Doherty Modelling Report Revised 10th August 2021

Abstract

Executive Summary – Doherty Modelling Report Revised 10th August 2021

• 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.
  • 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.
  • 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 – Addendum To Doherty Modelling Report Revised 10th August 2021

• 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%;
  • 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.

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