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03 Aug 2021

Doherty Institute Modelling Report for National Cabinet

The Doherty Institute Modelling Report to advise on the National Plan to transition Australia's National COVID Response can now be viewed here.

Recording of the Doherty Institute-hosted press conference with University of Melbourne Professor Jodie McVernon, Director of Doherty Epidemiology:

This work was contributed to by:

Doherty Institute
Professor Jodie McVernon, Dr David Price, Dr Trish Campbell

Curtin University and Telethon Kids Institute
Professor Nick Golding

University of Melbourne School of Mathematics and Statistics
Dr Chris Baker, Dr James Walker

University of Melbourne School of Computing and Information Systems
Associate Professor Nic Geard

University of Melbourne School of Population and Global Health
Dr Freya Shearer

Walter and Eliza Hall Institute
Dr Eamon Conway

Monash University
Dr Michael Lydeamore

UNSW Sydney School of Population Health
Associate Professor James Wood, Dr Nic Rebuli

La Trobe University
Dr Joel Miller

Executive Summary for the Doherty Modelling Report for National Cabinet

  • Models of COVID-19 infection and vaccination were used to define a target level of vaccine coverage for transition to Phase B of the National Plan. The model was based on the simplifying assumption of a single national epidemic, with COVID-19 transmission, severity and vaccine effectiveness as for the Delta variant. o Vaccine allocation scenarios were defined towards threshold coverage targets (16+ years) of 50/60/70/80%, noting achieved coverage to date has been largely concentrated in high-risk groups and elder populations in line with existing strategy;
    • We compared relative impacts on transmission and health outcomes of continuing the current risk focused strategy, with alternatives focused on reducing infection spread across the whole population. We included a scenario assessing the additional impact of increasing age eligibility for vaccination to 12+ years; 
    • Recognising that additional social measures would likely be required to constrain epidemic growth under different achieved coverage assumptions, we estimated ability of the ‘test, trace, isolate, quarantine’ approach and different bundles of public health and social measures to reduce transmission across the population; 
    • Clinical consequences of uncontrolled outbreaks were estimated by seeding infections at the time of reaching threshold levels of vaccine coverage, for the different allocation strategies. 
  • Stated objectives of the immunisation program enabling the transition to Phase B are to constrain severe outcomes within clinical capacity and reduce the intensity and length of requirement for socially and economically impactful public health and social measures. o For ‘baseline’ levels of social and behavioural restrictions, rapid epidemic growth is expected at 50 and 60% coverage, with more substantial transmission reduction by 70 and 80% targets. In these scenarios reduced effectiveness of the public health ‘test, trace, isolate, quarantine’ (TTIQ) response is anticipated due to high caseloads; 

    • Accordingly, extended and stringent social measures would likely be required to control epidemic growth if the transition to Phase B is made at 50% or 60% coverage; 
    • Supporting optimal public health TTIQ capacity by applying continuous low level social restrictions makes the requirement for stringent lockdowns unlikely at 70% population vaccine coverage, under transmission reducing allocation strategies; 
    • At this stage of the national COVID-19 vaccine rollout, extending eligibility to key transmitting age groups offers greatest potential to reduce transmission even at lower coverage, reducing workplace absenteeism, clinical cases and deaths across the whole population; 
    • Expanding the vaccine program to the 12-15 year age group has minimal impact on transmission and clinical outcomes for any achieved level of vaccine uptake; 
    • These findings are conditional on public health workforce and response capacity which varies nationally, population compliance with public health recommendations and orders, and persistence of immunity following infection or vaccination over a 6 months timeframe; 
    • Emergence of ‘vaccine escape’ variants will require re-evaluation of targets and associated requirements for public health measures. 
  • This phase of reporting defines aspirational coverage targets to minimise the consequences of community transmission. Achievement of these targets at small area level will be critical to ensure equity of program impact, as ongoing outbreaks in undervaccinated populations are reasonably anticipated from international experience. 
  • Particular attention should be paid to groups in whom socioeconomic, cultural and other determinants are anticipated to result in higher transmission and/or disease outcomes. 
  • Ongoing situational assessment of measured transmission potential and circulating SARS-CoV-2 variants in the Australian population over coming months will allow benchmarking of these hypothetical scenarios to guide real time policy decision making about the transition to Phase B of the National Plan. 

