Coronavirus (COVID-19): ONS Infection Survey – headline results – 21 May 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 or have been vaccinated

The Office for National Statistics (ONS) publish estimates for England, Wales, Northern Ireland and Scotland on their website.

The underlying data displayed in the charts in this publication is available in the reference tables on the ONS website.

All results are provisional and are subject to revision.

Due to low positivity rates, ONS are not able to produce sub-regional positivity estimates and analysis by single year of age for the four UK countries for this publication.

Due to lower positivity rates, ONS are also carrying out some additional checks on estimates of incidence (last published 7 May 2021). Therefore, incidence estimates will not be updated in this publication. For more information on how estimates of incidence are calculated please see COVID-19 Infection Survey: methods and further information.

Main Points

In the week 9 to 15 May 2021, the estimated percentage of the community population (private households) testing positive for COVID-19 in Scotland was 0.05% (95% credible interval: 0.02% to 0.10%). The percentage of people testing positive in the community has continued to decrease in the most recent week.

This equates to an estimated 2,700 people in the community population in Scotland that had COVID-19 at any given time (95% credible interval: 1,000 to 5,200), or around 1 in 1,960 people (95% credible interval: 1 in 5,160 to 1 in 1,010).

In the week 9 to 15 May 2021, estimates for the other nations of the UK are as follows:

• In England, the percentage of people testing positive shows early signs of an increase in the most recent week to 0.09% (95% credible interval: 0.07% to 0.11%), although rates remain low.
• In Wales, the percentage of people testing positive remained low in the most recent week at 0.02% (95% credible interval: 0.00% to 0.07%).
• In Northern Ireland, the trend in the percentage of people testing positive was uncertain in the most recent week at 0.06% (95% credible interval: 0.01% to 0.16%).

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

It is estimated that in the most recent week (9 to 15 May 2021), the percentage of the community population (private households) in Scotland that had the coronavirus (COVID-19) was 0.05%. A 95% credible interval for this figure is 0.02% to 0.10%.

In the same week, it is estimated that at any given time 2,700 people in the community population had COVID-19. A 95% credible interval for this figure is 1,000 to 5,200. This equates to around 1 in 1,960 people (95% credible interval: 1 in 5,160 to 1 in 1,010).

In Scotland, the percentage of people testing positive for COVID-19 in the community population has continued to decrease in the most recent week, as shown in Figure 1.

Modelled daily estimates of the percentage of people testing positive for COVID-19, and accompanying credible intervals, are represented in Figure 1 by the blue line and grey shading. The model smooths the series to understand the trend and is revised each week to incorporate new test results. Modelled daily estimates are used to calculate the official reported estimate.

Official reported estimates of the percentage of people testing positive for COVID-19, are based on the modelled estimate for the midpoint of the most recent week at the time of publication. Official reported estimates, and accompanying credible intervals, are represented in Figure 1 and Figure 2 by the point estimates (blue circles). In Figure 2, pale blue circles denote 14-day weighted estimates while the official reported weekly estimates are denoted by dark blue circles.

Figure 1: Modelled daily estimates and official reported estimates of the percentage of the community population in Scotland testing positive for COVID-19 between 4 April and 15 May 2021, including 95% credible intervals (see notes 2,3,4,5,6)

Figure 2 shows the trend in the official weekly reported estimates of the percentage of the community population testing positive for COVID-19 between 3 October 2020 and 15 May 2021. As shown in Figure 2, official reported estimates of positivity have been decreasing since mid-March. All official reported estimates of positivity displayed in this chart are available in the accompanying dataset on the ONS website.

Figure 2: Official reported estimates of the percentage of the community population in Scotland testing positive for COVID-19 between 3 October 2020 and 15 May 2021, including 95% credible intervals (see notes 3,4,5,6)

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

In the week ending 15 May 2021, the number of people testing positive in the community continued to decrease in Scotland. There are early signs of an increase in the number of people testing positive in England, although rates remain low. Positivity rates remain very low in Wales, which means that it is difficult to identify trends, since they are affected more easily by small changes in the number of people testing positive from week to week. The trend in the number of people testing positive in Northern Ireland in the most recent week is uncertain.

