Maximizing the probability that the 6-week lock-down in Victoria delivers a COVID-19 free Australia

Victoria is the unlucky state in a lucky country. Most other states and territories, except for NSW, have achieved elimination of community transmission of the pandemic virus SARS-CoV-2 (28 or more days of no locally acquired cases where the source is unknown). 

The situation in NSW is mixed and precarious.  On the one hand, NSW has a current outbreak of 40 cases (as of 16 July) linked to the Crossroads Hotel outbreak in Casula.   On the other hand, until 16 July there had only been three locally acquired cases of SARS-CoV-2 infection with no known source in the preceding 28 days suggesting NSW was on the cusp of elimination ¹  – but three cases have been reported on 16 July with no obvious link to the current outbreak.²  If NSW successfully contains this outbreak, it may resume its prior trajectory towards the elimination of local transmission, leaving Victoria isolated as the only state with community transmission.

It seems highly unlikely that states and territories that have eliminated local transmission will relinquish their status by freely opening borders and engaging with Victoria (and NSW if community transmission remains). Australia going forward as two separate systems (six or seven states and territories having eliminated the virus, one or two not) is a significant concern.  Both the Victorian economy (comprising 23.7% of Australia’s economy ³) and the wider Australian economy will be adversely impacted for a long period due to the public health measures necessary to prevent and control recurrent outbreaks arising from resurgent community transmission.  

There are three general strategic policy responses to the challenge of COVID-19: elimination, suppression, and mitigation (or herd immunity).  No response is free of economic, social and health harms – rather, it is about minimising harm.⁴  Society has largely rejected a mitigation response due to the high morbidity and mortality inherent (e.g. if 60% of the population were infected the number of deaths may exceed 100,000 under current case fatality rates ⁵).  We argue that explicitly pushing towards a strategy of elimination across all of Australia is optimal given where we are at and what we know. 

Elimination strategy

We know from New Zealand (NZ; population 5.0 million)⁶ and Taiwan (23.8 million)⁷ that elimination of community transmission is achievable in island jurisdictions, both having no reported community transmission for over two months as of 10 July.⁸  The advantage of elimination is that despite international border closures or strict quarantine, citizens can go about life with a near-normal functioning of their society and economy. 

Elimination has challenges.  First, there is the extra effort to achieve it – and the fact that aiming to achieve elimination does not guarantee success.  Second, having achieved elimination, there would be the constant risk of the virus re-entering due to quarantine breaches.  This is evident from the two quarantine hotels in Melbourne that leaked cases of SARS-CoV-2 infection.  However, if we learn from experience, there is no reason why the quarantine of arrivals to a country cannot be made near-perfectly secure with the appropriate processes and resources.

How sustainable a COVID-19 free nation with tight border controls is in the very long term is unclear, especially when international tourism begins to be resurrected in the rest of the world.  If an effective vaccine does not arrive in the next year or so, then at some point a COVID-19 free country may choose to pivot away from maintaining elimination towards suppression or mitigation. This could involve the easing of border restrictions and the return of disease spread but where the mortality and morbidity burden is substantially lessened through improved treatments and social mechanisms to protect the elderly and vulnerable (e.g. good ring-fencing of rest-homes).  The point now, though, is that living in a state or country that has achieved elimination is a far better option than suppression in the short- to medium-term, compared to the high likelihood of recurrent outbreaks precipitating recurrent lock-downs with attendant social and economic disruption.  For example, NZ used a more stringent lock-down that Australia in March to April 2020 (Supplementary Figure 1 - available in PDF), rapidly achieved elimination, and its economy appears to have bounced back more strongly than Australia’s, at least on the measure of payroll job estimates (Supplementary Figure 2 - available in PDF). So too, COVID-19 free Australian states and territories (e.g., Queensland) are benefiting economically, enabling the re-opening of business activity and hosting of national sporting teams and fixtures such as the AFL.

Is elimination achievable with a six-week lock-down?

Lock-downs are effective for pandemic control.^9,10 Our case for an explicit elimination strategy in Victoria, now, is that given the State is in lock-down for six-weeks there is only a marginal cost of ‘going hard’ with a rigorous public health response that increases the probability of achieving elimination.  But is elimination achievable within six-weeks? 