Executive Summary of the Transmission Reducing Scenario

  • Models of COVID-19 infection and vaccination were used to define a target level of vaccine coverage for transition to Phase B of the National Plan. The model was based on the simplifying assumption of a single national epidemic, with COVID-19 transmission, severity and vaccine effectiveness as for the Delta variant.
  • Our report for 30th July 2021 National Cabinet considered hypothetical age-based vaccine allocation scenarios underpinning coverage targets of 50, 60, 70 and 80%, to explore the population level impacts of strategies focused either primarily on direct protection or transmission reduction.
  • From the starting point of age-based coverage in Australia as of 12 July 2021, an ‘All adults’ allocation strategy that achieved high coverage in key transmitting populations (20-39 years) resulted in greatest reductions in harms across all age groups, regardless of vaccination status.
    • This hypothetical scenario was mapped to an implementable strategy consistent with the national COVID-19 immunisation programme, under which vaccines would be opened up to 30-39 year olds on 31 August 2021, and 16-29 years olds from 11 October, called ‘Transmission reducing’;
    • This strategy captured the benefits achieved under the previous preferred strategy, achieving a slightly lower TP by 70% coverage, and equivalence at 80%; o    Epidemic dynamics assuming baseline restrictions and partial TTIQ were very similar to the ‘all adults’ strategy;
    • Corresponding clinical outcomes were similar or improved at coverage of 60% or above.
  • Our main report highlighted the importance of maintaining optimal TTIQ responses in the context of ongoing ‘low’ public health and social measures to minimise rapid epidemic growth and escalation of severe disease outcomes, even in a highly immunised population;

    • This report compared epidemic dynamics and clinical outcomes for the ‘Transmission reducing’ strategy assuming either ‘baseline measures with partial TTIQ’ or ‘low PHSMs with optimal TTIQ’;
    • Infections and corresponding adverse consequences were reduced by several orders of magnitude, assuming ongoing light restrictions and sustained highly effective public health response capacity;
    • The ability to deliver this capacity is greatly assisted by the more even distribution of reported cases over the 6 months time window of reporting, given an absence of rapid epidemic escalation.
  • As in our previous report, the contingency of these outcomes on population behaviours including vaccine acceptance, co-operation with behavioural restrictions and active engagement and compliance with public health responses is critically important for achieving programmatic outcomes.
  • Our models assume a point source outbreak as the key initiating event for transmission. Given the low caseloads achieved under the ‘optimal TTIQ’ scenario and considered desirable in Phase B, the influence of imported infections on local epidemic dynamics merits further exploration in the next phase of modelling.

Erratum to Doherty Modelling Technical Report and Addendum

Update and further information on synthetic population size

The synthetic population used in the dynamic transmission model consists of a fixed number of households that represent the Australian population. The sizes of these households are chosen according to a distribution aligned with the average household size in Australia, and so the total simulation population changes slightly between simulations (the total changes by less than 0.05% from the average of 24,960,000). The difference in total population size from the census population estimate provided by Quantium does not materially impact epidemic dynamics and subsequent clinical outcomes. 

Corrections to clinical outcomes tables

Technical report

We identified an error in the code that summed clinical outcomes for reporting in Tables 5.4–5.7 which incorrectly computed the number of unvaccinated individuals in the 40–59 age group. Note that the underlying model code that produced the outputs is not affected by this error. The plots of these outcomes over time are unchanged (Figures 2.1–3.3).

As a result of these corrections (indicated in red below), the total numbers of clinical outcomes are marginally reduced in Tables 5.4–5.7, the trends are consistent.  

Secondly, when calculating the totals for Tables 5.1–5.3, both 60+ and 70+ age groups were summed, thereby ‘double counting’ the 70+ years outcomes. This error has been corrected.

Addendum - transmission reducing strategy

The errors in outputting summary tables for the Technical report were carried over to the second phase of work. The corresponding tables are 4.1-4.7.

The original tables and their corrections marked up in red are presented in the following pages for ease of comparison.