# Coronavirus (COVID-19): modelling the epidemic (issue no. 69)

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.

## 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 and from mid‑January onward. During mid-December to mid-January, the projections underestimated the number of infections, due to the unforeseen effects of the new variant.

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.

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, after which they include people in ICU over the 28 day limit.

Table 1. Probability of local authority areas exceeding thresholds of cases per 100K (26th September to 2nd October 2021), data to 13th September.
Probability of exceeding (cases per 100k)
Local Authority (LA) 50 100 300 500 1000 1500
Aberdeen City 75-100% 75-100% 50-75% 50-75% 25-50% 0-5%
Aberdeenshire 75-100% 75-100% 50-75% 50-75% 5-15% 5-15%
Angus 75-100% 75-100% 25-50% 15-25% 5-15% 0-5%
Argyll and Bute 75-100% 75-100% 50-75% 5-15% 0-5% 0-5%
City of Edinburgh 75-100% 75-100% 25-50% 15-25% 0-5% 0-5%
Clackmannanshire 75-100% 75-100% 50-75% 15-25% 5-15% 5-15%
Dumfries & Galloway 75-100% 75-100% 50-75% 25-50% 0-5% 0-5%
Dundee City 75-100% 75-100% 50-75% 50-75% 25-50% 5-15%
East Ayrshire 75-100% 75-100% 75-100% 50-75% 25-50% 5-15%
East Dunbartonshire 75-100% 75-100% 75-100% 25-50% 0-5% 0-5%
East Lothian 75-100% 75-100% 25-50% 5-15% 0-5% 0-5%
East Renfrewshire 75-100% 75-100% 75-100% 50-75% 5-15% 5-15%
Falkirk 75-100% 75-100% 50-75% 50-75% 5-15% 0-5%
Fife 75-100% 75-100% 75-100% 50-75% 25-50% 25-50%
Glasgow City 75-100% 75-100% 75-100% 15-25% 0-5% 0-5%
Highland 75-100% 75-100% 50-75% 25-50% 0-5% 0-5%
Inverclyde 75-100% 75-100% 75-100% 15-25% 5-15% 0-5%
Midlothian 75-100% 75-100% 25-50% 15-25% 15-25% 5-15%
Moray 75-100% 50-75% 15-25% 15-25% 5-15% 0-5%
Na h-Eileanan Siar 25-50% 5-15% 0-5% 0-5% 0-5% 0-5%
North Ayrshire 75-100% 75-100% 75-100% 50-75% 25-50% 0-5%
North Lanarkshire 75-100% 75-100% 75-100% 15-25% 5-15% 0-5%
Orkney Islands 25-50% 15-25% 0-5% 0-5% 0-5% 0-5%
Perth and Kinross 75-100% 75-100% 50-75% 25-50% 5-15% 0-5%
Renfrewshire 75-100% 75-100% 75-100% 25-50% 25-50% 15-25%
Scottish Borders 75-100% 75-100% 50-75% 15-25% 0-5% 0-5%
Shetland Islands 25-50% 25-50% 0-5% 0-5% 0-5% 0-5%
South Ayrshire 75-100% 75-100% 75-100% 50-75% 5-15% 0-5%
South Lanarkshire 75-100% 75-100% 75-100% 25-50% 5-15% 0-5%
Stirling 75-100% 75-100% 50-75% 15-25% 5-15% 0-5%
West Dunbartonshire 75-100% 75-100% 75-100% 50-75% 15-25% 5-15%
West Lothian 75-100% 75-100% 75-100% 50-75% 25-50% 5-15%

### What levels of Covid-19 are indicated by wastewater (WW) data?

Table 2 provides population weighted daily averages for normalised WW Covid-19 levels in the weeks beginning 31st August and 7th 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[13].

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[14]
w/b 31st August w/b 7th September w/b 31st August w/b 7th September
Aberdeen City 134 535 134 0%
Aberdeenshire 131 292 129 146 24%
Angus 212 120 212 120 43%
Argyll and Bute 0%
City of Edinburgh 187 141 187 141 96%
Clackmannanshire 226 237 226 184 92%
Dumfries & Galloway 108 208 108 197 37%
Dundee City 249 120 249 120 100%
East Ayrshire 196 312 196 144 57%
East Dunbartonshire 197 592 197 273 99%
East Lothian 187 130 187 130 65%
East Renfrewshire 273 61 273 61 95%
Falkirk 166 114 166 91 69%
Fife 127 402 121 175 52%
Glasgow City 232 328 232 209 94%
Highland 133 186 133 186 37%
Inverclyde 80 147 80 147 92%
Midlothian 196 141 196 141 73%
Moray 56 48 56 48 56%
Na h-Eileanan Siar 5 5 0%
North Ayrshire 101 129 101 78 93%
North Lanarkshire 121 798 121 368 26%
Orkney Islands 12 36 12 36 34%
Perth and Kinross 105 114 105 114 44%
Renfrewshire 286 66 286 65 13%
Scottish Borders 63 38 57 38 56%
Shetland Islands 4 2 4 2 29%
South Ayrshire 221 306 221 156 88%
South Lanarkshire 278 489 278 196 40%
Stirling 20 64 20 64 63%
West Dunbartonshire 101 593 101 272 48%
West Lothian 169 116 154 116 84%