Survey Name: Coronavirus (COVID-19) Infection Survey (CIS)
Frequency: Continuous with weekly publications.
How compiled: Estimates are derived from a sample survey in which private households are followed up on a weekly basis for five weeks, and then monthly thereafter.
Geographic coverage: UK
Number of participants: Enrolment for data collection via study worker home visits started on 26 April 2020 and ceased on 31 January 2022; at the time when enrolment stopped, there were 490,452 eligible individuals in 227,797 households enrolled in England, 30,146 eligible individuals in 14,346 households in Wales, 15,973 eligible individuals in 7,421 households in Northern Ireland and 48,351 eligible individuals in 23,941 households in Scotland. On 26 September 2022 after the move to remote data collection, a small number of invitations to enrol in the survey were sent to a new sample of households in Northern Ireland. The latest response rates, along with commentary, are found in our Coronavirus (COVID-19) Infection Survey: technical dataset, Tables 2a to 2f.
Achieved sample size: Between 1 May 2021 and 31 March 2022, an average of approximately 390,300 swab results per month (for estimating positivity rates) and 152,600 blood results per month (for estimating presence of antibodies) were analysed in the survey.
Target sample size: Between 1 April 2022 and 31 March 2023, the sample target is to achieve a maximum of 227,300 swab tests from individuals aged 2 years and over every 28 days in England, 15,650 in Wales, 10,050 in Northern Ireland and 23,300 in Scotland (276,200 in total across the UK every 28 days); this equates to approximately 300,000 swab tests in total across the UK per month.
The blood sample target, for the same period up to 31 March 2023, is to achieve up to 90,850 blood tests from individuals aged 8 years and over every 28 days in England, up to 6,300 in Wales, 4,150 in Northern Ireland and 9,200 in Scotland (110,500 in total across the UK every 28 days); this equates to approximately 120,000 blood tests in total across the UK per month.
In July 2022 we moved from collecting data and samples through home visits by a study worker to a more flexible remote data collection method.
In July 2022 we moved from collecting data and samples through home visits by a study worker to a more flexible remote data collection method. All questionnaires and swabs were completed remotely from 1 August 2022. This was to ensure that the survey remained as accessible and representative as possible for participants and allowed us to move to a more efficient method of data collection. Further information on these changes can be found in our blog post and our Coronavirus (COVID-19) Infection Survey: methods article.
This quality and methodology report contains information up to the end of 2022 on the quality characteristics (including the European Statistical System five dimensions of quality) of the statistics produced as outputs based on Coronavirus (COVID-19) Infection Survey data collected through study worker home visits up to July 2022 and remote data collection from August 2022, as well as the methods used to create them.
The information in this report will help you to:
- understand the strengths and limitations of the statistics
- learn about existing uses and users of the data
- reduce the risk of misusing data
- help you to decide suitable uses for the data
- understand the methods used to create the data
Important points about the Coronavirus (COVID-19) Infection Survey
In response to the COVID-19 pandemic, the Coronavirus Infection Survey was set up in April 2020 to estimate:
how many people across England, Wales, Northern Ireland and Scotland would have tested positive for COVID-19 infection, regardless of whether they report experiencing symptoms
the average number of new positive test cases per week
the number of people who would have tested positive for antibodies against SARS-CoV-2 at different levels
Only private residential households and their residents aged 2 years and over are included in the survey. People in hospitals, care homes and/or other communal settings are not included.
The Office for National Statistics (ONS) is currently working with the University of Oxford, IQVIA, Lighthouse Laboratory in Glasgow, UK Health Security Agency (UKHSA), the University of Manchester and the Wellcome Trust to run the COVID-19 Infection Survey in the UK.
Overview of the COVID-19 Infection Survey
The survey was launched in England on 26 April 2020 and was expanded to include Wales on 29 June 2020, Northern Ireland on 26 July 2020 and Scotland on 21 September 2020.
The survey is based on a random sample of households to provide a nationally representative survey. We ask everyone aged 2 years and over in each household sample to take a nose and throat swab. These are tested for SARS-CoV-2 using reverse transcriptase polymerase chain reaction (RT-PCR). This is an accredited test that is part of the national testing programme. These samples are collected so we can estimate the number of people who are infected.
The survey was designed to find out more about:
how the virus is transmitted in individuals who test positive on nose and throat swabs
whether individuals who have had the virus can be reinfected
the incidence of new positive infections
To address these questions, we collect data over time. Every participant is swabbed once; they are then invited to have repeat tests every week for another four weeks and then monthly.
To monitor antibodies against SARS-CoV-2 among the population, we ask a subset of participants aged 8 years and over to also give a sample of blood. This is done using a capillary finger prick method, demonstrated by a specially trained fieldworker, which is then undertaken by the participant. The blood samples are taken at enrolment and then every month. Initially, 20% of adults aged 16 years and over surveyed within our household sample were asked to provide a blood sample. From February 2021, we started asking a representative sample of the other adults recruited to the study to start giving blood samples at their monthly visits. In November 2021, we started collecting the blood samples from children aged between 8 and 15 years. In August 2022, all existing participants were invited to move from study worker home visits to remote data collection where participants complete the survey online or by telephone, and swab and blood samples are returned through the post (or by courier for some participants).