We examined four policy scenarios using an agent-based model, a type of micro-simulation of individuals.  The model accurately reflects the prior experience of both NZ and Australia ¹¹, and in this paper, we have adapted it to Victoria (including the case counts up to 14 July) (see Appendix for details - available in PDF). The four policy approaches were:

1. Standard, reflecting the first Australian lock-down (calibrated in ), with key parameters including 85% of people observing physical distancing; of those observing physical distancing, they do so 85% of the time; 30% of adult workers are essential workers; 93% of people asked to isolate doing so; 20% uptake of COVID-Safe App; but no closure of schools and no mask-wearing.    
2. Standard plus masks at 50%, above plus 50% of people wearing masks when in crowded indoor environments.
3. Stringent with masks at 50%, schools closed and essential workers restricted to 20% of workers, otherwise as above.
4. Stringent with masks at 90%, above with mask use increased to 90%.  

Figure 1 (available in PDF) shows the percentage likelihood of elimination in Victoria, defined as the date of clearance of infection by the last case. The observable moment of 28 days without a locally acquired case with no known contact follows by about another two weeks.

Under the ‘Standard‘ policy approach, there is no chance that all infected people will have cleared their SARS-CoV-2 infection by 19 August (six-weeks after lock-down started).  The probabilities for the other three policy approaches are 5% for ‘Standard plus masks at 50%’; slightly more at about 7% for ‘Stringent with masks at 50%’; and nearly 50% for ‘Stringent plus masks at 90%’. As an important aside, as we have updated these model runs in the week since lockdown began, the curves have been shifting to the right due to the ongoing high numbers of daily cases.  If Victoria gets on top of the current outbreak, these probabilities may improve, and the curves shift to the left – but also vice versa.  We will update these models on a regular basis.

Undertaking simulation modelling of SARS-CoV-2 policy options is challenging and the uncertainties are still large.  Nevertheless, our results lend weight to the proposition that elimination is achievable, and that mandatory wearing of masks can greatly assist its chances.

A ten-point plan to maximize the chance of elimination in Victoria

Box 1 (available in PDF) gives enhancements to the stay-at-home orders used in the first lock-down, that should see an increased probability of elimination being achieved.  The first and critical point is leadership. Victoria’s Premier Daniel Andrews (and indeed all State and Territory Premiers) should explicitly declare ‘elimination’ as the goal. This should be accompanied by increased transparency and target-setting including the appointment of an expert advisory group in order to increase trust in the process.  An explicit goal will more likely avert a public clamouring for premature opening up again as case-numbers fall and will recognise that these investments will have greater health and economic payoffs in the future.

Since the second lock-down in Victoria was announced (Tuesday 7 July), progress with aspects of this ten-point plan has already been made with the closure of schools (other than for year 11 and 12, and special needs students), encouragement to wear masks in indoor environments (though we argue this needs to be mandatory), and commitments to improve contact tracing capacity. 

Conclusion

We argue that Melbourne and Victoria should not waste the opportunity this lock-down presents.  By learning from the lessons on social and preventive measures to lower SARS-CoV-2 transmissibility,^9,10,12,13 and specifically the lessons from NZ,⁴ Taiwan and six of the eight Australia States and Territories that have achieved elimination, Victoria can increase its chances of also eliminating community transmission. 

There is the risk of failure – we cannot guarantee that our ten-point plan will achieve elimination, we cannot guarantee high compliance in measures by the Victoria population if a more stringent lock-down was imposed, and if the outbreak in NSW restarts community transmission then both NSW and Victoria will need to have elimination strategies for Australia to eliminate.  But we argue that it would be a bigger failure to not enhance the probability of elimination by augmenting the current lock-down now.  Our work and those of others who have independently considered the alternatives consistently demonstrates that elimination is possible, and if achieved optimal for health and long-term for the economy.^4,14-16

Finally, we can only make estimates as to what the future holds.  If a timely vaccine does not arrive, or treatments greatly improve, then for Australia to fully reintegrate with the rest of the world there may be a need to pivot to suppression and then mitigation. But we argue this is a decision better reserved for the future as the disease evolves and our knowledge improves; an Australia with no community transmission of SARS-CoV-2 in the near-term will be better and stronger for it.

References

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