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Publication - Statistics

Coronavirus (COVID-19): ONS Infection Survey - headline results - 29 January 2021

Published: 29 Jan 2021

Results from the ONS COVID-19 infection survey from 29 January 2021.

Published:
29 Jan 2021
Coronavirus (COVID-19): ONS Infection Survey - headline results - 29 January 2021

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 (17 to 23 January 2021), the percentage of the population in Scotland that had the coronavirus (COVID-19) was 0.92%. A 95% credible interval for this figure is 0.79% to 1.06%.

In the same week, it is estimated that at any given time 48,500 people in Scotland had the coronavirus (COVID-19). A 95% credible interval for this figure is 41,900 to 55,700.  This equates to around 1 in 110 people (95% credible interval: 1 in 125 to 1 in 95). The ratios presented are rounded to the nearest 5.

Modelled estimates suggest that the proportion of people that had the coronavirus (COVID-19) remained level 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 13 December 2020 and 23 January 2021, including 95% credible intervals (see notes 1,2,3,4,5)

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

The following data should be treated with caution. In particular, there are small numbers of positive test results detected in the survey, 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 UK variant of the virus across the UK was produced by Sarah Walker at the University of Oxford.

Swabs are tested for three 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 UK 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 UK variation in COVID-19 from mid-November, based on the higher levels of virus in these type of positives after this date. Prior to that, the data should not be read as being an indicator of the new UK variant. In contrast, the South African variant has an S-gene which is detectable with the current test, and will therefore be included in the ‘other’ types of COVID-19. Which of the ‘other’ types of COVID-19 are compatible with the South African variant cannot be identified from the swab PCR test alone.

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 UK variant.

ONS have improved their reporting of infections by different gene combinations. Please see the ONS blog for further details.

This analysis looks at the percentage of the people in the population testing positive for COVID-19 that are compatible with the new UK variant, those that test positive but are not compatible with the new variant, and those that test positive where levels of the virus in the sample are too low to be identifiable as either the new or existing variants of COVID-19. Cases that are classed as not identifiable are usually those where individuals have had the virus for a longer period of time. Increases in classed as not identifiable are consistent with an increase in infections acquired a longer time ago.

Figure 2 shows the estimated percentages of those testing positive that are compatible with the new UK variant, those not compatible with the new UK variant and other ‘not identifiable’ positive tests. The percentage of people in Scotland testing positive that are compatible with the new UK variant and the percentage of people in Scotland testing positive that were not compatible with the new UK variant have decreased in the latest week. The percentage of people testing positive where levels of the virus in the sample were too low for the variant to be identifiable is high in Scotland.

Figure 2: Estimated percentage of the population testing positive that are compatible with the new UK variant, not compatible with the new UK variant and other ‘not identifiable’ positive cases, in Scotland between 13 December 2020 and 23 January 2021. There is considerable uncertainty around these estimates (see notes 1,3,4,10)

*New variant compatible positives are defined as those that are positive on the N-gene and ORF1ab-gene, but not the S-gene.

**Positives that are not compatible with the new variant are defined as those that are positive on the S-gene, N-gene and ORF1ab-gene.

***Positives where levels of the virus in the sample are too low for the variant to be identifiable are defined as those that are positive with all other gene patterns. These definitions apply regardless of cycle threshold (Ct) value.

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 13 December 2020 and 23 January 2021 (See notes 1,2,4,5,6)

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 (17 to 23 January) 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

1.87% (1.79% to 1.95%)

1,018,700 (976,200 to 1,061,600)

1 in 55 (1 in 55 to 1 in 50)

Northern Ireland

2.01% (1.66% to 2.39%)

36,800 (30,500 to 43,900)

1 in 50 (1 in 60 to 1 in 40)

Scotland

0.92% (0.79% to 1.06%)

48,500 (41,900 to 55,700)

1 in 110 (1 in 125 to 1 in 95)

Wales

1.43% (1.19% to 1.70%)

43,600 (36,100 to 51,600)

1 in 70 (1 in 85 to 1 in 60)

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 (10 to 23 January 2021), the percentage of the population in Scotland that would have tested positive for the coronavirus (COVID-19) was 1.22%. A 95% confidence interval for this figure is 1.05% to 1.41%. 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 64,000 people in Scotland would have tested positive for the coronavirus (COVID-19) at any given time. A 95% confidence interval for this figure is 55,100 to 74,000. This equates to around 1 in 80 people (95% confidence interval: 1 in 95 to 1 in 70). The ratios presented are rounded to the nearest 5.

Figure 4: Weighted 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 2020 and 23 January 2021, including 95% confidence intervals (see notes 1,5,7,8,9)

Quality and methodology information

Fieldwork in Scotland has been scaled up to test 15,000 unique participants per fortnightly period. 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 will be produced when the sample size permits.

An estimate of the proportion of people in Scotland who would have tested positive for COVID-19 antibodies in December was published on 19 January 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.

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 64,892 swab tests from 26,912 people, with a total of 569 positive tests in 481 people from 425 households. In the latest two-week period, there were 25,125 swab tests from 19,146 people, with a total of 237 positive tests in 225 people from 194 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:

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 20 January (as shown by the dashed lines), as lab results for this period are still being processed at the time of publication. 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.

5. Modelled estimates are not directly comparable with the 14-day weighted estimates. The 14-day weighted estimates underpin the modelled estimates and are provided for context.

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

7. 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.

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.

10. Small numbers of new UK variant compatible positives detected in Scotland and the other devolved administrations are leading to considerable uncertainty surrounding these estimates.

11. This analysis uses weeks commencing Monday, and the most recent week this week is 18 to 24 January 2021. These time periods are used to align with other standard infectious disease surveillance as used by WHO in the main CT monitoring report.