Publication - Statistics

Coronavirus (COVID-19): ONS Infection Survey - headline results - 16 April 2021

Published: 16 Apr 2021

Results from the ONS COVID-19 infection survey from 16 April 2021.

Published:
16 Apr 2021
Coronavirus (COVID-19): ONS Infection Survey - headline results - 16 April 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

More detailed analysis will be available in future releases.

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, ONS are reviewing the methodology for providing sub-regional positivity estimates for the four UK countries. As a result, sub-regional estimates of positivity are not available for this publication.

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 (4 to 10 April 2021), the percentage of the population in Scotland that had the coronavirus (COVID-19) was 0.20%. A 95% credible interval for this figure is 0.12% to 0.30%.

In the same week, it is estimated that at any given time 10,500 people in Scotland had COVID-19. A 95% credible interval for this figure is 6,400 to 15,600. This equates to around 1 in 500 people (95% credible interval: 1 in 820 to 1 in 340).

In Scotland, the percentage of people testing positive for COVID-19 decreased in the most recent week.

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 population in Scotland testing positive for COVID-19 between 28 February and 10 April 2021, including 95% credible intervals (see notes 2,3,4,5,6)

Figure 1 shows that the official reported estimates of the percentage of the population testing positive for COVID-19 in Scotland have decreased in recent weeks.

Figure 2 shows the trend in the official weekly reported estimates of the percentage of the population testing positive for COVID-19 between 3 October 2020 and 10 April 2021. Since mid-January, official reported estimates of positivity have trended downwards. 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 population in Scotland testing positive for COVID-19 between 3 October 2020 and 10 April 2021, including 95% credible intervals (see notes 3,4,5,6)

Age analysis of the percentage of people testing positive for COVID-19 in Scotland

Figure 3 shows the percentage of people testing positive for COVID-19 by reference age, between 28 February and 10 April 2021. These estimates are based on modelled daily estimates of the percentage of the population testing positive for COVID-19 in Scotland by single year of age which are available in an accompanying dataset on the ONS website.

In those of nursery, primary and secondary school age, the percentage of people testing positive for COVID-19 appears to have decreased in recent weeks, although there is high uncertainty.

In older age groups, the percentage of people testing positive for COVID-19 has continued to decrease in recent weeks, although there is high uncertainty.

Caution should be taken in over-interpreting small movements in the latest trend. This is because as the percentage of people testing positive decreases, the positivity estimates by single year of age are subject to increased uncertainty as captured in the confidence intervals.

Figure 3: Modelled daily estimates of the percentage of the population in Scotland testing positive for COVID-19, by reference age, between 28 February and 10 April 2021, including 95% confidence intervals (see notes 2 ,5,6,7)

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

In Scotland, the percentage of people testing positive for COVID-19 has decreased in the most recent week, ending 10 April. In England and Northern Ireland, the percentage of people testing positive has also decreased in the most recent week. In Wales, the percentage of people testing positive has decreased in the most recent two weeks.

Figure 4: Modelled daily estimates of the percentage of the population testing positive for COVID-19 in each of the four nations of the UK, between 28 February and 10 April 2021, including 95% credible intervals (see notes 2,3,4,5,6)

*Northern Ireland

Figure 4 shows that the percentage of people testing positive in the four nations of the UK has decreased in recent weeks.

Table 1: Modelled estimates of the proportion of the population testing positive for COVID-19, and corresponding 95% credible intervals, between 4 and 10 April 2021 for the four nations of the UK (See notes 1,3,4,5)

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

0.21%
(0.18% to 0.24%)

112,600
(96,700 to 130,100)

1 in 480
(1 in 560 to 1 in 420)

Northern Ireland

0.14%
(0.05% to 0.28%)

2,600
(1,000 to 5,100)

1 in 710
(1 in 1,890 to 1 in 360)

Scotland

0.20%
(0.12% to 0.30%)

10,500
(6,400 to 15,600)

1 in 500
(1 in 820 to 1 in 340)

Wales

0.11%
(0.05% to 0.20%)

3,300
(1,500 to 6,000)

1 in 920
(1 in 2,080 to 1 in 510)

Number of new COVID-19 infections in Scotland

ONS have recently introduced estimates of incidence of polymerase chain reaction (PCR)-positive cases using a new method based which is based on the positivity estimate. This gives the rate at which new positives occur, and subsequently become detectable, within the population. ONS previously published results for England from 13 July to 28 November 2020 using a previous method.

