The strength of Sydney’s lockdown is crucial over the next 100 days

Sunday, Aug 22, 2021, 11:00 AM | Source: Pursuit

Laxman Bablani, Tim Wilson, Hassan Andrabi, Vijaya Sundararajan, Driss Ait Ouakrim, Patrick Abraham, Jason Thompson, Tony Blakely

Despite the strong restrictions in New South Wales, particularly in those Local Government Authorities (LGAs) of concern, COVID-19 case numbers are still growing exponentially at five per cent a day.

To turn the curve around, and perhaps even eventually eliminate COVID-19 in the community, the state needs to pull down the effective reproductive number (R 𝘦𝘧𝘧) – that is the number of people a single case infects – to below one, through a combination of vaccination, lockdowns, contact tracing and mask usage.

The strong restrictions in New South Wales, COVID-19 case numbers are still growing exponentially at five per cent a day. Picture: Getty Pictures

The New South Wales government has embraced increasing vaccinations and targeting younger, critical workers as the way through for Sydney.

Fifty per cent of Sydneysiders are already vaccinated with their first dose, and the government plans to ease restrictions – taking the higher vaccination rate into account – on 28 August.

But regional NSW is now also in lockdown, in an aggressive attempt to curtail the spread further than the already many seeding events.

To help with planning and expectation setting, we modelled 12 scenarios for combinations of:

- Three lockdown strengths: a strong, weak, and moderate lockdown modelled after Victoria’s restrictions. These were calibrated to the Victorian Stage 4, Stage 3, and in-between Stage 4 and 3 restrictions respectively. The current NSW lockdown is probably bounded by the moderate (Stage 3b) and weak (Stage 3) lockdowns in NSW, with pockets of Stage 4 (for example, those LGAs of concern in Sydney)

- Varying vaccine rollout: As planned in early July, prioritising essential workers, plus 25 per cent of adults less than 60 years old getting AstraZeneca urgently, and 50 per cent getting AstraZeneca rapidly. All scenarios achieve 80 per cent adult vaccination, then commence vaccinating five to 16-year-olds at a comparable speed.

Fifty per cent of Sydneysiders are already vaccinated with their first dose, and the government plans to ease restrictions. Picture: Getty Images

We estimated for these twelve scenarios:

- How long it would take to get to five cases per day: at which point we assume remaining cases are occurring in quarantine – when lockdown could be substantially eased

- What the number of hospitalisations and deaths might be in the 100 days from 1 August

Our modelling, pre-printed in the Medical Journal of Australia, uses an evolution of the same model we used for the Roadmap out of the Victorian second wave in a recent analysis of the optimal response considering both health and cost impacts, and the COVID-19 Pandemic Trade-offs online tool we developed.

In a nutshell, the news is not great – lockdowns matter more than the speed of the vaccine rollout in the short-term.

While we fully support speeding up the vaccine rollout in Sydney (and everywhere else in Australia), it’s a medium to long-term strategy – not a short-term strategy to end lockdowns.

Why? It takes time to get jabs in arms, it takes time to get an immune response, and it takes time to get people double dosed.

Our modelling results

If we look at Figure 1, it shows – as the solid lines – the 14-day average of cases per day in NSW for the 12 scenarios.

Figure 1: The number of cases per day, with different vaccination and restriction scenarios. Graphic: Supplied

There is large random variability whenever we rerun the model, like a time machine, due to the ‘stochastic’ or random nature of COVID. The wider bands around the solid lines, then, represent the uncertainty around our estimate: the fifth to 95th percentile of projected cases around our median projection.

We also projected when the key vaccination targets of 50 per cent, 70 per cent and 80 per cent of the adult population double dosed and fully protected could be.

These are shown on Figure 1 as well and are generally expected to be around early October for 50 per cent (say, 1 October for the 25 per cent AstraZeneca scenario), mid-November (12 November for the 25 per cent AZ), and mid-December (13 December for 25 per cent AZ).

NSW is currently vaccinating 22 per cent faster than June projections and we gauge that the state’s current restrictions sit on-the-whole between Stage 3 and Stage 4.

