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When Will New Zealand Have Herd Immunity?

The trans-Tasman bubble has been a step closer to normality with some semblance of overseas travel returning. Since then our borders have been shutting and re-opening to Australian states as new cases of Covid-19 emerge in their cities. This is not normality. With the emergence of the highly infectious delta variant threatening to pop the bubble completely, just how long will it be before things go back to normal for good?

Te Pūnaha Matatini scientists say four in five Kiwis need to get the jab before we can stop worrying about lockdowns and alert levels and start opening up again to the rest of the world. Until we hit this number, the modellers say that public health measures will be needed throughout all of our vaccine rollout to avoid deaths and serious illness.

The Government hopes to have most people vaccinated by the end of the year, but this will depend largely on compliance.

Lead author of the study, Nic Steyn says that the good news is increasing levels of vaccination will make maintaining an elimination strategy easier and allow the country to eventually move from relying on population-wide interventions like lockdowns to more targeted controls like contact tracing in the later stages of the rollout.

“We’re going to need to use the vaccine in conjunction with the other layers of protection that we have at the moment,” says Professor Michael Plank.

“This means that border restrictions, the Alert Level system, community testing and contact tracing will need to remain in place.”

The team used an age-structured model of COVID-19 transmission in Aotearoa New Zealand to estimate how increasing levels of immunity through vaccination can slow the growth of an outbreak. The models combined the latest available data on vaccine effectiveness with social contact survey data that estimates how much contact there is between people in different age groups across the country.

This modelling provides an indication of the potential for spread at a broad-scale national level and includes a range of scenarios at various stages of the vaccine rollout, from contained local outbreaks to an unmitigated epidemic.

Professor Shaun Hendy says that we’re still vulnerable to COVID-19 and will remain vulnerable even once the vaccine rollout is complete, but the results show that things will get better as the rollout progresses.

This modelling also includes the first New Zealand-specific estimates of the percentage of the population that needs to be vaccinated to reach population immunity. The lowest estimate of the population immunity threshold that the models produced was an 83% vaccination rate across the total population.

This was based on data from older variants of the virus with an estimated basic reproduction number of 4.5 and assuming the vaccine reduces transmission by 85%. Emerging data on newer more transmissible variants suggests a higher threshold, although this remains uncertain.

“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,” says Hendy.

“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.”

Vaccination rates will vary across Aotearoa New Zealand, so even if population immunity is reached nationally, communities with vaccination rates lower than the national average will remain at risk of hospitalisation and fatalities from COVID-19 outbreaks. Further modelling work will be needed to investigate this.

Professor Michael Plank cautions that 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.

“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.”

There is still a lot to learn about the Pfizer vaccine and its effectiveness in different population groups. These models will need to be updated as new data is collected internationally about vaccine effectiveness and transmissibility of new variants.

Plank says that these results deliver a clear message: As more of the population gets vaccinated, we still need to go as hard as we’ve ever done on testing, contact tracing, scanning in, hand sanitising and wearing masks.

“The vaccine rollout is good news, but life is not going back to normal for some time,” says Hendy.

Immunisation Advisory Centre’s Professor Peter McIntyre points out a limitation of the study is that it looks at a scenario that is very unlikely in practice – opening up the border without restrictions and modelling on five infected people entering daily for two years.

“The actual situation will almost certainly be that borders will open only in the context of high vaccine coverage and include entry conditions such as pre-testing and proof of vaccination status – this will give much more favourable outcomes than the authors suggest,” says McIntyre.

“We would like to see further refinement of this valuable model and its assumptions to better reflect the lowered risk of severe disease when there is high vaccine coverage in New Zealand and borders are cautiously opened to limited arrivals.

“This should better reflect the real-world experience of countries such as the UK and Israel, within the context of some community transmission and taking into consideration the reduction in severity of cases occurring in vaccinated people due to breakthrough infections.

“We can be confident that severe cases will be uncommon in a high vaccine coverage future.”

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