Coronavirus: New Zealand needs high uptake of vaccine to mitigate cases, hospitalisations, deaths once borders reopen - study

New Zealand will need a high total uptake of COVID-19 vaccines to help mitigate the numbers of cases, hospitalisations, and deaths once borders are reopened, a study says.

The modelling, led by Victoria University of Wellington in collaboration with the Institute of Environmental Science and Research (ESR), looked at various hypothetical scenarios that the potential effect the country's vaccination programme could have when borders reopen.

One scenario predicted that with 10 overseas cases daily and 90 percent total population vaccine uptake - including 0-15-year-olds - while targeting high-risk groups, there would be an estimated 11,400 total hospitalisations and 1030 total deaths. There would be a peak of 324 active and 36 new daily cases in hospitals. This modelling is based on a two-year open-border scenario without public health controls.

The paper, which is titled 'COVID-19 vaccine strategies for Aotearoa New Zealand: a mathematical modelling study' and published in The Lancet, suggests targeting high-risk groups for vaccinations will result in fewer hospitalisations and deaths, but a high number of cases compared to a strategy for reduced transmission. The modelling doesn't include the impact of public health controls, like lockdowns, so predicts a higher number of cases.

Professor Colin Simpson from Victoria University of Wellington says the modelling of predictions from different vaccination programme strategies to consider the number of cases, hospitalisations, and deaths over two years with open borders could help support New Zealand's vaccination strategy.

"The aims of the study were to predict how many people do you need to immunise for herd immunity, which age groups should be targeted first and in what order and what the number of cases, hospitalisations and deaths would look like under a number of different vaccine effectiveness, R0 and population coverage," he says.

The modelling found that reaching the herd immunity threshold based on the infection rate of the Delta variant was almost impossible.

"Based on a 90 percent vaccine effectiveness (VE) against disease and 80 percent VE against infection we would require at least 86.5 percent total population uptake (including children) for R0=4.5 (with high vaccination coverage for 30-49-year-olds) but that would jump to 98.1 percent uptake for R0=6 (the Delta variant)," Simpson says.

ESR chief scientist Dr Brett Cowan says the results show that vaccinating as many New Zealanders as possible will reduce the risk of widespread community outbreaks. As a result, vulnerable populations will have a greater chance of protection from the disease, but other public health and social measures will still be needed as part of a response.

"Vaccination modelling has been proved to help anticipate potential public health outcomes based on different vaccine effectiveness reported in clinical trials and 'real-world' studies and vaccination programme strategies," Simpson says.

"While the study was primarily developed with New Zealand in mind, our experience will also provide valuable insights to the international community to inform future actions."

Andrew Sporle, from the University of Auckland's department of statistics, says it is critical to include strategies to ensure Māori and Pasifika have maximum protection since they're at higher risk for hospitalisation and death from COVID-19.

"Prioritising vaccinations for those most at risk of severe outcomes from COVID-19 infection (including Māori and Pasifika) benefits the whole population as well as protecting those groups," he says.

"We know that opening the border will result in local cases of COVID. Minimising the resulting hospitalisations and deaths requires prioritisation of those groups and communities most at risk, as Australia and Canada have done."

Sporle adds that the risk of a border breach before vaccinations are complete means prioritisation must be a focus so it doesn't become a catch-up strategy.

There have been 148 cases in New Zealand's current outbreak so far, 41 of which were announced on Tuesday.