Over the last fortnight, the Australian state of Victoria has seen continued and sustained high COVID-19 cases, by Australian standards, with a concerning average of 321 reported cases per day from Friday to Monday inclusive.

On Thursday 22 July, face masks will become mandatory for metropolitan Melbourne and Mitchell Shire.

Also in the east of Australia, New South Wales sits on the brink of tipping over to join Victoria with uncontrolled community transmission.

Conjecture that Victoria's Premier Daniel Andrews may introduce a tougher lockdown is materialising, putting us somewhere between a Stage 3 and Stage 4 lockdown (currently closer to Stage 3).

Stage 3 restrictions require people to stay at home unless they are shopping for essentials, caregiving, exercising, working or studying (if it's not possible to do this from home). Added to this list is now mandatory mask wearing and 'encouragement' to not leave your immediate suburb.

But the option of a full Stage 4 lockdown remains if the numbers don't come down and/or we do not COVID-behave better.

Most notably, that would mean a strong or even severe tightening of the definition of essential workers and workplaces. Premier Andrews has already said they are undertaking a 'line-by-line' review of which workplaces and workers are generating excessive cases and outbreaks, for targeted tightening – which is sensible.

But the bottom line is we should only use Stage 4 lockdown bluntly when we have a worthy goal – not otherwise.

So, what is a Stage 4 lockdown? The short answer is that in the Australian context, we do not know.

But like so many things, we can borrow a page or two from the New Zealand COVID-19 playbook, where they call it Level 4.

Over there, it looks like:

1. Staying home
2. Tightly defining what an essential worker is (not tradies working on your long-planned home renovations). Essential health care workers include people working at care facilities, funeral homes and pharmacies, as well as doctors. Essential non-health care workers include those people working in the accommodation needed for essential workers, necessary courts, customs, those in essential construction (i.e. those necessary for supply chains – not just to stimulate the economy), fast-moving essential consumer goods (e.g. supplying supermarkets), and people in essential primary industries (e.g. dairy farming, horticulture).
3. Shutting down businesses with the exception of the essentials like supermarkets, pharmacies and GP surgeries (which obviously intersects with the definition of essential workers).

As New Zealand has proven, a Stage 4 lockdown is, without doubt, more effective at slowing the virus. The country's last reported a case of community transmission two-and-a-half months ago.

But we have to ask ourselves - what is the purpose of a Stage 4 lockdown? As with so many things with COVID-19 you need a goal, but this is so rarely articulated by our leaders.

There are two very good reasons which would justify a full-blown Stage 4 lockdown:

1. Because we are going for elimination.

2. Because our health services, and especially our ICU capacity, may be at threat of being over-run by caseloads.

Our recent research, published in the Medical Journal of Australia, quantified the probability of getting to elimination in six weeks of lockdown.

A Stage 4 lockdown plus (critically) 90 per cent of people wearing face masks when they're out of the home gets us to nearly a 50 per cent chance of eliminating community transmission by 19 August (the end date of the six-week lockdown).

But I cannot support, and do not think the community should support, a blunt Stage 4 lockdown 'just' to take the edge of the current case load and 'aggressively suppress' the virus when it will simply come back again.

Rather, we need 'smarter suppression' if we are not going to have a go at elimination.

Why?

Because of those severe economic impacts and the social consequences, it's frankly not worth it just to suppress the virus for a while, then have another surge and another lockdown; and again; and again.

This 'ground-hog day' is not a preferential way of living, at the cost of peoples' livelihoods and incomes.

If you are not going for elimination, then we should look to control the virus at a level that does not exceed our health services capacity.

In terms of ICU capacity in Victoria, about 840 additional ICU beds are being brought online, complementing the already freed up capacity achieved by reducing elective surgery. Call it 1000 beds.

Roughly speaking (it varies enormously by age), about two per cent of notified cases will need ICU, for about 10 days each. So that capacity would be exceeded with 5000 cases a day – ten or more times what we are at now.

But because you want a safety margin, you would probably go to Stage 4 to protect ICU capacity if 2,500 cases a day seemed likely without doing so – if defending ICU capacity was the goal. Obviously, these are quick calculations and need refinement as new information comes in.

So, only if the current enhanced Stage 3 lockdown does not keep us within that limit do we consider going into full-blown Stage 4 (if elimination is off the table).

Am I being callous? To some extent – yes.

I am implicitly advising that we 'accept' some level of infection and mortality and morbidity risk under enhanced Stage 3 lockdown, which will mean people dying.

This is the sharp edge of decision making. But at least it does not hit more young and lower socioeconomic people's livelihoods and futures harder than it needs to. These are tough trade-offs.

Any decision to go into full-blown Stage 4 lockdown must not be undertaken lightly, and our leaders must be very clear about the rationale for using it.

Mandatory mask wearing is a welcome move, because it is both effective and costs the economy nothing – great.

Likewise, selective tightening of essential workplaces may be justified, if mask wearing alone is not enough to slow the virus.

But if we are not going for elimination, we have to make sure the economic and social impacts of blunt Stage 4 lockdown are only unleashed if our health services are under threat.

Our leaders must be thinking about the clear and justifiable purpose for any tougher lockdowns.

Banner: Getty Images