BACK

How well can we model the path out of lockdowns?

This article was published on
August 6, 2021

This explainer is more than 90 days old. Some of the information might be out of date or no longer relevant. Browse our homepage for up to date content or request information about a specific topic from our team of scientists.

The road toward a life without major lockdowns in Australia looks a little clearer, with the federal government announcing COVID-19 vaccination targets of 70 per cent and 80 per cent amongst the eligible population as triggers for reduced restrictions.

The road toward a life without major lockdowns in Australia looks a little clearer, with the federal government announcing COVID-19 vaccination targets of 70 per cent and 80 per cent amongst the eligible population as triggers for reduced restrictions.

Publication

Doherty Modelling Report for National Cabinet 30 July 2021

Not peer-reviewed
This work has not been scrutinised by independent experts, or the story does not contain research data to review (for example an opinion piece). If you are reporting on research that has yet to go through peer-review (eg. conference abstracts and preprints) be aware that the findings can change during the peer review process
Peer-reviewed
This work was reviewed and scrutinised by relevant independent experts.

What our experts say

The road toward a life without major lockdowns in Australia looks a little clearer, with the federal government announcing COVID-19 vaccination targets of 70 per cent and 80 per cent amongst the eligible population as triggers for reduced restrictions.

This week, the Doherty Institute released the modelling it provided to the National Cabinet to help set those targets. The model is based on a single national Delta variant epidemic, and looks at the outcomes for Australia if we had 50, 60, 70 and 80 per cent of the population aged 16 and above vaccinated; how many cases, hospitalisations and deaths would occur and how well health departments would be able to test, trace, isolate and quarantine cases.

Professor Ivo Mueller, Co-Division Head of the Population Health and Immunity Division at The Walter and Eliza Hall Institute, told the AusSMC this modelling provided a framework for how Australia can transition from closed borders and frequent lockdowns to a more open way of life. 

"The presence of the highly infectious Delta strain means the current strategy of aggressive suppression with early, short and sharp lockdowns will need to continue until we reach 70 per cent vaccine coverage," Prof Mueller said.

He said 70 per cent was not a "magical threshold," with transmission potential continuously decreasing as vaccination coverage increases. This, Prof Mueller said, would lead to more effective and less frequent lockdowns and by the time 70 per cent of the eligible population is vaccinated short lockdowns and low-level public health measures should be enough to keep the virus under control,

"Only once we approach 80 per cent vaccine coverage, will life be able to gradually approach a ‘new normal’," he said.

A key takeaway from the Doherty modelling is that extending eligibility to "key transmitting age groups," such as younger people, has the potential to reduce transmission further even with less vaccination coverage.

The modelling showed that at this stage of the national COVID-19 vaccine rollout, extending vaccinations to all adults, rather than the oldest first, could lower rates of death and hospitalisations.

However, because Australia's current supply of vaccines is limited, the researchers went on to model how an 'all adults' scenario compared with the planned national COVID-19 rollout, under which vaccines would available for 30-39 year olds on 31 August 2021, and 16-29 years olds from 11 October - a strategy the modelling called ‘transmission reducing’.

This modelling found that a ‘transmission reducing’ vaccine strategy was marginally better than an ‘all adults’ strategy. 

Professor Robert Booy from the University of Sydney said the question of vaccinating the young or the old is a nuanced dilemma.

"The risk of dying from COVID-19 in your 30s is four times higher than for a teenager, the risk of dying from COVID-19 in your 50s is almost 40 times higher than a teenager. So what to do? The priority remains to get at-risk people vaccinated rapidly," Prof Booy said.

However, he said a person in their 30s might be a schoolteacher or an essential worker delivering food and therefore unable to restrict their movement.

"We need to think more outside the box. Efforts to dramatically improve supply need to continue so that current demand and need can be met," he said.

Associate Professor Alexandra Martiniuk from the University of Sydney said even younger people needed to be considered more when planning a path out of lockdowns. 

"Using vaccination targets in the >16 year age group as milestones for moving between phases of the opening-up plan misses an important cohort of the population – children."

The Doherty modelling did look at expanding the vaccine program to the 12-15 year age group, but if found that it would have minimal impact on transmission and clinical outcomes. A/Prof Martiniuk argues that we need a scenario modelling vaccination for all ages including kids under 12. 

"We need milestones to take into account children at all times," he said.

Professor Emma McBryde from the Australian Institute of Tropical Health and Medicine at James Cook University is also concerned about some assumptions in the modelling when it comes to children.

Her own vaccination modelling is based on an assumption that as many as 5 people could catch the virus from an infected person, a figure called the reproduction number. If that's the case, Prof McBryde says we need to vaccinate children to achieve herd immunity.

"The Doherty Institute assumes that the reproduction number (they call this transmission potential) is only 3.6. If we model this, we get the same findings but the results are highly sensitive to the assumed effective reproduction number." 

Professor Nikolai Petrovsky from Flinders University agreed that the Doherty model was likely to be very sensitive to how strongly vaccines reduced transmission, a question that we may not fully know the answer to yet. He said vaccine effects on transmission are notoriously hard to measure and in most cases should be considered no better than educated guesses. 

"Recent Covid-19 outbreaks have occurred in populations with high vaccination coverage approximating those predicted by the model.  With delta strain achieving exceptionally high transmission rates even amongst the vaccinated, the model and its assumptions need to be closely examined, before such data is relied upon to make any policy decisions," Prof Petrovsky said.

Prof Martiniuk said while we need to be cautious of the simplified nature of modelling, it was a good place to start when planning how to move forward.

