Coronavirus (COVID-19): ONS Infection Survey - headline results - 24 December 2020

The COVID-19 Infection Survey aims to measure:

• 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

More detailed analysis will be available when samples from the survey are large enough.

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

It is estimated that in the most recent week (12 to 18 December), the percentage of the population in Scotland that had the coronavirus (COVID-19) was 0.71%. A 95% credible interval for this figure is 0.55% to 0.88%.

In the same week, it is estimated that at any given time 37,100 people in Scotland had the coronavirus (COVID-19). A 95% credible interval for this figure is 28,900 to 46,300.  This equates to around 1 in 140 people (95% credible interval: 1 in 180 to 1 in 115). The ratios presented are rounded to the nearest 5.

Modelled estimates suggest that the proportion of people that had the coronavirus (COVID-19) has decreased in the most recent week, as shown in Figure 1.

Figure 1: Modelled estimates of the percentage of the population in Scotland testing positive for the coronavirus (COVID-19) between 07 November and 18 December, including 95% credible intervals (see notes 1,2,3,4,5,6,12)

Estimate of the percentage of those testing positive for the coronavirus (COVID-19) whose test results were compatible with the new variant

The following data should be treated with caution. In particular, there are small numbers of positive test results detected, leading to considerable uncertainty surrounding these estimates. There are further uncertainties given that not all cases that are positive on the ORF1ab and N genes will be the new variant. Data analysis on the prevalence of the new variant of the virus across the UK was produced by Sarah Walker at the University of Oxford.

Swabs are tested for 3 genes present in the coronavirus: N protein, S protein and ORF1ab. Each swab can have any one, any two or all three genes detected. Positives are those where one or more of these genes is detected in the swab other than tests that are only positive on the S-gene, which is not considered a reliable indicator of the virus if found on its own.

The new variant of COVID-19 has genetic changes in the S gene. This means the S-gene is no longer detected in the current test, and cases that would have previously been positive on all three genes are now positive only on the ORF1ab and the N gene (not the S gene).

There are also other reasons why a swab may be positive for only these two genes, including lower viral load in the sample, which is why we have always seen a small percentage of this type of positive result.

Absence of the S-gene appears to have become a reliable indicator of the new variation in COVID-19 from mid-November. Prior to that, the data should not be read as being an indicator of the variant.

There has recently been an increase in the percentage of positive cases where only the ORF1ab and N genes were found and a decrease in the percentage of cases with all three genes. This information can be used to approximate the growth of the new variant. 

In the latest week, beginning 14th December, analysis indicates that tests that are compatible with the new variant of the virus comprise 38% of positive tests in Scotland. This is compared with 7% in the previous week, beginning 7th Dec, and the previously reported 14% in the week beginning 9th December (see note 14). Small numbers of new variant compatible positives detected in Scotland are leading to considerable uncertainty surrounding these estimates.

Ad hoc data published by the ONS on the modelled estimates suggests that the majority of the recent decrease in Scotland could be due to a decrease in cases which have a pattern of gene positivity which is not compatible with the new variant of the virus (see note 13).

Figure 2 shows the percentage of positive cases in the week beginning 14 December where only the ORF1ab and N genes were found, that are compatible with the new variant of the virus. The highest percentages are seen in London, the South East and the East of England. 

Figure 2: Estimated percentage of positive cases which are compatible with the new variant (ORF1ab & N gene positive) based on people who have tested positive for Coronavirus (COVID-19) on nose and throat swabs, across the UK, by country, and by region of England during the most recent week, beginning 14 December) (see notes 1, 13)

Modelled estimate of the proportion of the population testing positive for the coronavirus (COVID-19) in each of the four nations of the UK  

Figure 3: Modelled estimates of the percentage of the population testing positive for the coronavirus (COVID-19) in each of the four nations of the UK, between 07 November and 18 December (See notes 1,4,5,6,7,8,12)

