Coronavirus (COVID-19): modelling the epidemic (issue no. 8)

Coronavirus (COVID-19): Analysis - Coronavirus (COVID-19): modelling the epidemic in Scotland (Issue No. 8)

Background

This is a report on the Scottish Government modelling of the spread and level of Covid-19. This updates the previous publication on modelling of Covid-19 in Scotland published on the 2 July 2020. The estimates in this document help the Scottish Government, the health service and the wider public sector plan and put in place what is needed to keep us safe and treat people who have the virus, e.g. to decide how many Intensive Care Beds (ICU) we need available for Covid patients.

Key Points

• Modelling is undertaken to look at the progression of the epidemic in Scotland and to inform the logistical response required.
• This is done over two time periods. Short term, for the next two weeks, and longer term. Both of these help the Scottish Government and wider public sector plan their response and help determine if the measures in place are working.
• Modelling by the Scottish Government estimates that on 3 July there were around 80 new infections and 1,000 people in Scotland who could be infectious with Covid-19. Both of these numbers have fallen significantly in the last week.
• The modelling forecasts that the number of infectious people, the number of cases, hospital and ICU use and deaths are all likely to continue to fall over the next two weeks.
• We currently use the value of R to talk about Covid-19 in Scotland. On 8 July, R in Scotland was estimated to be between 0.6 & 1.0.
• These forecasts were based on estimates of moving in to phase 2 guidance, implemented from 18 June. Changes associated with the move to phase 2 will not be fully seen until mid-July. The longer term forecasts will be closely monitored against actual cases over the next few weeks as the situation changes.

The Imperial College modelling code used by the Scottish Government to understand the progression of the epidemic in Scotland is driven by changes in the numbers of deaths occurring each day. We have reached a point where this number is very low. While this is really good news, it makes reporting of the R number less helpful in understanding how the epidemic is changing. A small, localised outbreak would result in a high R value, even though the number of cases would actually be very few. If the number of deaths drops further we will likely report trends in the numbers of new cases, the numbers of people who may be infectious, and the rate of change of these figures.

What the modelling tells us

Figure 1 shows how Rt has changed since February. Before the “stay at home” restrictions were put in place Rt was above 1, and most likely to have been between 4 and 6 before any interventions were put in place.

Source: Scottish Government modelled estimates using Imperial College model code,

Source: Actual data from https://www.nrscotland.gov.uk/statistics-and-data/statistics/statistics-by-theme/vital-events/general-publications/weekly-and-monthly-data-on-births-and-deaths/deaths-involving-coronavirus-covid-19-in-scotland

The Rt value estimated by the Scottish Government falls within the range of values estimated by other modelling groups and considered by SPI-M and SAGE (Figure 2). SAGE’s consensus view, as of 8 July, was that the value of Rt in Scotland was between 0.6 and 1.0.

Source: Scientific Advisory Group for Emergencies (SAGE).

The Scottish Government’s epidemiological model estimates that on 3 July there were around 80 new cases of Covid-19 in Scotland (see Table 1), while the number of people in Scotland who could be infectious on this date was around 1,000 (see Table 2). Our estimates indicate this number has been declining by around 30% each week, and will continue to decline at a similar rate over the next two weeks.

Table 1: Estimated daily number of new Covid-19 cases in Scotland.

Estimated new daily infections
Date	Mid	Lower	Upper
15 May	860	670	1,090
22 May	600	460	770
29 May	430	320	560
05 June	300	220	410
12 June	220	150	300
19 June	160	110	220
26 June	110	70	160
03 July	80	50	120
10 July	60	30	90
17 July	40	20	70

Table 2: Estimated number of people in Scotland who could be infectious.

Estimated Infectious Pool
Date	Mid	Lower	Upper	Percentage Weekly Change
15 May	10,900	8,600	15,500	-
22 May	7,600	5,900	11,000	-30%
29 May	5,300	4,100	7,800	-30%
05 June	3,800	2,800	5,600	-29%
12 June	2,700	2,000	4,100	-28%
19 June	1,900	1,400	3,000	-28%
26 June	1,400	900	2,300	-28%
03 July	1,000	600	1,700	-28%
10 July	700	400	1,200	-28%
17 July	500	300	900	-28%

Figure 3 shows the epidemiological model forecasts produced by the Scottish Government, given the present set of interventions. This epidemic curve continues to show signs of reducing.

Source: Scottish Government modelled estimates using Imperial College model code,

Source: Actual data from https://www.nrscotland.gov.uk/statistics-and-data/statistics/statistics-by-theme/vital-events/general-publications/weekly-and-monthly-data-on-births-and-deaths/deaths-involving-coronavirus-covid-19-in-scotland

The short-term forecasts produced by SAGE suggested that the number of hospital beds occupied by Covid-19 patients in Scotland would fall over the last two weeks of June (Figure 4). This was well within our Covid-19 hospital capacity of 4,250.

The short and medium term forecasts presented here are fitted to trends in the historical data. Because it takes time for infected people to develop symptoms, require hospitalisation, and either die or recover, we will not fully see the effect of moving into phase 2 in our modelling until mid-July.

Source: This figure has been produced by the Scottish Government using the forecast data provided by SAGE.

The medium-term forecasts produced by the Scottish Government (Figure 5 and 6) using the logistics model show a similar story with a steady decline in the number of people requiring hospitalisation from Covid-19. The logistical model also provides us with a medium term forecast of the number of ICU beds which may be required (Figures 7 and 8).

The three scenarios presented in Figures 5 and 6 for hospital demand and Figure 7 and 8 for ICU demand are for different levels of daily infections. In each case, we translate these into logistical forecasts which are used for planning purposes.

The Worse and Better scenarios should not be considered an upper and lower bound respectively. It is important to note, in particular, that for planning reasons many of the assumptions used are deliberately precautionary, and so it is reassuring that actual case data are lower than the modelled estimate in the past, as is the case with the ICU admissions demand forecast in figures 6 and 8.

The number of hospital beds in use (Figure 5) is tracking above the worse scenario. This is likely to be due to Covid patients remaining in hospital for longer than expected.

Source: Scottish Government modelled estimates using outputs from the Imperial College model code,

Source: Actual data from https://www.gov.scot/publications/coronavirus-covid-19-trends-in-daily-data/

Source: Scottish Government modelled estimates using outputs from the Imperial College model code,

Source: Actual data from https://www.gov.scot/publications/coronavirus-covid-19-trends-in-daily-data/

Source: Scottish Government modelled estimates using outputs from the Imperial College model code,

Source: Actual data from https://www.gov.scot/publications/coronavirus-covid-19-trends-in-daily-data/

Source: Scottish Government modelled estimates using outputs from the Imperial College model code,

Source: Actual data from https://www.gov.scot/publications/coronavirus-covid-19-trends-in-daily-data/

What next?

The modelled estimates of hospital and ICU use, and of the reproduction number Rt will be published each week. Further information can be found at https://www.gov.scot/coronavirus-covid-19