"The Doherty Institute Modelling Report for National Cabinet is useful in understanding how we might transition between the various phases of opening up. The modelling is useful in that it is based on Delta in terms of transmission, severity and vaccine effectiveness," she said.

"However we do need to be wary that the model is based on this being a “single national epidemic” in order to simplify the modelling. We also need to be wary of new variants of concern emerging beyond Delta, which of course, would affect any current model’s ability to predict future scenarios."

Context and background

Resources

Media briefing

Media Release

Expert Comments: 

Professor Robert Booy

It’s the end of August, and the end of the day, you’ve got one COVID-19 vaccine left. Two patients are booked in, one aged 30 and one aged 50. Do you vaccinate the 30-year-old to prevent transmission better or do you vaccinate the 50-year-old who is at greater risk of disease?

Teenagers are at very low risk of complications and indeed healthy 12 to 15-year-olds are not presently being recommended for routine vaccination (Only those with underlying problems)

The risk of dying from COVID-19 in your 30s is four times higher than for a teenager, the risk of dying from COVID-19 in your 50s is almost 40 times higher than a teenager.

So what to do? The priority remains to get at-risk people vaccinated rapidly.

However the person who is 30 may be a schoolteacher or a delivery person for essential food. Such professions are important to be vaccinated. But the decision to make by professionals is clearly nuanced.

We need to think more outside the box. Efforts to dramatically improve supply need to continue so that current demand and need can be met.

Associate Professor Alexandra Martiniuk

Using vaccination targets in the >16 year age group as milestones for moving between phases of the opening-up plan misses an important cohort of the population – children. While the Doherty Modelling does provide a scenario taking into account vaccination of children 12+ years, we need a scenario modelling vaccination for all ages (including <12 years) AND we need milestones to take into account children at all times.

The Doherty Institute Modelling Report for National Cabinet is useful in understanding how we might transition between the various phases of opening up. The modelling is useful in that it is based on Delta in terms of transmission, severity and vaccine effectiveness. However we do need to be wary that the model is based on this being a “single national epidemic” in order to simplify the modelling. We also need to be wary of new variants of concern emerging beyond Delta, which of course, would affect any current model’s ability to predict future scenarios.

Nikolai Petrovsky

Model predictions are only as good as their assumptions.  Any modelling result needs to be treated with a healthy degree of scepticism. A key issue is the data this model is relying upon to predict the size of any impact of vaccination on virus transmission. Vaccine effects on transmission are notoriously hard to measure and in most cases should be considered no better than educated guesses.

This model is likely to be highly sensitive to the input value of this vaccine transmission-reducing effect, which seems high and is based on just a single study result. It would be usual in such situations to do a sensitivity analysis where the value of this key input is varied to explore to what degree this could affect the modelling result. It would be good to see this done here.  For example what does the model predict, if the vaccine effect on transmissibility is instead set at the same value as the effect of the vaccine on virus infection?

Recent Covid-19 outbreaks have occurred in populations with high vaccination coverage approximating those predicted by the model.  With delta strain achieving exceptionally high transmission rates even amongst the vaccinated, the model and its assumptions need to be closely examined, before such data is relied upon to make any policy decisions.

Highly publicised models from the London School of Hygiene and other groups early last year all predicted that the pandemic would be over in 2-3 months and we now know just how wrong those models turned out to be. Disastrous public health decisions were taken by governments such as the UK based on those flawed models.

Professor Ivo Mueller

The very thorough modelling conducted by Professor McVernon and her team provides an excellent framework for safely guiding Australia from its current state of aggressive suppression of community transmission to a COVID-safe future where we are able live with few restrictions and be reconnected with the rest of the world. This will require striking a delicate balance between managing health risks, economic costs and personal freedom.

The presence of the highly infectious Delta strain means the current strategy of aggressive suppression with early, short and sharp lockdowns will need to continue until we reach 70 per cent vaccine coverage. However, this should not be seen as a ‘magical’ threshold. As vaccine coverage levels grow, transmission potential will continuously decrease and the efficacy of test, trace, isolate and quarantine (TTIQ) will increase, making short, sharp lockdowns both more effective and overall, less likely. Once we reach 70 per cent vaccine coverage, these and other low-level public health measures are predicted to be sufficient to contain the remaining transmission potential, where we can safely shift the paradigm from transmission suppression to minimising severe COVID-19 cases and deaths.  

Very sensibly, this paradigm shift will itself be a gentle one, with continued effective TTIQ and other low-level intervention and a careful, modest opening of international borders. Only once we approach 80 per cent vaccine coverage, will life be able to gradually approach a ‘new normal’. While clearing the high bar of 80 per cent coverage will require everybody to really do their share, the modelling by the Doherty team not only provides an evidence-based road map out of COVID-19 but offers reassurance that this can be done safely.

Emma McBryde

The Doherty modelling has many similarities with my own that I have on preprint and have been discussing over the last month and this online tool.

Once we target the elderly with the vaccine, the next best group is the young adults aged 18-30, not 60s, 50s, 40s etc.

If we only vaccinate 70 or 80 per cent we will need to have additional measures in place.

One finding that is very different to my work is the impact of teenagers on herd immunity. My group starts with an assumption that the effective reproduction number (before vaccination) could be as high as five, and if it is, then we need to vaccinate children to achieve herd immunity.

The Doherty Institute assumes that the reproduction number (they call this transmission potential) is only 3.6. If we model this, we get the same findings but the results are highly sensitive to the assumed effective reproduction number. The Doherty needs to clarify this and consider a range of reproduction numbers.

Q&A

No items found.