Doherty Modelling Assumptions Don’t Adequately Account for Changed TTIQ Capabilities

While the effectiveness of ‘test, trace, isolate, quarantine’ (TTIQ) is dependent upon case numbers, new analysis from the Australia Institute shows the Doherty Modelling does not adequately take this into account.

Key findings:

  • While most discussion is around vaccine rates and COVID cases at the time of transition to Phase B (‘opening up’) a significant issue with the modelling is its assumption that the efficacy of the test, trace, isolate, quarantine (TTIQ) does not decline in response to rising numbers of ‘mystery cases’.
  • At 70 per cent vaccine coverage, the contribution to driving down the infection rate (the transmission potential or TP) is roughly equally split between the TTIQ response and the effect of the vaccine itself.
  • That is, the path to a successful transition to Phase B has as much to with the effectiveness of TTIQ response as it does with vaccination rates. Put simply, if the TTIQ assumptions are optimistic then the results of the Doherty modelling will be excessively optimistic, and the frequency of future lockdowns and the likely number of deaths will be far higher than has been suggested.

“Our ability to ‘Test, Trace, Isolate and Quarantine’ people who have been exposed to Covid19 is the front line of Australia’s defence against Covid. It is our TTIQ workforce that stopped the Ruby Princess outbreak from infecting the whole country. But while Australia’s TTIQ is world class, it can and it will be overrun if case numbers are allowed to keep rising,” said Dr Richard Denniss, chief economist at the Australia Institute.

“The Doherty Modelling that the Prime Minister is relying on is literally built on the assumption that our TTIQ system is functioning well. While we hear a lot about the rollout of the vaccines, the Doherty Modelling makes clear that without a highly functioning TTIQ system we have no chance of stopping our country’s ICUs from being overwhelmed.

“The problem is that when the Doherty Modelling was prepared for National Cabinet on 4th June, the prospect of an outbreak as big as Melbourne’s second wave happening again seemed remote. And as a result the Doherty Modelling makes assumptions about the effectiveness of TTIQ based on the ability of contract tracers to do their job based on their performance in Melbourne last year. But now that the NSW outbreak is already that big, and the number of mystery cases is rising rapidly, it is simply unrealistic for the Prime Minister to keep relying on modelling that is based on the assumption that TTIQ effectiveness will hold up no matter how many cases there are in the community.

“While it is true that the Doherty Modelling suggests it doesn’t really matter how many active cases there are if we lift lockdowns and other restrictions, what few people realise is that the modelling only shows that because it assumes that TTIQ effectiveness won’t decline steadily as cases continue to rise. If that assumption is unrealistic, then so are the conclusions of the model.

“Given how important TTIQ is to the assumptions of the Doherty Modelling, there needs to be an open and transparent conversation about whether or not TTIQ capabilities are effected by the number of new cases and the number of mystery cases.”

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