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Publication - Research and analysis

Coronavirus (COVID-19): modelling the epidemic (issue no. 5)

Published
18 June 2020
From
Director-General Health and Social Care
Directorate
Constitution Directorate
Part of
Coronavirus (COVID-19) in Scotland
ISBN
9781839608445

Latest findings in modelling the COVID-19 epidemic in Scotland, both in terms of the spread of the disease through the population (epidemiological modelling) and of the demands it will place on the system, for example in terms of health care requirement

This document is part of a collection

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Coronavirus (COVID-19): modelling the epidemic in Scotland (Issue No. 5)

Background

This is an update on the Scottish Government modelling of the spread and level of Covid-19. This updates the previous publication on modelling the spread and level of Covid-19 in Scotland published on the 11 June 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 virus, e.g. to decide how many Intensive Care Beds (ICU) we need available for Covid patients.

Key Points

Modelling of the epidemic in Scotland is undertaken to look at the progression of the epidemic and to inform logistical response required.

This is done over two time periods. Short term, for the next two weeks, and longer term. Both these help to forecast Covid-19, which helps the public sector in Scotland plan their response and helps determine if the measures in place are working.

We use the value of R to talk about Covid-19 in Scotland. Up to the 17 June, R in Scotland was estimated to be between 0.6 & 0.8.

The modelling shows 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.

These forecasts were based on estimates of moving in to phase 1 guidance implemented from 29 May. However, due to the time associated with disease progression data used for the forecasting the data do not yet fully reflect changes associated with moving in to phase 1. The longer term forecasts will be closely monitored against actual cases over the next few weeks as the situation changes.

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.

Figure 1: Trends in Rt for Scotland, 2020
Figure 1. A graph showing the trends in the Rt value for Scotland over time, as calculated by the model. The graph show step changes downwards at the point when each intervention was introduced. This figure shows Rt to fall below 1.0 on the 23rd of March, when the “stay at home” advice was given.

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 17 June, was that the value of Rt in Scotland was between 0.6 and 0.8.

Figure 2. Estimates of Rt for Scotland, as of the 17 June, including 90% confidence intervals, produced by SAGE. The estimate produced by the Scottish Government is the 6 from left (yellow), while the SAGE consensus range is the right-most (black).
Figure 2. A graph showing the range of values which each of the academic groups reporting an R value to SAGE are likely to lie within, as of 17 June. The SAGE consensus, shown at the right hand side of the plot, is that the most likely “true” range is between 0.6 and 0.8.

Source: Scientific Advisory Group for Emergencies (SAGE).

The Scottish Government's epidemiological model estimates the number of infectious people in Scotland on 12 June to be around 2,900 (see Table 1). Forecasts indicate this number will decline over the following two weeks, and are promising, but is still at a level that could cause risk to the health service if onwards transmission rose rapidly. This figure is lower than was forecast previously, and reflects the fact that the number of deaths last week was lower than previously forecasted.

Table 1: Estimated number of infectious people in Scotland
Estimated Infectious Pool
Date Mid Lower Upper
15 May 11,600 8,600 15,400
22 May 8,200 5,900 11,100
29 May 5,800 4,000 8,200
05 June 4,100 2,700 5,900
12 June 2,900 1,800 4,300
19 June 2,000 1,200 3,200
26 June 1,400 800 2,300

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.

Figure 3: Scottish Government short-term forecast of the number of deaths from Covid-19 in Scotland, based on actual data (12 June 2020).
Figure 3. A barchart showing daily numbers of deaths caused by Covid-19 in Scotland between 12th March and 9 June, 2020. Overlain on this is the “estimated deaths” result from the model, which smooths out the cyclical weekly pattern in the reported numbers, due to fewer deaths being registered over a weekend. The model results suggest deaths in Scotland peaked around 19th April and have been steadily declining since then.

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 suggest that the number of hospital beds occupied by Covid-19 patients in Scotland will continue to fall over the next two weeks (Figure 4). This is well within our Covid-19 hospital capacity of 4,250.

These short-term forecasts 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 to phase 1 in our modelling until late June or early July.

Figure 4. Short-term forecast of hospital bed occupancy in Scotland as produced by SAGE (10 June).
Figure 4. A graph produced by the Scottish Government based on a short term forecasts provided by SAGE of hospital bed occupancy by Covid-19 patients in Scotland. The most likely scenario shows occupancy declining from around 1200 on 5 May to around 700 by 8 June, and is forecast to decline at a similar rate over the next two weeks.

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) using the logistics model show a similar story over the next few months, with a steady decline in the number of people requiring a hospital bed from Covid-19. The logistical model also provides us with a medium term forecast of the number of ICU beds which may be required (Figure 6).

These forecasts are based on the measures implemented under Phase 1 of Scotland's route map, which were implemented on 29 May. We will not be able to fully measure the effects of this change until late June. Instead, the medium term forecasts are based on observations from other European countries which have enacted measures similar to those within our Phase 1 measures. We will monitor the impact of these changes in the coming weeks, and how this will affect hospital demand.

The three scenarios presented in Figure 5 for hospital demand and Figure 6 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 scenario and Better scenario should not be considered an upper and lower bound. 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 demand forecast in Figure 6.

The number of hospital beds in use (Figure 5) is tracking above the worse scenario. This could be because patients are being kept in hospital for longer, particularly later in the epidemic, or actual cases are declining more slowly at this point in time.

Figure 5: Logistical model medium term forecast of number of people requiring a hospital bed from Covid-19 in Scotland, 2020
Figure 5. A graph showing the modelled forecast of the most likely number of people in Scotland requiring a hospital bed due to Covid-19 in the longer term, along with better and worse case scenarios. In this figure, the most likely number of people requiring hospital treatment declines from around 750 in mid-May, to below 50 in August. The actual number of cases is falling at a similar rate, but tracking above model predictions.

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/

Figure 6: Logistical model medium term forecast of number of people requiring an intensive care from Covid-19 in Scotland, 2020
Figure 6 A graph showing the modelled forecast of the most likely number of people in Scotland requiring intensive care due to Covid-19 in the longer term, along with better and worse case scenarios. In this figure, the most likely number of people requiring intensive care treatment declines from around 80 in mid-May, to around 5 in August.

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

Contact

Email: modellingcoronavirus@gov.scot

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