Tony Blakely: The five scenarios facing Victorians as Delta variant spreads

Victoria’s lockdown deadline is looming closer — and a graph has revealed how things could pan out over the next three months.

Tony Blakely, an epidemiologist and public health medicine specialist, is a professor at the University of Melbourne.

Lockdown 5 in Victoria was textbook. An outbreak occurred, contact tracing swung into action, cases were found, and over time the number of daily cases infectious in the community diminished to zero for three days, and we came out of lockdown.

Lockdown 6 is not going well. Victoria cannot seem to tip the balance, despite even stricter restrictions. In fact, the majority of cases found have been infectious while out in the community, and the numbers are going up rather than down. Even worse – and the reason for this bad trend – is the constant stream of mystery cases.

When there are mystery cases, there will, by definition, be future cases in the community.

I strongly support the Victorian government’s decision to go “really hard” with curfews, by closing playgrounds and childcare, and further reducing essential workplaces. It would be so much better to re-eliminate the virus, and wish for a near-normal life until the end of the year. But we need to be realistic. The elimination strategy always had, and continues to have, a planned shift to living with the virus once we are ready. Ideally at something like 80 per cent adult vaccination coverage (although personally I would prefer that to include children, as well).

Crowds enjoy a warm Sunday at St Kilda beach. Picture: NCA NewsWire/David Geraghty

The virus may be forcing our hand earlier than we planned. If the hard restrictions in Victoria do not lead to a dramatic improvement by September 2, such as fewer than five cases a day (which seems very unlikely), what should we do?

We could hang tough, and keep the number of infections as low as possible by staying in hard lockdown longer. But for how long? Victoria has been in lockdowns for more than 200 days now. The citizens are pretty sick of it, with growing concerns about the mental health effects, and the economic damage.

This is wickedly difficult decision-making for governments. It is about picking the least harmful course of action.

The Covid-19 Pandemic Trade-offs group I lead at the Melbourne School of Population and Global Health has done a lot of modelling of this pandemic, but not exactly the Victorian scenario leading up to September 2. Nor have the Doherty Institute and my collaborators in the Australian Covid-19 Modelling Initiative.

Victoria’s Covid numbers include a constant stream of mystery cases.

What I present here is not a model per se, but a framework to help us think our options through.

The first figure shows three scenarios, assuming we make some progress by September 2.

In the first scenario, we have hit 40 cases a day a week before September 2, then five cases a day by September 2 – so we come out of lockdown. And we get lucky with no further incursions and outbreaks from hotel quarantine or across the NSW border before the end of November, when 80 per cent of adults will be at least 10 days post their second dose, and we shift into phase C of the national plan. We all hope this will happen, but it does seem unlikely.

The second, orange, scenario starts out the same as the green scenario, but we have repeated monthly incursions requiring repeated fortnightly lockdowns (each of which works, so still an optimistic assumption in that regard – although it will get easier to stamp out outbreaks as vaccine coverage increases). Here we oscillate our way to the end of November with a total of about two months in hard lockdown.

The third scenario is where some progress has been made by September 2 – but only to the point of 30 cases a day, requiring another four weeks of hard lockdown to drive the case numbers down to five a day before stepping out of lockdown. Not great, but a return to zero transmission.

But what if we have not made any progress by September 2, and the case numbers are still around 50 a day (or worse)? What should we do then?

We could stay in hard lockdown and wait until the vaccine coverage tips the balance our way, and we tip to numbers going down. A scenario similar to the blue line in the first figure, but longer in lockdown.

There are other options, though.

Eliminating the virus is becoming more and more difficult everyday. Picture: NCA NewsWire/Andrew Henshaw

We could let go of hard lockdown and shift to a soft lockdown. A world where more time is allowed outside, no curfews, you can travel 10km or more from your home, some retail is reopened, and construction and other industries on the “essential” threshold are started up.

Using a Victorian-style stage 3 lockdown as an approximation for a soft lockdown, and using the apparent tipping point in terms of vaccine coverage when it is enough to cause numbers in our NSW modelling to start going down, I constructed the second figure. In this fourth “moonshot” scenario the numbers go up as expected, but then turn and decrease with increasing vaccine coverage.

This moonshot appears to work if the criterion for success is restricting daily case numbers to fewer than 500 on the way to the end of November. The flip side, though, is we are in a soft lockdown the whole time (or until early November). And it is – of course – risky and uncertain.

The fifth scenario is for a 2 percentage point higher daily change in cases at each point in time (that is, where I got the maths slightly wrong, or we got unlucky). It is enough to quickly see the case numbers hit 500 by virtue of exponential maths.

We hope that the numbers dramatically turn down in the next week, but we need to also start discussing what our options are come September 2, if the numbers have not dramatically decreased.