Publication - Research and analysis

Coronavirus (COVID-19): modelling the epidemic (issue no. 72)

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.

Coronavirus (COVID-19): modelling the epidemic (issue no. 72)
Technical Annex

Technical Annex

Epidemiology is the study of how diseases spread within populations. One way we do this is using our best understanding of the way the infection is passed on and how it affects people who catch it to create mathematical simulations. Because people who catch Covid-19 have a relatively long period in which they can pass it on to others before they begin to have symptoms, and the majority of people infected with the virus will experience mild symptoms, this "epidemiological modelling" provides insights into the epidemic that cannot easily be measured through testing e.g. of those with symptoms, as it estimates the total number of new daily infections and infectious people, including those who are asymptomatic or have mild symptoms.

Modelling also allows us to make short-term forecasts of what may happen with a degree of uncertainty. These can be used in health care and other planning. The modelling in this research findings is undertaken using different types of data which going forward aims to both model the progress of the epidemic in Scotland and provide early indications of where any changes are taking place.

The delivery of the vaccination programme will offer protection against severe disease and death. The modelling includes assumptions about compliance with restrictions and vaccine take-up. Work is still ongoing to understand how many vaccinated people might still spread the virus if infected. As Covid-19 is a new disease there remain uncertainties associated with vaccine effectiveness. Furthermore, there is a risk that new variants emerge for which immunisation is less effective.

How the modelling compares to the real data as it emerges

The following charts show the history of our modelling projections in comparison to estimates of the actual data. The infections projections were largely accurate during October to mid-December 2020 and from mid‑January 2021 onwards. During mid-December 2020 to mid‑January 2021, the projections underestimated the number of infections, due to the unforeseen effects of the new variant.

Figure 11. Infections projections versus actuals, for historical projections published between one and two weeks before the actual data came in.
Figure 11. A combination line and scatter graph comparing infections projections against actuals.

Hospital bed projections have generally been more precise than infections estimates due to being partially based on already known information about numbers of current infections, and number of people already in hospital. The projections are for number of people in hospital due to Covid-19, which is slightly different to the actuals, which are number of people in hospital within 28 days of a positive Covid-19 test.

Figure 12. Hospital bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.
Figure 12. A combination line and scatter graph comparing hospital bed occupancy projections against actuals.

As with hospital beds, ICU bed projections have generally been more precise than infections. The projections are for number of people in ICU due to Covid-19. The actuals are number of people in ICU within 28 days of a positive Covid-19 test up to 20 January 2021, after which they include people in ICU over the 28 day limit.

Figure 13. ICU bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.
Figure 13. A combination line and scatter graph comparing ICU occupancy projections against actuals.
Table 1. Probability of local authority areas exceeding thresholds of cases per 100K (17th to 23rd October 2021), data to 4th October.
Probability of exceeding (cases per 100k)
Local Authority (LA) 50 100 300 500 750 1000
Aberdeen City 75-100% 75-100% 25-50% 15-25% 5-15% 0-5%
Aberdeenshire 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
Angus 75-100% 25-50% 0-5% 0-5% 0-5% 0-5%
Argyll and Bute 75-100% 75-100% 5-15% 0-5% 0-5% 0-5%
City of Edinburgh 75-100% 75-100% 25-50% 25-50% 15-25% 15-25%
Clackmannanshire 75-100% 50-75% 5-15% 0-5% 0-5% 0-5%
Dumfries & Galloway 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
Dundee City 75-100% 75-100% 25-50% 15-25% 5-15% 0-5%
East Ayrshire 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
East Dunbartonshire 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
East Lothian 75-100% 50-75% 5-15% 0-5% 0-5% 0-5%
East Renfrewshire 75-100% 75-100% 15-25% 0-5% 0-5% 0-5%
Falkirk 75-100% 75-100% 25-50% 15-25% 0-5% 0-5%
Fife 75-100% 75-100% 25-50% 25-50% 15-25% 5-15%
Glasgow City 75-100% 75-100% 25-50% 25-50% 25-50% 25-50%
Highland 75-100% 50-75% 0-5% 0-5% 0-5% 0-5%
Inverclyde 75-100% 75-100% 5-15% 0-5% 0-5% 0-5%
Midlothian 75-100% 75-100% 5-15% 0-5% 0-5% 0-5%
Moray 75-100% 50-75% 15-25% 15-25% 5-15% 5-15%
Na h-Eileanan Siar 50-75% 25-50% 0-5% 0-5% 0-5% 0-5%
North Ayrshire 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
North Lanarkshire 75-100% 75-100% 25-50% 25-50% 25-50% 15-25%
Orkney Islands 5-15% 0-5% 0-5% 0-5% 0-5% 0-5%
Perth and Kinross 75-100% 75-100% 15-25% 5-15% 0-5% 0-5%
Renfrewshire 75-100% 75-100% 25-50% 15-25% 5-15% 0-5%
Scottish Borders 75-100% 50-75% 5-15% 0-5% 0-5% 0-5%
Shetland Islands 15-25% 0-5% 0-5% 0-5% 0-5% 0-5%
South Ayrshire 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
South Lanarkshire 75-100% 75-100% 25-50% 25-50% 15-25% 15-25%
Stirling 75-100% 75-100% 15-25% 0-5% 0-5% 0-5%
West Dunbartonshire 75-100% 75-100% 5-15% 0-5% 0-5% 0-5%
West Lothian 75-100% 75-100% 50-75% 25-50% 5-15% 0-5%

What levels of Covid-19 are indicated by wastewater data?

Table 2 provides population weighted daily averages for normalised WW Covid-19 levels in the weeks beginning 21st and 28th September, with no estimate for error. This is given in Million gene copies per person, which approximately corresponds to new cases per 100,000 per day. Coverage is given as percentage of LA inhabitants covered by a wastewater Covid‑19 sampling site delivering data during this period[14].

Table 2. Average daily cases per 100k as given by WW data.
Local authority (LA) Average daily WW case estimate,
with outliers included
Average daily WW case estimate,
with outliers removed
Coverage[15]
w/b 21st September w/b 28th September w/b 21st September w/b 28th September
Aberdeen City 34 83 34 83 80%
Aberdeenshire 33 99 33 92 51%
Angus 85 80 85 80 56%
Argyll and Bute 3%
City of Edinburgh 91 84 91 84 96%
Clackmannanshire 132 125 92%
Dumfries & Galloway 60 60 3%
Dundee City 85 97 85 97 100%
East Ayrshire 163 105 163 105 69%
East Dunbartonshire 118 96 118 96 99%
East Lothian 81 84 81 84 56%
East Renfrewshire 146 58 146 58 95%
Falkirk 45 91 45 91 69%
Fife 78 95 78 95 52%
Glasgow City 128 90 128 90 98%
Highland 85 40 85 40 37%
Inverclyde 62 23 62 23 92%
Midlothian 91 90 91 90 88%
Moray 44 44 14%
Na h-Eileanan Siar 32 32 0%
North Ayrshire 74 44 74 44 85%
North Lanarkshire 101 152 101 152 69%
Orkney Islands 16 12 16 12 34%
Perth and Kinross 152 152 9%
Renfrewshire 89 62 89 62 57%
Scottish Borders 22 23 22 23 51%
Shetland Islands 4 2 4 2 29%
South Ayrshire 156 109 156 109 77%
South Lanarkshire 146 80 146 80 69%
Stirling 21 27 21 27 63%
West Dunbartonshire 118 78 118 78 98%
West Lothian 87 146 87 146 84%

Contact

Email: modellingcoronavirus@gov.scot