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Coronavirus (COVID-19): modelling the epidemic (issue no.99)

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.

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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 from mid-January 2021 until mid-December 2021, from which point the projections have underestimated the number of infections, due to the unforeseen effects of the Omicron variant. The same is true for the hospital beds projections, however the ICU beds projections have overestimated the actual figures since mid-December 2021, due to the lower severity of Omicron.

Figure 23. Infections projections versus actuals, for historical projections published between one and two weeks before the actual data came in. The vertical line shows when the projections move to being based on wastewater estimates.
A combination line and scatter chart showing infections projections versus actuals, for historical projections published between one and two weeks before the actual data came in.

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 24. Hospital bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.
A combination line and scatter chart showing hospital bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.

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 25. ICU bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.
A combination line and scatter chart showing ICU bed projections versus actuals, for historical projections published between one and two weeks before the actual data came in.

How has the reporting of the Scottish Contact Survey changed?

Following the change in publication frequency to fortnightly, panels A and B of the Scottish Contact Survey have been merged into one panel and surveys are now run fortnightly. There has been a short transition period where only panel A results were reported (issue numbers 97 and 98) and monitoring of the merged panels took place.

Figure 26 shows that the mean contacts for the merged panels (A and B) are similar to the mean contacts for panel A as previously reported and fit within the confidence intervals.

Figure 26: Mean contacts per day for adults in Scotland (truncated to 100 contacts per participant) for panel A and merged panels.
A bar chart showing mean contacts per day for adults in Scotland (truncated to 100 contacts per participant) for panel A and merged panels.

Breaking down the contacts into settings shows that equivalent results are reported for panel A compared to the merged panels across age groups as shown in Figure 27.

Figure 27: Mean contacts per day for adults in Scotland (truncated to 100 contacts per participant) by age group and setting for panel A and merged panels.
A series of bar charts showing mean contacts per day for adults in Scotland (truncated to 100 contacts per participant) by age group and setting for panel A and merged panels

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 ending 3rd May and 10th May 2022, with no estimate for error. This is given in Million gene copies per person per day. Coverage is given as percentage of inhabitants in each local authority covered by a wastewater Covid‑19 sampling site delivering data during this period[16].

Table 2. Average Covid-19 wastewater levels (Mgc/p/d) [17]
Local Authority (LA) w/e 3rd May w/e 10th May Coverage
Aberdeen City 56 53 99%
Aberdeenshire 76 72 46%
Angus 33 62 68%
Argyll and Bute 65 23%
City of Edinburgh 65 165 98%
Clackmannanshire 38 46 70%
Dumfries and Galloway 42 41 38%
Dundee City 36 75 100%
East Ayrshire 140 101 69%
East Dunbartonshire 96 115 99%
East Lothian 69 149 65%
East Renfrewshire 89 69 95%
Falkirk 42 80 96%
Fife 58 53 75%
Glasgow City 80 71 98%
Highland 54 90 44%
Inverclyde 49 108 98%
Midlothian 65 150 88%
Moray 75 85 70%
Na h-Eileanan Siar 62 0%
North Ayrshire 128 74 92%
North Lanarkshire 110 58 95%
Orkney Islands 78 34%
Perth and Kinross 61 94 45%
Renfrewshire 62 67 97%
Scottish Borders 124 38 40%
Shetland Islands 10 29%
South Ayrshire 145 83 88%
South Lanarkshire 81 69 87%
Stirling 37 63%
West Dunbartonshire 246 57 98%
West Lothian 26 74 95%

Contact

Email: sgcentralanalysisdivision@gov.scot

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