10 percent of infected people - so there's a lot of luck involved, as Wellington recently found out when an Australian tourist infected with Delta visited the city, but doesn't appear to have infected any Kiwis.
The Delta variant is unlikely to be the last. If there was one which knocked the effectiveness of the Pfizer vaccine down to 50 percent, leaving it to rip through New Zealand unchecked would kill around 4300 people, even with 90 percent vaccine coverage, the modelling showed; while the Delta variant would likely kill 77 people if the borders were opened once the vaccine rollout was complete, without ongoing measures to stop it.
The UK's experience
COVID-19 Response Minister Chris Hipkins told The AM Show on Wednesday the figures were "crystal ball-gazing".
"If we look at our friends in the UK they've got quite high vaccination coverage, certainly of the first dose; they're still working their way through second doses. But they've had to delay removing some of the restrictions because they're dealing with another outbreak of this Delta variant... they're still seeing outbreaks and still seeing quite a high number of cases."
The UK has had one of the fastest vaccine rollouts, but is still far from herd immunity - just 48 percent are fully vaccinated, and many with the AstraZeneca vaccine, which has shown just 60 percent efficacy against the Delta variant in studies - lower than the Pfizer/BioNTech jab's efficacy of about 88 percent.
Case numbers are rising again, going from about 2000 a day in May to 20,000 now - a rise eerily similar to what preceded the massive wave in the new year.
But the vaccination rate is high enough now to prevent many people getting sick, with all the vaccines still able to reduce that risk somewhat.
"What that shows is vaccination is really important," said Hipkins. "But we're still going to be dealing with potential outbreaks for a while, and vaccination rates around the rest of the world are actually really important to us here in New Zealand, and we're seeing some countries that have barely started vaccinating yet. We all, as a global community, need to be concerned about that."