We use many different techniques to estimate the number of people testing positive for SARS-CoV-2 (the virus that causes the coronavirus (COVID-19) disease) and the presence of antibodies above different levels, broken down by different characteristics, including age and region. Results are adjusted to be representative of the community population and to help mitigate possible biases from non-consent and non-response.
Uses and users of the COVID-19 Infection Survey
The UK Government, Welsh Government, Northern Ireland Executive and Scottish Government are the main users of the COVID-19 Infection Survey. Our statistics are used to track the progress of the pandemic in the UK and to help inform decisions about coronavirus restrictions and related policies. Matters such as restrictions and policies related to the pandemic are devolved, with the Welsh Government, Northern Ireland Executive, and Scottish Government using data from the survey to inform these decisions.
The Welsh Government, Department of Health (Northern Ireland Executive), and Scottish Government use results from the survey to analyse and describe trends and changes in the pandemic for Wales, Northern Ireland and Scotland, respectively.
The results of the COVID-19 Infection Survey contributed to the Scientific Advisory Group for Emergencies' (SAGE) estimates of the rate of transmission of the infection (often referred to as "R") and continues to contribute to epidemic estimates produced by the UK Health Security Agency. The survey also continues to provide important information about the socio-demographic characteristics of the people and households who have contracted COVID-19. Results are used to inform policy in government, providing an evidence base for decisions around changes to restrictions, helping with monitoring and surveillance, and with planning for services and vaccinations.
Other users include academics and health researchers, who conduct research and analysis of the pandemic, the characteristics of those testing positive (such as their occupation, work location, travel status and symptoms reported) and any possible inequalities associated with those.
The media also report widely on the COVID-19 Infection Survey data and the public are interested in the statistics produced by the survey to help understand trends in the percentage of people testing positive. The survey is also used by international audiences, such as the World Health Organization (WHO), who use the data to help measure the pandemic globally.
The data can be used for:
estimating the number and proportion of current positive cases in the community, including cases where people do not report having any symptoms
identifying differences in numbers of positive cases and changes in them over time between different areas and regions
estimating the number of new cases and change over time in positive cases
estimating the presence of antibodies in the population at different levels and how these change over time
The data cannot be used for:
measuring the number of cases and infections in care homes, hospitals and/or other communal settings
providing information about recovery time of those infected
Strengths and limitations of the COVID-19 Infection Survey
These statistics were initially produced rapidly in response to developing world events. The Office for Statistics Regulation (OSR), on behalf of the UK Statistics Authority, reviewed them in May 2020 and again in March 2021 against important aspects of the Code of Practice for Statistics and regarded them as consistent with the Code's pillars of trustworthiness, quality and value.
One of the survey's main strengths is that survey subjects are a random sample of the population with a large sample size. This means that unlike other sources, such as national testing programmes, which includes primarily people reporting symptoms or their close contacts, the COVID-19 Infection Survey also identifies people not reporting symptoms. The survey presents timely estimates weekly or fortnightly on a range of domains of interest such as the presence of antibodies at different levels and symptoms reported, and social factors like return to work and travel.
The estimates presented in our weekly bulletin and monthly bulletins contain uncertainty. Although the statistics produced as outputs from the survey data are our best estimates, they should not be regarded as completely accurately reflecting the unknown true numbers we are trying to measure. There are many sources of uncertainty including:
uncertainty in the test results; false negatives and false positives can exist
in the estimates because we have sampled only a proportion of the population
potential non-response bias, which may not be fully mitigated by the methods used to adjust for this
uncertainty in the models used; some models borrow strength across smaller population groups and there could be possible incoherence between modelled estimates and the underlying truth
quality of data collected in the questionnaire and the method of the test administration
Information on the main sources of uncertainty are presented in our methods article and in our blog; Accuracy and confidence: why we trust the data from the COVID-19 Infection Survey.Nôl i'r tabl cynnwys
Assessment of user needs and perceptions
The processes for finding out about uses and users, and their views on the statistical products.
We hold regular weekly meetings with key departments across government, ensuring we keep up to date with changing user needs. We have a clear process for reviewing, prioritising, and responding to user requests. This is to ensure we balance the public good of the request with the resource required to meet it. In addition, the questionnaire is regularly reviewed, which allows new information and questions (for example, which type of vaccination people have received) to be added to the questionnaire in a timely way.
We receive feedback on our analysis from the UK Government. We welcome feedback and encourage users to provide feedback in our releases by including the following text:
We are continuously refining and looking to improve our modelling and presentations. We would welcome any feedback via email: email@example.comNôl i'r tabl cynnwys
The Coronavirus (COVID-19) Infection Survey analysis was produced by the Office for National Statistics (ONS) in collaboration with our research partners at the University of Oxford, the University of Manchester, UK Health Security Agency (UKHSA) and Wellcome Trust. Of particular note are:
Sarah Walker - University of Oxford, Nuffield Department for Medicine: Professor of Medical Statistics and Epidemiology and Study Chief Investigator
Koen Pouwels - University of Oxford, Health Economics Research Centre, Nuffield Department of Population Health: Senior Researcher in Biostatistics and Health Economics
Thomas House - University of Manchester, Department of Mathematics: Reader in Mathematical Statistics
This study is jointly led by the ONS and the University of Oxford, working with UKHSA and Lighthouse Laboratories for the collection and testing of test samples.Nôl i'r tabl cynnwys
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