Coronavirus and self-isolation after being in contact with a positive case in England: methodology

The Test and Trace Contacts Insights Survey was compiled in response to policy questions on the level of adherence with self-isolation requirements, the prevalence of behaviour that poses a risk of transmitting the coronavirus (COVID-19), and the impact of self-isolation on well-being and finances. It was produced, run and analysed in a collaboration between the Department for Health and Social Care (DHSC), Public Health England (PHE) and the Office for National Statistics (ONS).

This survey was specifically designed to obtain information on people who have been identified as a contact of someone who had tested positive for COVID-19 and who are at or near the end of their 10-day self-isolation period. The ONS experts were consulted on questionnaire design.

The survey respondents (adults aged 18 years and over) were randomly selected using the Contact Tracing and Advisory Service (CTAS) database, held by NHS Test and Trace. This list was created by NHS Test and Trace to record information about people who have been in contact with someone who had tested positive for COVID-19. The sample was limited to those who had provided a valid telephone number and who had been entered onto the CTAS database at the point of sampling. Respondents were contacted by telephone and all answers were self-reported.

The first COVID Test and Trace Contacts Insights Survey (Wave 1) was run from 1 to 6 March 2021. Results for all surveys conducted to date can be found in our series of Coronavirus and self-isolation after being in contact with a positive case in England bulletins.

Estimates and margins for error

Percentages in these releases are weighted to address age, sex and regional bias in response rates.

As with all surveys, the estimates have an associated margin of error. Significance testing and confidence intervals have been used to test for differences. Where a difference is statistically significantly different, we can be more confident that the difference really exists.

The statistics contained in this release are Experimental Statistics.

Self-isolation

Self-isolation refers to not leaving your home because you have been informed by NHS Test and Trace that you are a contact of a person who has had a positive test result for the coronavirus (COVID-19). You must stay at home and complete 10 full days of isolation. It is a legal requirement to self-isolate if you have been notified to by the NHS Test and Trace service. Your isolation period includes the date of your last contact with them and the next 10 full days. If you develop symptoms, stay at home and arrange to have a polymerase chain reaction (PCR) test for COVID-19.

In addition to staying home, if you are self-isolating you should not receive visitors, unless the purpose of the visit is to provide essential care.

For further information please see NHS guidance When to self-isolate and what to do.

Understanding self-isolation requirements

Respondents to the survey were asked what activities they believed were allowed during self-isolation (from a list of reasons for leaving the home). For the purpose of our analysis, respondents were categorised as having fully understood self-isolation requirements if they did not select any reason for leaving the home that was not permitted during self-isolation (such as going to work or to the shops).

To improve the accuracy of estimates, reasons presented to respondents were updated in Wave 4 (19 to 24 April 2021) and Wave 7 (28 June to 3 July 2021).

In Wave 4, a question on leaving the house for medical reasons was changed to specify “non-emergency medical reasons” (instead of any medical reasons).

In Wave 7, one reason was added (“for other essential household responsibilities, for example taking children to school, dog walking, or taking someone to hospital”) and one reason was also removed (“to help or provide care for a vulnerable person”). These amendments were applied to better reflect common reasons for leaving the house, as reported by respondents in previous waves.

Adherence to self-isolation requirements

Respondents were categorised as having adhered to self-isolation requirements if they:

• did not leave their home during self-isolation, except to get or return a test for coronavirus (COVID-19), or for emergency medical treatment or hospital attendance

• did not receive any visitors during self-isolation, except for visitors supporting their personal care

Prior to Wave 7 of the survey, respondents were not able to select “for emergency medical treatment or hospital attendance” as a reason for leaving the house. This means respondents may have been mis-categorised as non-adherent if they left for that reason only, despite this reason being permitted.

In previous surveys, the only permitted reason respondents could report for leaving their accommodation was “to get or return a test for coronavirus (COVID-19)”.

The addition of the “emergency medical treatment or hospital attendance” response option had no statistically significant impact on estimates of adherence to self-isolation produced for Wave 7.

Individuals who left their home or had visitors for other reasons may have been adherent with the requirements if they did so because of exceptional circumstances. While we have made improvements to reduce misclassification of non-adherence, some may still occur. This means a small number of individuals may be miscategorised as non-adherent.

Adherence is measured until the end of self-isolation, or until the point of the survey if isolation has lasted beyond 10 days because of developing symptoms.

Contact with non-household members

Contact with non-household members was defined as either physical contact for any length of time or being within two metres (six feet) of someone for at least a few minutes during a trip out of the house. Having visitors to the home was also counted as contact with non-household members if these visitors were not providing personal care.

The main strengths of the COVID Test and Trace Contacts Insights Survey include:

• timely production of data and statistics that can respond quickly to changing needs, as the questions included are reviewed for each survey

• the sample was stratified to be representative of the age, sex and regional distribution of the population being sampled and percentages are based on weighted counts representative of the population

• quality assurance procedures are undertaken throughout the analysis stages to minimise the risk of error

• confidence intervals have been used to determine whether differences across time periods and groups are statistically significant

The main limitations of the COVID Test and Trace Contacts Insights Survey include:

• because of the limited period in which fieldwork took place, it is difficult to reach a large number of people and therefore the overall sample size for the survey is limited

• the behaviour during self-isolation is self-reported and may be subject to recall-bias, which influences how accurately respondents are able to recall past events and experiences; most interviews took place within four days of the end of self-isolation to reduce this bias

• the Experimental Statistics presented contain uncertainty; as with all survey data based on a sample, there is an element of uncertainty as they are susceptible to respondent error and bias

• because of the nature of the target population, in which a large proportion of contacts self-isolating are members of the same household, it is possible that the sample could include multiple members of the same household