So, we think the moderate lockdown strategy with 25 per cent of adults receiving AstraZeneca is the best indicator of what could happen.

It takes 124 days, or until 3 December (and a 90 per cent uncertainty interval of 18 October to 23 January) to get to less than five cases per day.

Even under the most ambitious scenario – a hard lockdown, well implemented across all of NSW, with the most rapid vaccine rollout with 50 per cent of adults less than 60 getting AstraZeneca – ‘only’ got to five cases per day on 18 Oct 2021.

On the other hand, the scenario with the weakest Stage 3 lockdown and the June vaccine projection gets us to five cases a day on 24 Feb 2022.

Figure 2: The number of infections, hospitalisations, and deaths in the 100 days from August 1. 90 per cent uncertainty intervals available at pre-print, and tend to range from 20 per cent to five times the medians given here for weak and moderate lockdown, and 50 per cent to twice the medians for strong lockdown. Graphic: Supplied

There are some key patterns here.

First, it’s the strength or severity of the lockdown (not the speeding up vaccination) that makes the most difference to the timing and peak of the epidemic curves.

Across vaccination scenarios, a strong lockdown sees daily cases less than five per day end sometime in October, a moderate lockdown in December, and a weak lockdown in February. While the pace of vaccination helps and more so with weaker lockdowns, the difference in timing to get to five cases is in days rather than months.

Now, if we look at Figure 2, this gives us heatmaps of the numbers of estimated cases, hospitalisations and deaths for each vaccination strategy and stage combination, for a timeline of 100 days post 1 August.

The most substantial reduction in cases, hospitalisations and deaths is a result of tightening restrictions; looking vertically down the columns in the heatmaps, rather than at the faster vaccination strategies presented horizontally across the rows.

For example, for deaths under the ‘business-as-usual’ (BAU) vaccine rollout following the June vaccine projections, median deaths increase 33-fold from 79 under strong lockdown to 2570 under weak lockdown.

Under a strong lockdown, median deaths only increase by seven per cent, from 74 under a rapid vaccine rollout (AZ to 50 per cent of 16 to 60-year-olds) to 79 under a BAU vaccine rollout. Again, the difference in magnitude is much larger as we change lockdown strategies.

The most substantial reduction in cases, hospitalisations and deaths is a result of tightening restrictions. Picture: Getty Images

For a rollout where an additional 25 per cent of 16 to 60-year-olds take AZ, infections go down from 191,000 in Stage 3 to 9,320 – nearly 20 times. However, even with the weak Stage 3 lockdowns, increasing the pace of vaccinations only reduces caseloads by roughly a third (from 331,000 under the July scenario to 191,000 under the 50 per cent AstraZeneca rollout).

We are most definitely not saying that a restriction in caseloads because of vaccination shouldn’t be strived for - it must. We are saying that the fact remains that in the short term, it’s the strength of restrictions that’s going to make a real difference.

What our modelling reveals about the NSW endgame

Despite the inherent uncertainty of modelling, it reveals key insights about where NSW the restrictions are headed.

Even with the current lockdown and appeals for vaccination, our modelling suggests that should NSW still be aiming for elimination it is unlikely to occur until late 2021, in the most plausible combination of scenarios.

However, one possibility is that the state will get to high vaccine coverage and aim to contain COVID without further lockdowns when vaccine thresholds are met (i.e. a version of ‘living with the virus’).

The National Plan sets these at 70 per cent and 80 per cent of adults vaccinated, which are sensible targets to have in mind for some lessening of restrictions. However, the brutal truth is that – as we learn more about Delta in other countries – children will also need to vaccinate, and targets in excess of 90 per cent will be ‘best’.

Accelerating the vaccine rollout is important for the medium-term in NSW (and the rest of Australia), as it will make the population more resilient to outbreaks later in 2021 and into 2022 when we open the borders.

But, over the next few months, it’s the strength of lockdown (or public health and social measures) that has the greatest potential impact for reducing COVID-19 hospitalisations and deaths.

Banner: Getty Images