New Doherty modelling advises ‘medium’ Covid restrictions until Australia reaches 80% vaccination

·6 min read
<span>Photograph: Mike Bowers/The Guardian</span>
Photograph: Mike Bowers/The Guardian

New Doherty Institute modelling presented to national cabinet warns that maintaining “medium” public health and social measures would be “prudent” until Australia reaches 80% vaccination if caseloads are high – with “medium” measures previously defined as including stay-at-home orders except for work, study and other essential purposes.

The institute – which conducted the modelling informing Australia’s four-phase reopening plan – has updated its work after a dispute erupted within the federation about whether or not it was safe to ease restrictions once 70% of Australians over the age of 16 were vaccinated.

While the New South Wales premier, Gladys Berejiklian, warned other premiers to stick to the reopening plan agreed by national cabinet, and ease restrictions at 70%, other leaders in other states queried whether the first run of modelling was a reliable barometer of the risks because it assumed low case numbers.

However, following Friday’s national cabinet meeting, Scott Morrison said the Delta variant “can be managed” at 70% and 80% vaccination of the over-16 population, when combined with “appropriate” public health and social measures (PHSM), as well as test, trace, isolate and quarantine practices.

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“Doherty’s sensitivity analysis showed that, even if an outbreak starts with hundreds or thousands of cases, its original conclusions for transitioning to Phases B and C of the National Plan at 70% and 80% vaccination rates respectively remain robust,” the prime minister said in his statement.

“The sensitivity analysis confirms that, when daily case numbers are in the tens or hundreds, movement to Phase B can be achieved with vaccination rates of 70% when combined with low-level PHSMs and partial (test, trace, isolate, quarantine) TTIQ or alternatively baseline PHSMs and optimal TTIQ.

Guardian Australia has seen a summary of the new sensitivity analysis, which was presented ahead of Friday’s meeting, noting it also warned Australia’s political leaders the Covid-19 pandemic will continue to be “a fire fought on multiple fronts”.

According to that summary, the Doherty Institute considered three scenarios where coronavirus cases were in the tens, the hundreds and thousands.

In the scenario where 70% of Australians over 16 were vaccinated and there was an increase from tens to hundreds of seeded infections while governments maintained baseline public health and social measures with partial testing and tracing capability, the simulated epidemic “completes” in 180 days.

But when thousands of cases were seeded at 70% with the same assumptions about public health measures and testing and tracing, the size of the epidemic increased “significantly” because “the window in time between 70% and 80% coverage is sufficient to allow early epidemic growth from high numbers”.

Doherty says much less impact on the overall size of epidemics was observed when its three seeding scenarios – tens, hundreds and thousands – were introduced once 80% of Australians over 16 were vaccinated.

Given the “observed sensitivity” associated with high seeding infections at the 70% vaccination threshold, it says “the ongoing application of medium public health and social measures” would be “prudent in such cases, at least until the 80% coverage threshold is achieved”.

In its original modelling, “medium” public health and social measures were defined as stringent capacity restrictions, group size limits and stay-at-home orders except for work, study or other essential purposes.

In the new analysis, the Doherty Institute notes that at high caseloads it would be very difficult for governments to maintain optimal testing, tracing, isolation and quarantine practices. There would be a need to maintain flexibility to strengthen public health and social measures either “generally or locally” to regain epidemic control.

“The Doherty modelling confirms that with high vaccination and appropriate TTIQ and PHSMs to constrain outbreaks, overall cases and deaths are expected to be similar in order of magnitude to annual influenza,” Morrison said in his statement.

In 2019, the year before the pandemic, 953 Australians died after contracting influenza. The five year average before 2019 was 403.8 deaths.

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The report that went to Scott Morrison, the premiers and chief ministers emphasises the three scenarios it considered are “clearly and deliberately artificial” because their purpose is to inform “high level policy strategy”.

“Their key message is to highlight the importance of a combination of timely public health responses … and ongoing social and behavioural measures to contain transmission, even in highly immunised populations,” the institute says.

The epidemiological experts warn that “in reality, the national Covid-19 epidemic has been, and will continue to be, a fire fought on multiple fronts”.

“Bridging of this high-level strategy to implementation requires attention to localised risk determinants, differential impacts of public health and social measures, small area reporting of vaccine coverage and optimisation of testing, tracing, isolation and quarantine and public health responses to address local outbreaks.”

The main scenarios modelled in the first Doherty report estimated how rapidly and how far a single outbreak involving 30 individuals would spread through the Australian population at the time of transition to phase B of the national plan – which is when 70% of the adult population is vaccinated.

NSW is currently reporting daily case numbers over 1,000, Victoria in the hundreds, and the Australian Capital Territory in the tens.

The federal health secretary, Prof Brendan Murphy, updated the nation’s leaders on the health system’s capacity to deal with Covid as the country transitioned into the next phases of the national plan, as well as the capacity of test, trace, isolate and quarantine practices, given its importance to the plan’s success.

Those updates were not released, with Morrison noting the “leaders discussed in detail the health system capacity within jurisdictions, with further analysis to come back to the next meeting of national cabinet”.

The leaders did agree to integrate people’s Covid vaccination history into the state and territory check-in apps. That will act as a quasi-vaccination passport, with the apps to be used as part of the state and territory public health orders. There was no time limit offered on how long that will be required.

The cabinet won’t meet again until 1 October, when Morrison has returned from his visit to the US.

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