The new incidence method uses an estimate of the length of time for which an individual will test positive, based on modelling the time from first positive to first subsequent negative test in the survey. This estimate is used alongside the positivity model to produce an estimate. For more information on the new method of incidence please see the updated methods article on the ONS webpage.

The reference date used for the official estimates of incidence of PCR-positive cases is 31 March 2021, 10 days prior to the end of the positivity reference week (4 to 10 April). This is necessary as estimates later than this date are more likely to change as additional data is received.

In Scotland, during the week 28 March to 3 April 2021, it is estimated that there were 1.69 new PCR-positive COVID-19 cases per 10,000 people per day (95% credible interval: 0.57 to 3.03). This equates to 890 new positive cases in Scotland per day (95% credible interval: 300 to 1,600). Incidence of new PCR positive COVID-19 cases decreased in the week up to 3 April 2021.

Modelled daily estimates of incidence rates, and accompanying credible intervals, are represented in Figure 5 by the blue line and grey shading. The model smooths the series to understand the trend and will be revised to incorporate new test results. Modelled daily estimates are used to calculate the official reported/indicative estimate.

Official reported/indicative estimates of incidence rates, are based on the modelled estimate for the midpoint of the week. Indicative estimates are denoted by pale blue circles, while the official estimates are denoted by dark blue circles, in Figure 5 and 6. Indicative estimates were produced using the new positivity-based incidence method, and are presented on the official estimates charts, but were not previously published.

Figure 5: Modelled daily estimates and official reported/indicative estimates of incidence rates in Scotland between 28 February and 3 April 2021, including 95% credible intervals (see notes 2,4,5,6)

Figure 5 shows that the official reported estimates of incidence rates of COVID-19 in Scotland have decreased in recent weeks.

Figure 6 shows the trend in the weekly estimates of incidence rates of COVID-19 between 25 October 2020 and 3 April 2021. As shown in Figure 6, estimates of incidence rates have decreased since mid-March. All official reported estimates of incidence rates displayed in this chart are available in the accompanying dataset on the ONS website.

Figure 6: Official reported/indicative estimates of incidence rates in Scotland between 25 October 2020 and 3 April 2021, including 95% credible intervals (see notes 4,5,6)

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 which coronavirus 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. Other variants, including B.1.525 (first identified in Nigeria), may also have this same pattern of gene positivity. At present these are rare in the UK so this group will continue to be described as compatible with the UK variant, but this will be continually reviewed.

When there is a high viral load (for example, when a person is most infectious) absence of the S-gene in combination with the presence of the other two genes (ORF1ab and N genes) is a reliable indicator of the UK variant in COVID-19. However, as the viral load decreases (for example, if someone is near the end of their recovery from the infection), the absence of the S gene is a less reliable indicator of the new UK variant.

In contrast the South African (B.1.351) and Brazilian (P.1 and P.2) variants have an S-gene that is detectable with the current test and will therefore be included in the “not compatible with new variant” group of COVID-19, where the virus level is high enough to identify this. Which of these types of COVID-19 are compatible with these variants cannot be identified from the swab PCR test alone. You can read more about the new UK variant in the blog published by ONS.

In Scotland, the percentage testing positive compatible with the UK variant decreased in the week ending 10 April 2021. In the same week, the trend in the percentage of people testing positive compatible with the UK variant decreased in Northern Ireland, was level in England and uncertain in Wales.

Detailed analysis on the new 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.

Further information

The latest estimate of the proportion of people in Scotland who would have tested positive for COVID-19 antibodies was published on 14 April 2021 on the Scottish Government website and ONS website.

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.

An article on the symptom profile of strong positive cases in the UK was published on the ONS website on 8 April 2021. The article includes estimates of the percentage of people testing positive that reported symptoms and the likelihood of reporting specific symptoms. 

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 57,873 swab tests from 29,749 people, with a total of 175 positive tests in 165 people from 138 households. In the latest two-week period, there were 14,710 swab tests from 14,314 people, with a total of 32 positive tests in 32 people from 25 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. Any ratios presented are rounded to the nearest 10.
  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.
  7. 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.