More than a month after the first case of the current COVID-19 outbreak in New South Wales was identified, the State Government is still struggling to contain a surge in infections driven by the highly contagious Delta variant.
That outbreak spilt over to Victoria, sending it into a fifth lockdown, which has now been extended by another seven days. So, how long could the current NSW outbreak last?
Where does NSW stand in terms of restrictions?
The New South Wales (NSW) Government introduced the first set of restrictions on 26 June consisting of mask-wearing indoors (outside of the home), limiting exercise to 10 people outside, take-away food only with food courts remaining open and a general recommendation to work from home where practicable.
By 9 July, further restrictions were introduced with travel authorised within the local government authority (LGA) of residence or within a 10-kilometre radius and exercise authorised outside with only one other person.
The assumption was that these measures, combined with contact tracing, testing and increasing (albeit slow) vaccination uptake, would be sufficient to control the outbreak.
Unfortunately, the 216 new community cases identified over the weekend, show that the hoped-for turning of the tide was not taking place.
This triggered what for many watching was a long-awaited tightening of lockdown rules and the announcement of new restrictions, including suspension of all construction work and the closure of retail stores in greater Sydney until the end of July.
This worsening of the epidemiological situation was predicted by many experts over the last few weeks.
Modelling released from the University of Sydney on 15 July suggests that the current level of social distancing is insufficient to drive case numbers down quickly. Similarly, modelling from the Burnet Institute shows that the restrictions in place as of 12 July might prevent the daily case numbers from increasing, but are insufficient to eliminate community transmission.
Our new modelling backs up these concerns and provides direct estimates (with uncertainty) of the amount of time it might take NSW to get to a 14-day average of five cases per day (which was the target set for Victoria in July 2020) for generic stages of restrictions and lockdowns.
What does our modelling do?
We used an updated version of the agent-based RoadMap model developed in 2020 to assist the Victorian Government during the second COVID-19 wave to simulate the evolution of the current NSW outbreak under different policy scenarios.
We estimated the amount of time associated with different COVID-19 policies to bring the 14-day average number of cases to five.
Why an average of five per day?
Firstly, this is the threshold used successfully in the Victorian second wave in 2020 to come out of lockdown. Secondly, at five cases overall per day, we assume (and hope) that the majority of all cases are occurring among known contacts in isolation – or put another way, that cases occurring in the community are near zero.
Our model simulates three different policies:
1) Stage 2: minimal restrictions in place corresponding to the situation early in the NSW Delta outbreak
2) Stage 3: approximating restrictions in place in NSW, between 9 and 17 July (a soft lockdown)
3) Stage 4: Victorian style hard lockdown, put in place in NSW on 18 July.
The graphs below show 10,000 runs of each scenario. It’s a bit like rerunning a time machine over and over again to capture the randomness or stochasticity in how COVID-19 behaves.
Each red or faint line represents one of these 10,000 time-machine runs. The black line represents the median or average across all model runs. The dotted lines bound the fifth to 95th percentile, giving us a 90 per cent range of what might happen from here.
That said, nothing is certain about COVID-19 – so we have to show that uncertainty in our modelling.
How long will it take to bring the current outbreak under control?
Our modelling suggests the Stage 4 lockdown now put in place will reduce the number of daily cases to five or less (averaged over 14 days) within 5.8 weeks – with a range of 4.6 to 7.5 weeks.
Under Stage 3 restrictions or soft lockdown, we estimate reaching that target number would have taken 8.3 weeks – with a range of five to 14 weeks. But notice that the time to successfully reach that number is wider and much more uncertain.
A Stage 2 scenario would likely have taken an unpleasant 14 weeks to get cases down to five per day, even allowing (as our model does) for increasing vaccine coverage at the same time.
But the range of times is now very uncertain (or stochastic) – if NSW had been lucky, it may have taken only six weeks, but had it been unlucky it could have taken 27 weeks or eight months.
The problem is that the Delta variant is a game-changer. It’s harder to control and eliminate and makes policies other than hard lockdown very uncertain and hard to predict.
So NSW made the hard decision to go into hard lockdown – but it’s the right thing to do. It will take less time to get rid of the virus, and add more certainty about how long it will take.
Let’s now hope luck is on NSW’s side again, and the lockdown is more like four weeks than seven weeks.
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