Covid 19 coronavirus: Extremely difficult – New herd immunity estimates revealed

New Zealand may never reach herd immunity against Covid variants like the deadly Delta strain or be able to completely abandon public health measures, new estimates show.

Modelling from research centre Te Pūnaha Matatini indicates 83 per cent of New Zealanders will have to be vaccinated against less transmissable virus strains for measures like lockdowns and 14-day quarantine to be no longer needed.

However, the modelling – which was yet to be peer reviewed – suggests 97 per cent of Kiwis would need both Pfizer jabs to abandon such measures if the country was hit by a wave of a strain as transmissable as the Delta variant.

Te Pūnaha Matatini professor Shaun Hendy said it was “pretty unlikely” New Zealand would reach that level of vaccine coverage, which he said would require previously unseen uptake rates.

“Time will tell and it may not be achievable.

“Life’s not going to go back to 2019 any time soon.”

Herd immunity, also known as population immunity, is when enough people have immunity, either from vaccination or a past infection, to stop uncontrolled spread.

These were the first New Zealand-specific herd immunity estimations, replacing a generic 75 per cent target referenced previously.

Hendy said the 83 per cent estimate, which could see immunity achieved against strains similar to the Alpha variant, was less relevant now, given the Delta variant – which was 40 per cent more transmissable than Alpha – had become globally dominant.

As New Zealand increased vaccination coverage, Te Pūnaha Matatini researchers said virus spread prevention could be achieved by a combination of testing, shorter quarantine periods or home isolation for people from low-risk countries.

Fellow Te Pūnaha Matatini professor Michael Plank was quick to reinforce that reaching either vaccination target would not suddenly give Kiwis default protection against virus strains.

“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.

“It will be more of a gradual relaxation of border measures alongside continued
testing and contact tracing measures.”

Plank explained that while New Zealand might meet a national vaccination target, other communities which were hard to access or more at-risk to the virus might have lower rates of vaccination.

National Hauora Coalition clinical director Dr Rawiri Jansen (Ngāti Raukawa, Ngāti Hinerangi) said border controls shouldn’t be lifted until the rollout was more equitable.

“[Te Pūnaha Matatini] notes that communities with ‘relatively low vaccine coverage’ or ‘high contact rates’ will remain vulnerable to major outbreaks,” he said.

“Māori communities are exposed to both of these risks currently with a vaccination programme that is underperforming in terms of equity.

“Until vaccination rates in the order of 90 per cent of eligible Māori are achieved, opening of the borders would likely be catastrophic.”

University of Otago epidemiologist Nick Wilson said Te Pūnaha Matatini’s estimations were “plausible”, but predicted the targets would be “extremely difficult” to reach.

National potential vaccine uptake, as per the latest Ministry of Health update, was 80 per cent – up from 77 per cent in April. Māori potential uptake was 75 per cent, up from 71 per cent.

Wilson wasn’t overly concerned that anticipated uptake was currently lower than Te Pūnaha Matatini’s estimates, given New Zealand was experiencing what was a global pattern of increasing vaccine acceptance.

However, he stressed the need to vaccinate children as soon as it was deemed safe to, in an effort to boost population immunity.

The Ministry of Health was expected to respond to the report today when it was made public.

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