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
Swab sample – used for estimating positivity rates and incidence Since October 2020, approximately 150,000 people tested per fortnight in England, 15,000 in Scotland, 9,000 in Wales and 5,000 in Northern Ireland.
Blood sample – used for estimating presence of antibodies Target is to achieve up to 125,000 people giving blood samples every month in England, and up to 7,500, 5,500 and 12,000 per month in Wales, Northern Ireland, and Scotland respectively through to April 2022. This target increased in February 2021 to allow better monitoring of the effect of vaccinations.
Last revised: 16 July 2021
This quality and methodology report contains information on the quality characteristics (including the European Statistical System five dimensions of quality) of the statistics produced based on Coronavirus (COVID-19) Infection Survey (CIS) data, 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
understand the methods used to create the data
help you to decide suitable uses for the data
reduce the risk of misusing data
The coronavirus (COVID-19) pandemic is having a profound impact across the UK. In response to the pandemic, the Coronavirus (COVID-19) Infection Survey was set up from April 2020 to estimate:
how many people across England, Wales, Northern Ireland, and Scotland test positive for COVID-19 infection at a given point in time, regardless of whether they report experiencing symptoms
the average number of new positive test cases per week
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
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 institutional settings are not included. In these settings, rates of COVID-19 infection are likely to be different. More information about rates of COVID-19 in care homes can be found in Impact of coronavirus in care homes in England: 26 May to 19 June 2020.
The Office for National Statistics (ONS) is currently working with the University of Oxford, University of Manchester, Public Health England, Wellcome Trust, IQVIA and the Lighthouse Laboratory at Glasgow to run the COVID-19 Infection Survey in the UK.Back to table of contents
Overview of the Coronavirus (COVID-19) Infection Survey
The Coronavirus (COVID-19) Infection 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 25 September 2020.
Nose and throat swab samples
The COVID-19 Infection Survey is based on a nationally representative survey with a random sample. We ask everyone over the age of 2 years in each household sample to have 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.
We need to know 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; and about the incidence of new positive tests in individuals who have not been exposed to the virus. 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 until April 2022.
To capture data about people who have had COVID-19 but have since recovered and to assess the impact of vaccinations, we aim to ask adults aged 16 years or older to also give a sample of blood, 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 over 16 years old surveyed within our household sample were asked to provide a blood sample. However, to monitor the impact of vaccination on both immunity and infection, 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.
The target is to achieve up to 125,000 people giving blood samples every month in England, and up to 7,500, 5,500 and 12,000 per month in Wales, Northern Ireland, and Scotland respectively (150,000 in total across the UK) through to April 2022.
Estimating those testing positive and antibodies
We use a number of different techniques to estimate the number of people testing positive for SARS-CoV2 (the virus that causes the coronavirus (COVID-19) disease) and the presence of antibodies, broken down by different characteristics (age, region and so on). The results are adjusted for a number of characteristics such as age, sex and region 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, Scottish Government, Welsh Government and Northern Ireland Executive are the main users of the COVID-19 Infection Survey. They use our statistics to track the progress of the COVID-19 pandemic in the UK and to help inform decisions about COVID-19 restrictions and related policies.
Matters such as COVID-19 restrictions and policies related to the COVID-19 pandemic are devolved and the Scottish Government, Welsh Government and Northern Ireland Executive use data from the COVID-19 Infection Survey to inform these decisions.
The Welsh Government, Department of Health (Northern Ireland Executive) and the Scottish Government use the results from the COVID-19 Infection Survey to analyse and describe trends and changes in the COVID-19 pandemic for Wales, Northern Ireland, and Scotland respectively.
The results of the COVID-19 Infection Survey contribute to the Scientific Advisory Group for Emergencies (SAGE) estimates of the rate of transmission of the infection, often referred to as "R". The survey also provides important information about the socio-demographic characteristics of the people and households who have contracted COVID-19. Results are also 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 COVID-19 pandemic, the characteristics of those testing positive (such as their occupation, work location, travel status and symptoms experienced) and any possible inequalities associated with those.
The media also report widely on the COVID-19 Infection Survey data and the general public are interested in the statistics produced by the COVID-19 Infection 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 Organisation (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 number of people testing positive for COVID-19 antibodies
The data cannot be used for:
measuring the number of cases and infections in care homes, hospitals and/or other institutional settings
providing information about recovery time of those infected
Strengths and limitations of the COVID-19 Infection Survey
These statistics have been produced rapidly in response to developing world events. The Office for Statistics Regulation, 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.
Sample size and selection
One of the survey's main strengths is the fact the survey subjects are a random sample of the population with a large sample size. This means that unlike other sources such as NHS Test and Trace, for example, which includes only people reporting symptoms, the COVID-19 infection survey also picks up those people not reporting symptoms too.
The survey presents very timely estimates either weekly or fortnightly on a range of domains of interest such as the presence of antibodies and symptoms experienced. The survey questionnaire is also adaptable as social factors change like return to work and travel.
As well as low prevalence rates limiting analysis, the estimates presented in our weekly bulletin and monthly article 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 estimate. There are many sources of uncertainty including:
uncertainty in the test (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 weighting
uncertainty in the models used (some models borrow strength and there could be possible incoherence between modelled estimates and point-in-time estimates)
the quality of data collected in the questionnaire
Information on the main sources of uncertainty are presented in our methodology article and in our Accuracy and Confidence: why we trust the data from the COVID-19 Infection Survey blog.Back to table of contents
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 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 ensuring 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 vaccine people have received) to be added in a timely way.
We receive feedback on our analysis from the UK government and the Scientific Advisory Group for Emergencies (SAGE). 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.com.
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, Public Health England (PHE) 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 Department for Health and Social Care (DHSC) working with the University of Oxford and Lighthouse laboratories to collect and test samples.
Our weekly COVID-19 Infection Survey bulletin provides headline estimates for the percentage of people testing positive for COVID-19 in England, Wales, Northern Ireland and Scotland, as well as an estimate of the incidence rate for England.
We also publish a fortnightly article on the characteristics of people testing positive for COVID-19 and a separate fortnightly article including antibody and vaccination data by UK country and region for England.
The Department of Health (Northern Ireland) publishes results from the COVID-19 Infection Survey that describe COVID-19 infections in Northern Ireland.
The Scottish Government publishes results from the COVID-19 Infection Survey that describe COVID-19 infections in Scotland.Back to table of contents
Contact details for this Methodology
Telephone: +44 (0)1633 651689