Doherty Institute modelling for COVID Delta variant made public

By Melissa Coade

Wednesday August 4, 2021

The Doherty Institute report for the federal government explains what it will take for Australia to move from ‘suppression’ to ‘post vaccination phase’.
The Doherty Institute report for the federal government explains what it will take for Australia to move from ‘suppression’ to ‘post vaccination phase’. (AAP Image/Lukas Coch)

A 24-page document produced by the Doherty Institute at the request of the federal government has been released to help explain what it will take for Australia to move from ‘suppression’ to ‘post vaccination phase’.

The prime minister hosted a press conference on Tuesday with Professor Jodie McVernon, Doherty Institute director of epidemiology, to explain the modelling and assumptions underpinning Australia’s four-stage COVID response plan.

“The world is in a serious battle with the Delta strain of COVID-19. This has been a long war against this virus, and there have been many, many battles, and this is a fierce one when it comes to the Delta strain,” Morrison said, suggesting that all Australia’s planning and response safeguards of the past 18 months needed to be adapted for the more virulent strain.

“We have been charting out [a course to a way of living with the virus] since the beginning of this year when I first tasked Secretary Phil Gaetjens to work with his counterparts in the states and territories.”

“That process led to the first piece of work that was done by the Doherty Institute that was based on the Alpha strain. And, at the end of June, it was very clear that we would have to go back and do it again because of the Delta strain,” Morrison said. 

The biggest way the Delta variant of the virus has altered what will now be the accepted course of action when there is a COVID outbreak is that it has made swift lockdowns nationally acceptable. The pm also invited treasurer Josh Frydenberg to explain how the economy would could lift from the imposition of public health restrictions and lockdown measures once particular vaccination targets had been achieved. 

“Until we get to 70% and above vaccination rates, the economic imperative is that governments need to move fast [and lockdown communities] to get on top of those cases,” Frydenberg said. 

“If they don’t, we see lengthier and more sever lockdowns, which have a much more significant economic cost.”

It is the combination of vaccination ‘threshold coverage targets’, ongoing lockdowns and other measures to reduce infection spread within the community that the Doherty Institute modelling is based on. The modelling considered variables for ‘test, trace, isolate, quarantine’ capabilities and how they would impact the transmission of the virus among groups of people. 

Crucially, the modelling has also identified that vaccinating the most mobile group of people aged 20-39 year-olds can lead to the ‘greatest reduction in harms across all age groups’. Since June, research coming from Norway and the US has described a ‘mix-vaccination strategy’ (targeting vaccination of vulnerable populations first, and then the most social groups) as one of the most effective approaches in reducing pandemic-related deaths. 

Morrison underscored the serious potential COVID-19 death toll Australia has so far avoided in his address multiple times, reminding people that 30,000 citizens would be dead today if it were not for the decision to close international borders to most travellers early in 2020. 

“I want to thank all those working in the Health Department and around the country – the GPs, the pharmacists, those working in the state hubs – who are getting these vaccines out every single day,” Morrison said. 

The prime minister went on to invoke national Olympic pride in the challenge to get as many people vaccinated against COVID-19 as possible, urging people who are yet to get the jab to achieve ‘the triple gold that Australia is looking for’.

“It can be done. It will be done. I have great faith in the Australian people to get this done, a tremendous faith in their determination and their motivation to get this job done,” Morrison said. 

Vaccination of the Australian people should be regarded as a continuum

The Doherty Institute researchers estimated the clinical consequences of uncontrolled virus outbreaks in Australia once population targets for vaccination had been achieved at 50%, 60%, 70% and 80% vaccine coverage. It is based on these scenarios that the phases of Australia’s four-stage COVID response plan have been given transition thresholds, which mean once specific vaccination targets are achieved the country can move into its next phase.

“The targets that are part of this plan, the vaccination targets of 70% to get to the next phase and 80% to Phase C, are based on the world’s best scientific analysis and economic advice,” Morrison said.

But Professor Jodie McVernon stressed that it was important for people to think about vaccination rates as a continuum.

“We’ve heard a lot of discussion of herd immunity thresholds and some magic number where there’s a binary cut off and above that life is life as normal and below that we haven’t achieved our goal. Vaccination coverage is a continuum,” she said.

“Every Australian who is immunised helps to protect themselves, their families and their community, and that will help to reduce transmission and negative health outcomes.”

“We look at the outcomes of vaccination to reduce transmission potential, at this point in the population strategy, as a way of reducing those adverse outcomes,” McVernon added. 

Professor McVernon said that the modelling provided by the Doherty Institute would be complemented with ongoing assessment of different SARS-CoV-2 variants circulating in the Australian community. She added that an ongoing assessment of measured transmission potential over the coming months would allow the government to benchmark the hypothetical scenarios, and guide ‘real-time policy decision making’ about when Australia was ready to move into ‘phase b’ of the national plan. 

“What we’re proposing now is a shift in strategy at this phase of the program having got to where we have, and vaccination alone is a very big part of the answer but it is not the whole answer. We must maintain ongoing public health and social measures.”

Part of the government’s change in strategy will be to update its vaccination timetable to accelerate vaccination for people in the 30-39 age group. Currently Australians in the 20s and 30s will become eligible for the Pfizer vaccine from September-October 2021. But that may change this week, according to reports from Michelle Grattan, once the government confirms incoming numbers for Pfizer supplies. 

What about the children?

Responding to the newly released Doherty Institute modelling, University of Sydney’s Professor Alexandra Martiniuk expressed concern about a missing group of the Australian population who were not included in the scenarios — children under 16 years.

“While the Doherty Modelling does provide a scenario taking into account vaccination of children 12+ years, we need a scenario modelling vaccination for all ages (including <12 years) AND we need milestones to take into account children at all times,” Martiniuk said.

The epidemiologist noted that the modelling was useful in that it considered the Delta virus strain in terms of transmission, severity and vaccine effectiveness but cautioned the modelling was limited because it was simplified to represent a ‘single national epidemic’. She also explained that it reasonably could not consider the impact of new and different strains of the virus, should they emerge.

“The [modelling] for National Cabinet is useful in understanding how we might transition between the various phases of opening up. However we do need to be wary that the model is based on this being a ‘single national epidemic’ in order to simplify the modelling,” Martiniuk said.

“We also need to be wary of new variants of concern emerging beyond Delta, which of course, would affect any current model’s ability to predict future scenarios.”

Professor Ivo Mueller, WEHI’s joint head of population health and immunity division, also cautioned that 70% vaccination coverage of the Australian population, to move into phase B of the national plan, should not be regarded as a ‘magical threshold’. He explained that changing public health settings would occur gradually and a life that most resembles what it used to be with all the freedoms pre-pandemic was much more likely once Australia had achieved 80% vaccination coverage.

“As vaccine coverage levels grow, transmission potential will continuously decrease and the efficacy of test, trace, isolate and quarantine (TTIQ) will increase, making short, sharp lockdowns both more effective and overall, less likely. 

“Once we reach 70% vaccine coverage, these and other low-level public health measures are predicted to be sufficient to contain the remaining transmission potential, where we can safely shift the paradigm from transmission suppression to minimising severe COVID-19 cases and deaths,” Mueller said.

“Very sensibly, this paradigm shift will itself be a gentle one, with continued effective TTIQ and other low-level intervention and a careful, modest opening of international borders.”

Mueller added that while achieving 80% vaccination coverage was a high bar, the Doherty team’s modelling offered an evidence-based road map out of COVID-19 and reassurance that in Australia this can be done safely.

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