Photo: Ministry of Health

When will NZ have herd immunity against Covid-19? – Expert Reaction

New modelling shows 83 per cent of all New Zealanders need to be vaccinated against Covid-19 before we can safely remove public health restrictions. 

The estimate is based on data from earlier variants of the virus, so the percentage may need to be even higher now with faster-spreading variants. Te Pūnaha Matatini scientists say it means public health measures like masks, Alert Levels, and border restrictions will need to remain in place for the vaccine rollout.

The SMC asked experts to comment on the research.

Dr Rawiri McKree Jansen (Ngati Raukawa, Ngati Hinerangi), Clinical Director, National Hauora Coalition, comments:

“The paper addresses an important issue for Aotearoa. So far our pandemic response has achieved significant success and has largely protected communities from severe illness and death from Covid. The vaccination programme can contribute further protections, indeed must contribute further protections before the borders are opened.

“The authors note that communities with ‘relatively low vaccine coverage’ or ‘high contact rates’ will remain vulnerable to major outbreaks. Māori communities are exposed to both of these risks currently with a vaccination programme that is underperforming in terms of equity. The paper is a timely provocation to ensure that a pro-equity vaccination programme is resourced.

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

Conflict of interest statement: “I am a member of the Expert Panel for Stuff’s The Whole Truth – Covid-19 Vaccination editorial project.”

Associate Professor Malcolm Campbell, GeoHealth Laboratory, University of Canterbury, comments:

“In essence, the key message from this paper is that more vaccinated people is better for all of us. We need to reach ‘herd immunity’, the protection that comes from immunity through successful vaccination or from previous infection. The modelling also shows that vaccination leads to fewer hospitalisations and fatalities.

“As we get closer to herd immunity, there is a chance of a new normal emerging. However, we still need to vaccinate as many people as we can, at least 4 out of 5, and some of the modelled scenarios give us a clear signal that a new ‘normal’ may not be as close as we might think. This is especially important if lots of us aren’t vaccinated and a highly transmissible variant emerges. Even once we are vaccinated, we still need to carefully think about following public health messages and measures.

“As with any model, the assumptions scientists make are key to understanding the results. Often, assumptions change or evolve as more evidence emerges, allowing models to be refined. This means that models aren’t ‘right’ or ‘wrong’ but based on the best evidence at a point in time under uncertain circumstances. For example, new variants of the virus may well change any modelling assumptions. This is a relatively normal part of the modelling process. At present, the models of the unequal coverage of vaccine by socio-economic group or by local geographies across NZ, haven’t been developed as this is even more complex.”

No conflict of interest declared.

Professor Peter McIntyre, medical advisor, Immunisation Advisory Centre, comments:

“This is a valuable study by a highly experienced modelling group. Its most important message is “whether or not New Zealand reaches a theoretical population immunity threshold, the higher vaccination coverage is, the more protection the population has collectively against … COVID-19, and the easier it will become to control outbreaks.”

“However, a limitation of the study, which leads to some unnecessarily scary conclusions, is that it looks at a scenario that is very unlikely in practice, i.e. 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.

“We strongly agree with the authors that “it will be essential to continue to vaccinate as many people as possible to minimise the combined potential for transmission and health impacts” and that “until the vaccine rollout is complete, retaining the elimination strategy will protect people who have not yet been vaccinated and decrease … use of the alert level system.”

“However, 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.”

No conflict of interest declared.

Andrew Sporle, Senior Research Fellow (Honorary), Department of Statistics, University of Auckland, comments:

“This new modelling demonstrates how important it will be to protect those at higher risk of infection and poor outcomes as the vaccine rollout continues. Groups with more people who can’t get vaccinated due to age or health issues will need higher levels of vaccine coverage to prevent deaths and hospitalisations.

“We already know that Māori and Pacific people have had much worse outcomes from the current epidemic. Those populations also have much larger proportions of their population who aren’t old enough for vaccination or who have pre-existing conditions. While improving the vaccination coverage for Māori and Pacific adults needs to be urgently addressed, there also needs to be increased support for local initiatives to protect local communities and improving contact tracing in case of an outbreak.”

Conflict of interest statement: “Andrew Sporle is a social statistician who works with Te Pūnaha Matatini and other groups on COVID-19 outcomes and vaccination modelling as well as disinformation sources. He was not involved in the work for this particular paper.”

Professor Mick Roberts, Professor in Mathematical Biology, Massey University, comments:

“The reopening of New Zealand’s borders makes it inevitable that some persons infected with Covid-19 will enter the country. The concept of herd immunity means that, on average, each infected person would infect less than one other person, and therefore a major outbreak would be prevented. The only way to achieve herd immunity (without having a major outbreak) is by achieving a high level of vaccination coverage.

“A new model from Te Pūnaha Matatini calculates the vaccine coverage needed to achieve herd immunity in New Zealand. The model is deterministic and age structured, but still relies on best-guess estimates of parameter values, some of which are poorly known. The model suggests that if the basic reproduction number is 4.5 (alpha variant) or 6.0 (delta variant) then vaccine coverage needs to be 83% or 97% respectively. Using a standard ‘back of the envelope’ rule the results would be 85% and 91% respectively. These are overall population estimates, so if those under 15 years old are not protected, the higher figure is not attainable. Mask wearing, testing and tracing will be necessary for some time yet.”

No conflict of interest.