Coronavirus (COVID-19) infection survey (positivity estimates): 25 to 31 July 2021
Analysis of the proportion of people testing positive for COVID-19 for 25 to 31 July 2021.
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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.
Proportion of people in Wales who had COVID-19
For the week of 25 to 31 July 2021 it is estimated that 0.43% of the community population had COVID-19 (95% credible interval: 0.27% to 0.63%).
This equates to around 1 in 230 individuals (95% credible interval: 1 in 370 to 1 in 160), or an estimated 13,000 people in total (credible interval: 8,200 to 19,000).
The trend of the percentage of people testing positive has decreased in the most recent week.
Since the estimates are based on a relatively low number of positive tests, caution should be taken in over-interpreting any small movements in the latest trend.
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.
Since the end of May, infections compatible with the Delta variant have been the most common across all four UK countries. Because of this high proportion of a single variant, we have not included any breakdown by variant. We last published our main variant analysis on the 23 July 2021, where more detail can be found. Infections by variant will continue to be monitored and the analysis will be re-introduced if the situation changes.
The percentage of people testing positive for COVID-19 by single year of age since 20 June 2021 for Wales, Northern Ireland and Scotland is provided in the ONS dataset.
Estimates for incidence in Wales
Estimates of incidence of PCR-positive cases have been re-introduced for England, Wales, Northern Ireland and Scotland following a period of low positivity. The estimates are available in the data tables on the ONS website.
The incidence of new infections (the number of new infections in a set period of time) helps us understand the rate at which infections are growing within the population and supports the main measure of positivity (how many people test positive at any given time) to provide a fuller understanding of the coronavirus (COVID-19) pandemic.
For more information on the method of estimating incidence please see the methods article on the ONS website.
In Wales, during the week ending 17 July 2021, it is estimated that there were 3.96 new positive coronavirus (COVID-19) cases per 10,000 people per day (95% credible interval: 2.49 to 5.86).
This equates to 1,200 new positive cases in Wales per day (95% credible interval: 760 to 1,800).
When prevalence is very low it may not be possible to produce a reliable estimate.
Estimates for the countries of the UK
At the midpoint of the most recent week (25 to 31 July 2021), the estimated percentage of the community population with COVID-19 across the UK ranged from 0.43% in Wales to 1.87% in Northern Ireland.
The percentage of people testing positive has increased in Northern Ireland, whilst the percentage of people testing positive has decreased in England, Wales and Scotland 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.
The following tables summarise positivity rates and incidence rates across the UK countries.
| Positivity rates (95% Credible Interval) |
|||
|---|---|---|---|
| Wales | 0.43% (0.27 to 0.63) |
1 in 230 people (1 in 370 to 1 in 160) |
13,000 people (8,200 to 19,000) |
| England | 1.32% (1.23 to 1.42) |
1 in 75 people (1 in 80 to 1 in 70) |
722,300 people (671,800 to 775,900) |
| Scotland | 0.82% (0.63 to 1.03) |
1 in 120 people (1 in 160 to 1 in 95) |
43,000 people (33,100 to 54,000) |
| Northern Ireland | 1.87% (1.35 to 2.48) |
1 in 55 people (1 in 75 to 1 in 40) |
34,400 people (24,700 to 45,500) |
Source: Coronavirus (COVID-19) Infection Survey, Office for National Statistics
| Incidence (95% Credible Interval) |
||
|---|---|---|
| Wales | 3.96 cases per 10,000 people per day (2.49 to 5.86) |
1,200 new cases per day (760 to 1,800) |
| England | 13.69 cases per 10,000 people per day (12.53 to 14.94) |
74,400 new cases per day (68,300 to 81,400) |
| Scotland | 6.77 cases per 10,000 people per day (4.10 to 9.70) |
3,600 new cases per day (2,200 to 5,100) |
| Northern Ireland | 10.70 cases per 10,000 people per day (7.44 to 14.89) |
2,000 new cases per day (1,400 to 2,700) |
Source: Coronavirus (COVID-19) Infection Survey, Office for National Statistics
Definitions
Cases compatible with variants
Cases identified as being compatible with known variants of Covid-19 based on their genetic patterns. Variants of Concern and Variants of Interest have recently been relabelled by the World Health Organisation (WHO). 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.
Incidence
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 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.
When comparing Test and Trace (T&T) and estimates from the COVID-19 Infection Survey it is important to note that T&T provides information on the number of tested people who are positive, while the estimates in this release are based on the total number of people who are currently infected at a particular point in time (prevalence). Estimates from this survey are also based on a random sample of the population, including people who are asymptomatic. Asymptomatic individuals are less likely to go for a test and therefore appear in the T&T data.
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.
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
13 August 2021
Contact details
Statistician: Lisa Bloemberg
Telephone: 0300 025 0166
Email: kas.covid19@gov.wales
Media: 0300 025 8099
SFR 230/2021