What’s new?
Today’s announcement of 644 new local cases in NSW is the third day of substantially higher than the model estimates. Both the Gompertz and the Richards’ growth curve models are underestimating. An interpretation of this is that the current growth dynamics, on average, have worsened. This coincides with the major outbreak in Western Sydney.
The projections for the cumulative number of local cases for today, from the Gompertz and Richards’ growth models 7 days ago underestimated by 12% and 9% respectively. The same from 14 days ago underestimated by 18% and 17% respectively. As the Richards’ growth curve model has consistently had lower predictive error for over two weeks, I’m going to preferentially refer to the Richards’ model again for the time being for the short-term projections.
The model projections suggest that the outbreak is continuing to grow, with the rate of growth increasing. According to the model, we are likely to see around 4500-5000 new cases over the next 7 days, but if we factor in the probable level of underestimate, that pushes it up to 6000 new cases in the coming week. The current lock down conditions in Greater Sydney, planned to end 29 August 2021, have just been extended to the end of September 2021 in an announcement by the NSW Government today. Without major changes control of transmission, this might not be plausible.
In response to the evolving Delta outbreak, NSW will extend the current lockdown in Greater Sydney until the end of September, and introduce new rules targeting the local government areas of concern, where the vast majority of new cases are emerging. pic.twitter.com/KoKwl0GCin
— NSW Health (@NSWHealth) August 20, 2021
A range of further restrictions, including a 9 PM to 5 AM curfew in the LGAs of “concern”, and restriction of exercise being only 1 hour, were also announced, starting from next Monday. These new policies, and the recent state-wide lockdown and some enhanced restrictions in Greater Sydney might make a difference in the medium term but there is some question about their implementation.
Context and timeline of the Sydney 2021 outbreak
I started these analysis in early July with the new COVID-19 (delta variant) outbreak in Sydney NSW. The purpose of these charts is that they provide some projections into the near future. Hopefully this allows for some data driven expectations. The Sydney 2021 series starts on 12 July 2021 and are available here: https://vitualis.com/?page_id=4071
The current Sydney outbreak started on 17 June 2021 – the first day with reports of community cases in Bondi. The NSW Government formally commenced stay-at-home orders (“lockdown”) for a number of inner-Sydney regions on 25 June 2021 at 2359, having announced this earlier in the day. This was then broadened 18 hours later to cover the whole of Greater Sydney. The initial plan was a two-week lockdown, with it potentially being lifted on 9 July 2021.
By 9 July 2021, there were early signs that COVID-19 transmission was worsening. On 15 July 2021, the lockdown was again extended until the end of July, and then extended again, this time for four weeks on 28 July 2021. Conditions at the end of the month for people living with South Western, and Western Sydney included a limit to travelling only 5 km from home, masks at all times outside, with only essential workers allowed to leave the local government area, with the requirement of a COVID-19 swab every 72 hours.
On 14 August 2021, in the setting of rapidly rising case counts, further restrictions were announced. The 5 km radius limit (from 10 km) was extended to all residents in Greater Sydney, starting 16 August 2021. Permits were also now required to leave the Greater Sydney area. Furthermore, stay-at-home orders were extended to across the entire state of NSW.
On 20 August 2021, the lockdown in Greater Sydney was extended to the end of September, and a range of new restrictions introduced in the “hot” LGAs including an overnight curfew, restricting outdoor exercise to an hour a day, closure of most businesses except for click-and-collect, and increased policing powers.
Projection of new daily cases, and cumulative counts of COVID-19 with data up to 20 August 2021
What is this?
The top image is a chart of the cumulative (total) COVID-19 cases in NSW, starting from 17 June 2021, and the lower image is a chart of the daily new cases. Only local cases are included (i.e., excluding cases identified in quarantine). Projections are given for the next 7 days. It should be noted that estimates have high levels of uncertainty beyond a few days and must be interpreted cautiously.
The projections are made using a model by fitting the cumulative case data since 17 June 2021 to a Richards’ growth curve using non-linear regression. The dark central dashed lines are the model estimates, with 95% confidence intervals of the estimate. On the lower chart, the colour gradations can be understood as the degree of uncertainty in the model projections.
Gompertz and Richards’ growth curve
The Gompertz function is a type of sigmoid, or “S”-shaped curve. It’s been around since the early 19th century and was initially used to describe and model demographic mortality curves, and hence, well known to actuaries. The Gompertz function can also be used to accurately model biological growth (e.g., epidemics, tumour size, enzymatic reactions). I have chosen to use this model to help with creating insights as earlier in the pandemic, it was found to be useful in modelling cumulative cases of COVID-19 from the Chinese outbreaks (Jia et al. arXiv:2003.05447v2 [q-bio.PE]).
The Richards’ growth curve (or the generalised logistic function), which is a broad family of sigmoid (S-shaped) curves that can describe well many types of growth, including epidemics. It has also been demonstrated to have utility in modelling COVID-19 outbreaks in 2020 (Lee et al. PLoS One 2020 doi: 10.1371/journal.pone.0236860).
Why the changes?
I’ve undertaken some assessment of the degree of predictive error in both the Gompertz and Richards’ growth curve models. These charts compare the 7- and 14-day total case projections of the models, to what actually occurred in reality 7 and 14 days later. For interpretation, above the 0% error line means that the model provided an over-estimate compared to reality, and below the 0% error line an underestimate.
Both models were in early July providing substantial underestimates, which is what I suspected when I started this series. The Richards’ growth curve model has struggled with the fit to the data, with poor predictive accuracy in July. Both models have been gradually underestimating further since the beginning of August. By mid-late August, the Richards’ growth curve model was clearly outperforming the Gompertz model.
Daily case trends
Comparison between the Gompertz and Richards’ growth curve model projections, along with smoothed data trends (7-day simple moving average, and GAM) with data up to 20 August 2021
The generalised additive model gives a descriptive “reality check” to the models. The GAM can be considered as an advanced smoothed trend of the daily counts. Compared to the GAM, both models seem to be underestimating currently, but especially so the Gompertz model.
Want to know more?
Primary data source is from NSW Health for daily new cases. The analysis is performed using RStudio Cloud using R version 4.1.0.