Table 1: Modelled estimates of the proportion of the population testing positive for coronavirus (COVID-19), and corresponding 95% credible intervals, for the most recent week (12 to 18 December) for the four nations of the UK

Nation	Estimated percentage of population that had COVID-19	Estimated number of people who had COVID-19	Estimated ratio of people who had COVID-19
England	1.18% (1.12% to 1.25%)	645,800     (610,100 to 683,100)	1 in 85 (1 in 90 to 1 in 80)
Northern Ireland	0.55% (0.33% to 0.81%)	10,100 (6,100 to 14,900)	1 in 180 (1 in 300 to 1 in 125)
Scotland	0.71% (0.55% to 0.88 %)	37,100 (28,900 to 46,300)	1 in 140 (1 in 180 to 1 in 115)
Wales	1.72% (1.34% to 2.15%)	52,200 (40,800 to 65,300)	1 in 60 (1 in 75 to 1 in 45)

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

It is estimated that in the most recent 14-day period (05 December to 18 December), the percentage of the population in Scotland that would have tested positive for the coronavirus (COVID-19) was 0.97%. A 95% confidence interval for this figure is 0.77% to 1.20%. Averaging estimates of the proportion of people in Scotland that would have tested positive for coronavirus (COVID-19) over the latest 14-day period can mask changes that have occurred in the most recent week.

In the same 14-day period, it is estimated that an average of 51,000 people in Scotland would have tested positive for the coronavirus (COVID-19) at any given time. A 95% confidence interval for this figure is 40,600 to 63,200. This equates to around 1 in 105 people (95% confidence interval: 1 in 130 to 1 in 85). The ratios presented are rounded to the nearest 5.

Figure 4: 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 26 September and 18 December 2020, including 95% confidence intervals (see notes 1,5,9,10,11,12)

Quality and methodology information

Fieldwork in Scotland 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 and establishing the average number of new infections per week.

An estimate of the proportion of people in Scotland who would have tested positive for COVID-19 antibodies in November was published on the 14 December on the Scottish Government and ONS website.

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.

Estimates for Scotland do not include data for Orkney, Shetland or the Western Isles due to operational issues. We are working to resolve these issues as soon as possible.

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.

In the latest six-week period, there were 44,791 swab tests, with a total of 338 positive tests in 257 people from 211 households. In the latest two-week period, there were 16,366 swab tests, and a total of 122 positive tests, in 109 people from 98 households.

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:

• Welsh Government
• Department of Health in Northern Ireland

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

Notes

1. Results are provisional and subject to revision.

2. The blue line and shading represent the modelled trend and credible intervals based on the latest data. The point estimates and error bars represent the official weekly estimates and their credible intervals, which are based on the modelled estimate for the midpoint of the week at the time of publication.

3. 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.

4. There is more uncertainty around estimates after 15 December (as marked by the dashed vertical line), as lab results for this period are still being processed at the time of publication.

5. Modelled estimates are not directly comparable with the 14 day weighted estimates.

6. Additional swab tests that become available after this publication are included in subsequent models, meaning that modelled estimates can change as additional data are included.

7. The lines represent the modelled trends for each of the four nations based on the latest data.

8. Credible intervals have not been included in this chart.

9. 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.

10. 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.

11. 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.

12. Estimates for Scotland do not include data for Orkney, Shetland or the Western Isles due to operational issues. We are working to resolve these issues as soon as possible.

13. Ad hoc data published by the Office for National Statistics on the modelled estimates of ORF1ab+N and other variants, Scotland, Wales and Northern Ireland can be found at the following link: All data related to coronavirus (covid-19) - Office for National Statistics (ons.gov.uk)

14. Ad hoc data published by the Office for National Statistics on the percentage of COVID-19 cases that are positive for ORF1ab and N genes can be found at the following link: Percentage of COVID-19 cases that are positive for ORF1ab and N genes - Office for National Statistics (ons.gov.uk)