Coronavirus (COVID-19): modelling the epidemic (issue no.91)

Latest findings in modelling the COVID-19 epidemic in Scotland.


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 have the Covid-19 scenarios been developed?

The Covid-19 scenarios have been developed using the same Scottish Government model used for our usual Medium Term Projections modelling. The scenarios we provide look at what could happen for planning purposes, not to forecast what will happen.

The scenarios take the actuals data and project these forward using the same underlying assumptions as are currently used to model Omicron and are included in the SPI-M-O consensus. These assumptions are then adapted as follows:

  • Immune world – assumes a lower transmission than currently due to high levels of natural and vaccine acquired immunity.
  • Waning world – assumes the same transmission level with vaccine effectiveness waning over time[13].
  • Polarised world - assumes that behavioural changes beginning to occur. Unvaccinated adults and those with one dose are assumed to return to pre-pandemic behaviours within six weeks. Adults with two doses are assumed to return to these behaviours over 12 weeks, with those with three doses assumed to have no significant changes in behaviour.
  • Variant world (vaccine escape) – assumes a variant with similar transmissibility and vaccine escape characteristics as Omicron and same severity as Delta.
  • Variant world (vaccine escape & increased severity) – assumes a variant with similar transmissibility and vaccine escape characteristics as Omicron and a higher severity than Delta.

The assumptions are based on our most up to date knowledge, but do not include the effect of future changes in treatment of Covid-19 e.g. widespread use of antivirals or changes in behaviour in response to high levels of infections e.g. in Variant world.

To reflect the uncertainty the modelling the graphs provide a range for possible outcomes.

As more information becomes available we will revise the modelling where appropriate.

Which local authorities are likely to experience high levels of Covid-19 in two weeks' time

Table 1: Probability of local authority areas exceeding thresholds of cases per 100K (6th March to 12th March 2022). Data to 21st February.
Probability of exceeding (cases per 100K)
Local Authority (LA) 50 100 300 500
Aberdeen City 75-100% 75-100% 75-100% 50-75%
Aberdeenshire 75-100% 75-100% 75-100% 75-100%
Angus 75-100% 75-100% 75-100% 75-100%
Argyll and Bute 75-100% 75-100% 75-100% 25-50%
City of Edinburgh 75-100% 75-100% 75-100% 15-25%
Clackmannanshire 75-100% 75-100% 75-100% 25-50%
Dumfries & Galloway 75-100% 75-100% 25-50% 5-15%
Dundee City 75-100% 75-100% 75-100% 50-75%
East Ayrshire 75-100% 75-100% 50-75% 25-50%
East Dunbartonshire 75-100% 75-100% 75-100% 25-50%
East Lothian 75-100% 75-100% 75-100% 25-50%
East Renfrewshire 75-100% 75-100% 75-100% 25-50%
Falkirk 75-100% 75-100% 75-100% 25-50%
Fife 75-100% 75-100% 75-100% 25-50%
Glasgow City 75-100% 75-100% 75-100% 25-50%
Highland 75-100% 75-100% 75-100% 75-100%
Inverclyde 75-100% 75-100% 75-100% 25-50%
Midlothian 75-100% 75-100% 75-100% 25-50%
Moray 75-100% 75-100% 75-100% 25-50%
Na h-Eileanan Siar 75-100% 75-100% 25-50% 15-25%
North Ayrshire 75-100% 75-100% 50-75% 25-50%
North Lanarkshire 75-100% 75-100% 75-100% 25-50%
Orkney Islands 75-100% 75-100% 75-100% 75-100%
Perth and Kinross 75-100% 75-100% 75-100% 25-50%
Renfrewshire 75-100% 75-100% 75-100% 25-50%
Scottish Borders 75-100% 75-100% 50-75% 25-50%
Shetland Islands 75-100% 75-100% 75-100% 75-100%
South Ayrshire 75-100% 75-100% 50-75% 25-50%
South Lanarkshire 75-100% 75-100% 75-100% 25-50%
Stirling 75-100% 75-100% 75-100% 25-50%
West Dunbartonshire 75-100% 75-100% 75-100% 25-50%
West Lothian 75-100% 75-100% 75-100% 25-50%

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 14th February and 21st February 2022, with no estimate for error. This is given in Million gene copies per person 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 Covid-19 wastewater levels (Mgc/p/d) [15].
Local authority (LA) w/e 14th February w/e 21st February Coverage
Aberdeen City 133 145 99%
Aberdeenshire 129 106 51%
Angus 105 163 56%
Argyll and Bute 61 56 23%
City of Edinburgh 126 81 98%
Clackmannanshire 170 159 81%
Dumfries & Galloway 54 61 30%
Dundee City 105 175 100%
East Ayrshire 64 38 69%
East Dunbartonshire 92 64 99%
East Lothian 109 83 74%
East Renfrewshire 37 32 95%
Falkirk 58 67 96%
Fife 96 68 82%
Glasgow City 66 50 98%
Highland 94 84 48%
Inverclyde 60 52 98%
Midlothian 117 83 88%
Moray 70 52 70%
Na h-Eileanan Siar 0%
North Ayrshire 22 25 93%
North Lanarkshire 88 70 92%
Orkney Islands 43 0%
Perth and Kinross 91 85 45%
Renfrewshire 41 47 97%
Scottish Borders 56 72 59%
Shetland Islands 22 74 29%
South Ayrshire 57 41 88%
South Lanarkshire 66 65 61%
Stirling 45 31 63%
West Dunbartonshire 86 72 98%
West Lothian 85 77 94%

Contact

Email: modellingcoronavirus@gov.scot

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