Coronavirus (COVID-19): ONS Infection Survey - headline results - 13 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 testing positive for COVID-19

This week modelled estimates have been introduced for Scotland, please note that these are not directly comparable with the 14 day weighted estimates.

It is estimated that in the most recent week (31 October to 6 November), the percentage of the population in Scotland testing positive for the coronavirus (COVID-19) was 0.75%. A 95% credible interval for this figure is 0.54% to 1.00%.

In the same week, it is estimated that 39,700 people in Scotland had the coronavirus (COVID-19). A 95% credible interval for this figure is 28,300 to 52,800.  This equates to around 1 in 135 people (95% credible interval: 1 in 185 to 1 in 100). The ratios presented are rounded to the nearest 5.

Modelled estimates suggest that positivity rates in Scotland have increased over the last six weeks, but it is too early to say whether rates are levelling off.

In the latest six-week period, there were 19,796 swab tests, and a total of 112 positive tests, in 77 people from 59 households. In the latest two-week period, there were 7,714 swab tests, and a total of 57 positive tests, in 52 people from 39 households.

Figure 1: Modelled estimates of the percentage of the population in Scotland testing positive for the coronavirus (COVID-19) between 26 September and 6 November, including 95% credible intervals (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 3 November, 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 testing positive for COVID-19

It is estimated that in the most recent 14-day period (24 October to 6 November), the percentage of the population in Scotland testing positive for the coronavirus (COVID-19) was 1.06%. A 95% confidence interval for this figure is 0.77% to 1.42%.

In the same 14-day period, it is estimated that an average of 55,900 people in Scotland had COVID-19. A 95% confidence interval for this figure is 40,600 to 75,000. This equates to around 1 in 95 people (95% confidence interval: 1 in 130 to 1 in 70). The ratios presented are rounded to the nearest 5.

Figure 2: Weighed estimates of the percentage of the population in Scotland testing positive for the coronavirus (COVID-19) by non-overlapping 14-day periods between 26 September and 6 November 2020, including 95% confidence intervals (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.

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