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Antibody data for Wales is now published fortnightly in a separate release.

The Coronavirus (COVID-19) Infection Survey (CIS) is run across the whole of the UK and aims to estimate:

  • how many people have the infection over a given time (positivity)
  • how many new cases occur over a given period (incidence)
  • how many people have antibodies to COVID-19

The survey helps track the extent of infection and transmission of COVID-19 among people in private residences, referred to as the community population.

No estimates of incidence are published this week as additional checks are being carried out due to low positivity rates.

Proportion of people in Wales who had COVID-19

For the week of 23 to 29 May 2021 it is estimated that 0.10% of the community population had COVID-19 (95% credible interval: 0.03% to 0.19%).

This equates to around 1 in 1,050 individuals (95% credible interval: 1 in 2,910 to 1 in 520), or an estimated 2,900 people in total (credible interval: 1,000 to 5,800).

There are early signs of an increase in the percentage of people testing positive in the most recent week in Wales.

As positivity rates are currently low in Wales, it is difficult to identify trends as they are more easily affected by small changes in the number of people testing positive from week to week.

Since the estimates are based on a relatively low number of positive tests, there is some uncertainty and the results should be interpreted with caution. Continued publication of official estimates will be determined by the ability to produce credible estimates.

Further information on the classification of positive cases can be found on the Office for National Statistics (ONS) website.

Please note that there is a greater lag in data from the infection survey than from other sources such as Public Health Wales.

Chart showing the official estimates for the percentage of people testing positive through nose and throat swabs from 18 April to 29 May 2021. There are early signs of an increase in the percentage of people testing positive in the most recent week.

Cases compatible with the UK and other variants continue to be monitored via the survey. Data on cases compliant with different variants can be found in the technical dataset on the ONS website.

Estimates for the countries of the UK

At the midpoint of the most recent week (23 to 29 May 2021), the estimated percentage of the community population with COVID-19 remained low across the UK, ranging from 0.10% in Wales to 0.16% in England.

However the percentage of people testing positive in England has increased in the most recent week. Rates have likely increased in the last two weeks in Scotland, though the trend is uncertain in the most recent week.

There is some uncertainty around the individual point estimates for the nations. Estimates for the last few days of the series, shown as dashed lines in the chart below, have more uncertainty.

Chart showing the official estimates for the percentage of people testing positive through nose and throat swabs from 18 April to 29 May 2021 for the four countries of the UK.
Table 1: Positivity rates across UK countries for the week 23 to 29 May 2021
  Positivity rates
(95% Credible Interval)
Wales 0.10%
(0.03 to 0.19)
1 in 1,050 people
(1 in 2,910 to 1 in 520) 
2,900 people
(1,000 to 5,800)
England 0.16%
(0.13 to 0.19)
1 in 640 people
(1 in 760 to 1 in 540)
85,600 people
(71,900 to 100,900)
Scotland 0.15%
(0.08 to 0.24)
1 in 680 people
(1 in 1,270 to 1 in 420)
7,700 people
(4,100 to 12,500)
Northern Ireland 0.12%
(0.04 to 0.26)
1 in 800 people
(1 in 2,290 to 1 in 380)
2,300 people
(800 to 4,800)

Source: Coronavirus (COVID-19) Infection Survey, ONS


Cases compatible with variants

Cases identified as being compatible with known variants of COVID-19 based on their genetic patterns. Further information on the identification of variants in the survey can be found on the ONS website.

Community population

This survey covers people living in private households only and this is referred to as the community population. Residents in hospitals, care homes and/or other institutional settings are excluded.

Confidence intervals

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. Overlapping confidence intervals indicate that there may not be a true difference between two estimates.

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.


The number of new infections over a period of time.

Modelled estimates

Estimates of positivity from this survey are based on statistical modelling of the underlying data. The model smooths the series to understand the trend and is revised each week to incorporate new test results.

Point estimates

The headline point estimates are based on the modelled trend and are reflect the most representative reference point for the given week.

Positivity rate

The estimated proportion of people who test positive for coronavirus (COVID-19) at a point in time, with or without symptoms, based on nose and throat swabs.

Quality and methodology information

The results of the survey are based on self-administered nose and throat swabs provided by participants to the study. A subgroup of participants also provide blood test, taken by trained field staff.

As well as looking at overall incidence, positivity and antibody level, the survey will be used to examine the characteristics of those testing positive for COVID-19 and the extent to which those infected experience symptoms. The results are for private households only and do not apply to those in hospitals, care homes or other institutional settings. This is referred to as the community population.

The survey covers all the countries of the UK, enabling estimates to be calculated for each country individually, and in time the UK as a whole.

Fieldwork started first in England on 26 April 2020 meaning there is more cumulative data available for England enabling more detailed analysis at present. Fieldwork began in Wales on 29 June 2020 followed by Northern Ireland on 26 July 2020 and Scotland on 21 September 2020.

It is important to note that there is a significant degree of uncertainty with the estimates. This is because, despite a large sample of participants, the number of positive cases identified is small. Estimates are provided with 95% credible or confidence intervals to indicate the range within which we may be confident the true figure lies.

The modelled estimates are carried out afresh each week using the previous 6 weeks’ data. The model works by smoothing the series to understand the trend and is revised each week to incorporate new test results. This means that the latest estimate for an earlier period may be different to the official estimate that was produced at the time. Chart 1 shows the latest modelled trend and the official (point) estimates that were published at the time.

The Office for National Statistics (ONS) publishes weekly statistical bulletins and references tables and periodic statistical articles which include results for England, Wales, Northern Ireland and Scotland as they become available.  The estimates for Northern Ireland and Scotland are published by the respective administrations, as we do here for Wales. 

Further information about quality and methodology can be found on the ONS website and the survey pages on the Oxford University site.

Well-being of Future Generations Act (WFG)

The Well-being of Future Generations Act 2015 is about improving the social, economic, environmental and cultural well-being of Wales. The Act puts in place seven well-being goals for Wales. These are for a more equal, prosperous, resilient, healthier and globally responsible Wales, with cohesive communities and a vibrant culture and thriving Welsh language. Under section (10)(1) of the Act, the Welsh Ministers must (a) publish indicators (“national indicators”) that must be applied for the purpose of measuring progress towards the achievement of the Well-being goals, and (b) lay a copy of the national indicators before Senedd Cymru. The 46 national indicators were laid in March 2016.

Information on the indicators, along with narratives for each of the well-being goals and associated technical information is available in the Well-being of Wales report.

Further information on the Well-being of Future Generations (Wales) Act 2015.

The statistics included in this release could also provide supporting narrative to the national indicators and be used by public services boards in relation to their local well-being assessments and local well-being plans.

Next update

11 June 2021

Contact details

Statistician: Lisa Bloemberg
Telephone: 0300 025 0166

Media: 0300 025 8099

SFR 171/2021