Publication - Statistics

Coronavirus (COVID-19): ONS infection survey results - 20 November 2020

Published: 20 Nov 2020

Results from the ONS COVID-19 infection survey from 20 November 2020.

Published:
20 Nov 2020
Coronavirus (COVID-19): ONS infection survey results - 20 November 2020

The COVID-19 Infection Survey aims to establish:

  • how many people test positive for COVID-19 infection at a given point in time, regardless of whether they report experiencing coronavirus symptoms
  • the average number of new infections per week over the course of the study
  • the number of people who test positive for antibodies, to indicate how many people are ever likely to have had the infection

Samples from the survey are not yet large enough to support more detailed analysis than is provided below.

Modelled estimate of the proportion of people in Scotland that had the coronavirus (COVID-19)

It is estimated that in the most recent week (8 to 14 November), the percentage of the population in Scotland that had the coronavirus (COVID-19) was 0.64%. A 95% credible interval for this figure is 0.45% to 0.86%.

In the same week, it is estimated that at any given time 33,800 people in Scotland had the coronavirus (COVID-19). A 95% credible interval for this figure is 23,600 to 45,300.  This equates to around 1 in 155 people (95% credible interval: 1 in 225 to 1 in 115). The ratios presented are rounded to the nearest 5.

Modelled estimates suggest that the proportion of people in Scotland that had the coronavirus (COVID-19) increased throughout most of October, and now appear to have levelled off.

In the latest six-week period, there were 25,326 swab tests, and a total of 146 positive tests, in 95 people from 74 households. In the latest two-week period, there were 8,900 swab tests, and a total of 60 positive tests, in 51 people from 40 households

Figure 1: Modelled daily estimates of the percentage of the population in Scotland that had the coronavirus (COVID-19) between 4 October and 14 November, including 95% credible intervals (see notes 1,2,3,4)

1. Results are provisional and subject to revision.
2. Because of the relatively small number of tests and a low number of positives in the sample, credible intervals are wide and therefore results should be interpreted with caution. The model used to provide these estimates is a Bayesian model: these provide 95% credible intervals. A credible interval gives an indication of the uncertainty of an estimate from data analysis. 95% credible intervals are calculated so that there is a 95% probability of the true value lying in the interval.
3. There is more uncertainty around estimates after 11 November (as marked by the dashed line), as lab results for this period are still being processed at the time of publication.
4. Modelled estimates are not directly comparable with the 14 day weighted estimates.

14-Day weighted estimates of the proportion of people in Scotland who would have tested positive for the coronavirus (COVID-19)

It is estimated that in the most recent 14-day period (1 to 14 November), the percentage of the population in Scotland who would have tested positive for the coronavirus (COVID-19) was 0.96%. A 95% confidence interval for this figure is 0.70% to 1.28%.

In the same 14-day period, it is estimated that an average of 50,300 people in Scotland would have tested positive for COVID-19 at any given time. A 95% confidence interval for this figure is 36,600 to 67,400. This equates to around 1 in 105 people (95% confidence interval: 1 in 145 to 1 in 80). The ratios presented are rounded to the nearest 5.

Figure 2: Weighed estimates of the percentage of the population in Scotland that would have tested positive for the coronavirus (COVID-19) by non-overlapping 14-day periods between 20 September and 14 November 2020, including 95% confidence intervals (see notes 5,6,7,8,9)

5. These results are provisional and subject to revision.
6. Weighted estimates are provided with 95% confidence intervals to indicate the level of uncertainty around them. A confidence interval gives an indication of the degree of uncertainty of an estimate, showing the precision of a sample estimate. The 95% confidence intervals are calculated so that if we repeated the study many times, 95% of the time the true unknown value would lie between the lower and upper confidence limits. A wider interval indicates more uncertainty in the estimate.
7. Modelled estimates are not directly comparable with the 14 day weighted estimates.
8. Estimates are weighted to be representative of the population in Scotland that live in private-residential households in terms of age (grouped), sex, and region.
9. The 14-day non-overlapping time periods presented in this publication are updated to work backwards from the most recent 14 days available. Time periods presented overlap with those presented in previous publications, therefore direct comparisons are not possible.

Fieldwork is being scaled up with the aim of testing 15,000 participants per fortnightly period. This will enable more detailed analysis, such as examining the characteristics of those testing positive for COVID-19, establishing the average number of new infections per week, and the number of people who test positive for antibodies, to indicate how many people are ever likely to have had the infection.

How this data can be used

The data can be used for:

  • estimating the number of current positive cases in the community, including cases where people do not report having any symptoms

The data cannot be used for:

  • measuring the number of cases and infections in care homes, hospitals and other institutional settings
  • estimating the number of positive cases and new infections in smaller geographies, such as towns and cities
  • providing information about recovery time of those infected
  • producing a UK estimate; ONS now have estimates for England, Scotland, Wales and Northern Ireland, but these cannot be added up or averaged to understand the UK infection rate

Methodology

The results are based on nose and throat swabs provided by participants to the study, obtained from fieldwork which started in Scotland on 21 September 2020.

The results are for private households only, and do not apply to those in hospitals, care homes or other institutional settings. The population used in this analysis relates to the community population aged two years and over.

The Infection Survey bulletins available on the ONS website also include results for England, Wales and Northern Ireland.

The Welsh Government and the Department of Health in Northern Ireland also publish results from the COVID-19 Infection Survey for Wales and Northern Ireland respectively:

Further details on the methodology used can be found on the ONS website.

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