Figure 3: Modelled daily estimates of the percentage of the community population testing positive for COVID-19 in each of the four nations of the UK, between 4 April and 15 May 2021, including 95% credible intervals (see notes 2,3,4,5,6)

*Northern Ireland

Table 1: Modelled estimates of the proportion of the community population testing positive for COVID-19, and corresponding 95% credible intervals, between 9 and 15 May 2021 for the four nations of the UK (See notes 1,3,4,5)

Nation	Estimated percentage of the population that had COVID-19	Estimated number of people who had COVID-19	Estimated ratio of people who had COVID-19
England	0.09% (0.07% to 0.11%)	49,000 (38,800 to 60,300)	1 in 1,110 (1 in 1,410 to 1 in 900)
Northern Ireland	0.06% (0.01% to 0.16%)	1,200 (300 to 2,900)	1 in 1,550 (1 in 6,810 to 1 in 630)
Scotland	0.05% (0.02% to 0.10%)	2,700 (1,000 to 5,200)	1 in 1,960 (1 in 5,160 to 1 in 1,010)
Wales	0.02% (0.00% to 0.07%)	700 (100 to 2,000)	1 in 4,340 (1 in 26,110 to 1 in 1,530)

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

A new variant of COVID-19 was identified in the UK in mid-November 2020. The UK variant (B.1.1.7) of COVID-19 has changes in one of the three genes that COVID-19 swab tests detect, known as the S-gene. This means in cases compatible with the UK variant, the S-gene is not detected by the current test and has the pattern ORF1ab+N (S gene negative) in our main variant analysis. Other variants of concern – including both B.1.617.2 (first identified in India) and B.1.351 (first identified in South Africa) – are positive on all three genes, with the pattern ORF1ab+S+N. If there is an increase in the prevalence of any of these strains, this will show up in our analysis as an increase in cases “Not compatible with the UK variant”. This main variant analysis can therefore differentiate between these two groups of variants (ORF1ab+N positive or ORF1ab+S+N positive), but cannot differentiate between variants that have the same gene pattern for the three genes that COVID-19 swab tests detect. More information on individual variants and where they were first detected is available on the government variant web page.

Other variants, including B.1.525 (first identified in Nigeria), may also have the same pattern of gene positivity as the UK variant. At present this variant is rare in the UK so this group will continue to be described as compatible with the UK variant, but ONS will continue to keep this under review. You can read more about the UK variant in the blog published by ONS.

The percentage of people in the community population testing positive that are compatible with the UK variant decreased in Scotland and Wales and increased in England in the week ending 15 May 2021. The trend was uncertain in Northern Ireland in the same week. The trend in the percentage of people testing positive that are not compatible with the UK variant was uncertain in the most recent week across all four UK countries, and remains low.

Detailed analysis on the UK variant is no longer included in the Scottish Government COVID-19 Infection Survey publication. This data will continue to be published in the accompanying reference tables on the ONS website. Trends in different variants will continue to be monitored over the coming weeks and the charts will be reintroduced into the publication if there is a variant that can be identified using survey tests that appears to be affecting the trends in the percentage of people testing positive for COVID-19.

Whole genome sequencing is undertaken on positive tests from this survey, where possible, more information on this can be found in the latest article on the ONS website.

This analysis was produced by research partners at the University of Oxford.

Further information

The latest estimate of the proportion of community population aged 16 and over in Scotland who would have tested positive for COVID-19 antibodies was published on 13 May 2021 on the Scottish Government website and ONS website.

An article on how often individuals are reporting social contact with other people outside their own household, either socially distanced or physical contact was published on 20 May 2021 on the Scottish Government website and ONS website.

An article on the symptom profile of strong positive cases in the UK was also published on 20 May 2021. The article includes estimates of the percentage of people testing positive that reported symptoms and the likelihood of reporting specific symptoms by nation. Analysis for Scotland was last published on the Scottish Government website on 5 May 2021. The article on the ONS website also includes analysis on the likelihood of testing positive for COVID-19 by patient-facing or non-patient facing job roles.

An article on the prevalence of ongoing symptoms following coronavirus (COVID-19) infection in the UK was published on the ONS website on 1 April 2021. The article includes estimates of the prevalence of self-reported "long COVID", and the duration of ongoing symptoms following confirmed coronavirus infection, using UK Coronavirus (COVID-19) Infection Survey data to 6 March 2021.

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
• estimating the number of new cases and change over time in positive cases

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

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 51,230 swab tests from 30,215 people, with a total of 72 positive tests in 67 people from 53 households. In the latest two-week period, there were 15,208 swab tests from 14,264 people, with a total of 7 positive tests in 7 people from 7 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. Any ratios presented are rounded to the nearest 10. These ratios do not represent a person's risk of becoming infected, since risk of infection depends on a number of factors such as contact with others or whether a person has been vaccinated.
2. There is more uncertainty around estimates for the latest three reported days (as shown by the vertical dashed line), 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 is included.
3. Modelled estimates are not directly comparable with the 14-day weighted estimates provided in the accompanying dataset on the ONS website. The 14-day weighted estimates underpin the modelled estimates and are provided for context. 14-day weighted estimates are not directly comparable with the weekly modelled estimates due to the differing methodology, however they have been included in Figure 2 as they were reported as the official estimates for Scotland before the weekly modelled estimates became available.
4. 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.
5. 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.
6. 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.