23 Aug 2021
Explainer - Doherty Institute Modelling Report for National Cabinet
The Commonwealth Government commissioned a consortium of modellers led by the Doherty Institute- to advise on the plan to transition Australia's National COVID Response. The first phase of work has played a major role in ongoing Australian pandemic planning, contributing to recent decisions by policymakers to have a staged “re-opening” once two-dose COVID-19 vaccine coverage reaches 70% and then 80% of the population. Models provide useful frameworks to explore ‘what if’ scenarios that can support decision making for uncertain futures. Here we clarify the purpose and limitations of the modelling.
The report’s purpose
An important goal of policy makers is to reduce the harms associated with COVID-19, which includes social and economic harms caused by lockdowns. However, lifting them and seeing people interact and mix again brings a risk of more COVID-19 transmission.
The initial report models a range of scenarios which policy makers can use to gain insight into the disease impact of their decisions. Guided by these scenarios, as well as information from other sources about the adverse impacts of interventions, policy makers can formulate a strategy most likely to minimise the overall harm to society.
The scenarios
The report provides a range of hypothetical epidemic scenarios. It considers what an ‘Australia-wide’ outbreak might look like under different interventions. We compare what happens if the outbreak starts at the time of achieving 50%, 60%, 70% or 80% two-dose coverage of the population aged 16 years and older (16+). The model assumes vaccine uptake continues to rise in every scenario, reaching a final endpoint of approximately 90% of the 16+ population. It also assumes that the vaccine uptake is the same through the entire Australian population and that there are no pockets of lower vaccination. Hence, the model gives insight into when population-wide lockdowns will not be necessary, but careful local interventions may still be needed.
The primary question the modelling addresses is, “what would spread look like under ‘baseline interventions’ (which are no stay-at-home orders, and minimal social distancing) combined with a test, trace, isolate and quarantine (TTIQ) program?” We found that at 50% or 60% vaccination coverage, even with a very good TTIQ program, the infection spread would be very rapid. These scoping scenarios were deliberately ‘artificial’ as in reality, additional social measures would have been imposed to limit infection and disease at these levels. However, they let us compare that scenario with the same conditions at 70% and 80% coverage, where the spread was observed to be much slower.
Even at 70% and 80% adult coverage, vaccines can’t completely stop COVID-19 spreading through the population. For example, even at 80% coverage, if TTIQ is only partially effective, hundreds of COVID-19 deaths are anticipated over the following six months, with about a third of these occurring in vaccinated people (who make up the majority of the population). These deaths can be substantially reduced if TTIQ is highly efficient (termed ‘optimal’), and/or by applying a low level of ongoing social restrictions that limit infection spread and support the public health response. It leads us to conclude that in combination with timely TTIQ, supported by continuous low-level social restrictions, vaccines can substantially reduce the likelihood that we would need to resort to wide-scale lockdowns in the future.
A secondary question addressed is, “what strategy for prioritising vaccination to certain groups results in the best outcomes?” We found that once the highest risk groups (in particular the oldest age groups) have been offered vaccination the overall impact of the program can be maximised by redirecting the focus to younger adults who are responsible for much of the spread. This helps slow the growth of an epidemic and because there’s less COVID-19 around, both vaccinated and unvaccinated people are better protected while vaccination coverage continues to increase.
Policy impact
In response to the initial report and related work by Treasury, governments have made several policy decisions. Most notably, the National Cabinet decided to build their plans around a staged re-opening as double-vaccination (two dose) levels cross the key thresholds of 70% and 80% of the 16+ population. The explicit aim during these phases is to keep cases low, to maximise health and economic benefits. They also adjusted the distribution strategy of the vaccine rollout to open up access to young adults at an earlier timepoint.
Expectations
While we believe large-scale lockdowns will eventually become a thing of the past, we need to be cautious in our approach, continuously evaluating the situation, with flexibility to adapt our approaches to new information (e.g. the decision by ATAGI to allow children 12+ to be vaccinated, or the possible emergence of a new variant).
The modelling is intended to provide insight for high-level strategy, and, as such, considers the Australian population as a whole. Hence, there is much more work to fully understand population heterogeneity, including how to manage outbreaks in settings where transmission may be higher, such as schools, remote communities and areas with lower vaccine coverage.