# Coronavirus (COVID-19): modelling the epidemic (Issue No. 65)

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 (29th August to 4th September 2021), data to 16th August.
Local Authority (LA) Probability of exceeding (cases per 100k)
50 100 300 500
Aberdeen City 75-100% 50-75% 5-15% 0-5%
Aberdeenshire 75-100% 25-50% 5-15% 0-5%
Angus 75-100% 25-50% 15-25% 5-15%
Argyll and Bute 75-100% 50-75% 15-25% 15-25%
City of Edinburgh 75-100% 50-75% 25-50% 25-50%
Clackmannanshire 75-100% 50-75% 25-50% 25-50%
Dumfries & Galloway 75-100% 75-100% 50-75% 25-50%
Dundee City 75-100% 25-50% 5-15% 5-15%
East Ayrshire 75-100% 50-75% 15-25% 5-15%
East Dunbartonshire 75-100% 50-75% 15-25% 15-25%
East Lothian 75-100% 50-75% 15-25% 15-25%
East Renfrewshire 75-100% 50-75% 15-25% 15-25%
Falkirk 75-100% 25-50% 15-25% 5-15%
Fife 75-100% 75-100% 25-50% 5-15%
Glasgow City 75-100% 75-100% 50-75% 25-50%
Highland 75-100% 50-75% 5-15% 5-15%
Inverclyde 75-100% 75-100% 25-50% 15-25%
Midlothian 75-100% 25-50% 5-15% 0-5%
Moray 50-75% 25-50% 5-15% 0-5%
Na h-Eileanan Siar 25-50% 15-25% 0-5% 0-5%
North Ayrshire 75-100% 75-100% 25-50% 15-25%
North Lanarkshire 75-100% 75-100% 50-75% 25-50%
Orkney Islands 0-5% 0-5% 0-5% 0-5%
Perth and Kinross 75-100% 50-75% 15-25% 5-15%
Renfrewshire 75-100% 75-100% 25-50% 15-25%
Scottish Borders 75-100% 25-50% 15-25% 5-15%
Shetland Islands 5-15% 0-5% 0-5% 0-5%
South Ayrshire 75-100% 50-75% 15-25% 5-15%
South Lanarkshire 75-100% 75-100% 25-50% 25-50%
Stirling 75-100% 25-50% 5-15% 0-5%
West Dunbartonshire 75-100% 50-75% 15-25% 5-15%
West Lothian 75-100% 50-75% 15-25% 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 3rd and 10th August, 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[16].

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[17]
w/b 3rd August w/b 10th August w/b 3rd August w/b 10th August
Aberdeen City 25 22 25 22 80%
Aberdeenshire 18 18 16 18 27%
Angus 39 25 39 25 56%
Argyll and Bute 0%
City of Edinburgh 47 47 47 47 96%
Clackmannanshire 49 75 36 75 70%
Dumfries & Galloway 13 26 11 26 38%
Dundee City 49 32 49 32 100%
East Ayrshire 23 65 23 65 57%
East Dunbartonshire 38 36 38 36 99%
East Lothian 47 47 47 47 56%
East Renfrewshire 26 49 26 49 89%
Falkirk 25 49 25 49 43%
Fife 29 49 29 49 36%
Glasgow City 35 52 35 52 98%
Highland 21 24 21 24 31%
Inverclyde 23 23 92%
Midlothian 47 47 47 47 73%
Moray 19 23 13 23 42%
Na h-Eileanan Siar 0%
North Ayrshire 18 38 18 38 85%
North Lanarkshire 84 43 76 43 88%
Orkney Islands 9 3 9 3 34%
Perth and Kinross 3%
Renfrewshire 26 16 26 16 57%
Scottish Borders 31 16 28 16 40%
Shetland Islands 1 1 1 1 29%
South Ayrshire 23 60 23 60 84%
South Lanarkshire 48 62 48 62 52%
Stirling 17 11 17 11 53%
West Dunbartonshire 38 36 38 36 48%
West Lothian 24 24 24 24 55%