Research and analysis

UKHSA testing and tracing statistics: information for users

Updated 15 October 2020

Testing

Why the figure for number of tests processed and the figure for number of people tested is not the same

The number of tests processed (and the time taken to do so) is not the same as the number of people tested. For clinical reasons, some people are tested more than once. Therefore, the number of tests will be higher than the number of people tested.

There are several valid reasons why some individuals may be tested more than once, including:

• regular testing of health and care workers
• individuals who have the virus, test positive and have a subsequent test to see if they can return to work
• participation in early trials comparing healthcare-professionals administered tests with self-administered tests, resulting in 2 results on the same day
• participation in research studies conducting routine tests on the same people each month
• some tests may come back as ‘void’ and a retest would be issued
• individuals with early symptoms who test negative, develop more symptoms and are retested and found to be positive

The figure for the number of people tested counts the people tested for COVID-19 at least once in pillars 1 and 2 within the reporting week. This figure removes duplicate tests within the reporting week, so an individual is only counted once even if they have had multiple tests (including tests under different pillars). If a person is tested under both pillar 1 and pillar 2, then only the pillar they were first tested under is counted.

Why the total for people tested or people testing positive in all reporting weeks and the cumulative people tested or cumulative people testing positive are not the same

The figure for the number of people tested counts the people tested for COVID-19 at least once in pillars 1 and 2 within the reporting week. This figure removes duplicate tests within the reporting week, so an individual is only counted once even if they have had multiple tests (including tests under different pillars). Similarly, the figure for the number of people testing positive counts the people with at least one positive result for COVID-19 within the reporting week.

The cumulative number of people tested is the number of people who have been tested at least once since Test and Trace launched. Similarly, the cumulative number of people testing positive is the number of people who have tested positive for COVID-19 at least once since Test and Trace launched.

The total of people tested will not be the same as the cumulative number of people tested, as people may be tested in several reporting periods so will appear more than once if all of the weeks were summed together. The same is true for people testing positive.

Regional breakdowns of testing figures are available in the daily coronavirus dashboard

Breakdowns for positive cases are given in the daily coronavirus dashboard. This currently gives counts at a lower tier local authority level. Weekly counts at middle super output area (MSOA) level are now also available. Regional breakdowns for number of tests processed are currently only available for pillar 2 tests in England. These can be found as part of the weekly NHS Test and Trace statistics. Contact tracing is now available at upper tier local authority level as part of the weekly NHS Test and Trace statistics.

Why testing figures do not indicate the spread of the virus in the UK

Data collected for the Test and Trace programme is primarily for operational purposes and was not designed to track the spread of the virus.

While it is now possible to calculate a positive rate and this is included in the release, people tested is still not a random sample and is instead focused on people who may be more likely to have COVID-19, so should therefore not be used as a measure of prevalence for the general public. Additionally, it will not be an accurate measure of prevalence for the general public as some people will be asymptomatic and not all people who experience COVID-19 symptoms will get tested. This means that the people transferred into NHS Test and Trace only make up a proportion of the total population who have COVID-19. More information about the estimated number of people who have COVID-19 at a given time can be found in the national flu and COVID-19 weekly surveillance report and ONS coronavirus infection survey pilot.

Why figures for people testing negative are not currently available

Figures for the number of people testing negative are not currently available for England or for UK level figures. Tests come back non-positive because they have tested negative or because the test was void. In addition, some individuals who are tested multiple times may have had both a positive and a negative test. Because of this, the number of people testing negative cannot be derived by subtracting the total number of people who have tested positive from the total number of people tested.

Why the figures for number of home kits processed are so much lower than the number of tests sent out

The figures for number of tests processed for home kits in the Test and Trace weekly release is lower than the number of tests sent out (given in the daily statistics page) for several reasons:

• the daily statistics for pillar 2 are given for the whole UK, whereas the Test and Trace release is just for England
• home testing kits will often take longer to send, administer, process and release results. The Test and Trace data takes the time a person inputs that they took their test, whereas the daily testing figures gives the date the test was sent out
• not all home testing kits that are sent out will be completed

Tracing

The number of people transferred to NHS Test and Trace does not always align with the number of people testing positive, due to the time taken for this data to be transferred and data not being de-duplicated

The number of people transferred to NHS Test and Trace does not always align with the number of people testing positive for the following reasons:

• there is a lag to the time between a sample being taken, tested and reported, and being passed from the UK Health Security Agency (UKHSA) to the contact tracing system. This means that weekly totals transferred to NHS Test and Trace may include people who were tested in the previous week
• once a person has a confirmed positive test result for coronavirus, this person is transferred to NHS Test and Trace and a case is opened for them. There is not a rigorous process for de-duplication of the data being passed into the contact tracing system each day, as the emphasis is on transferring these cases quickly, and so some people may have their case transferred to the contact tracing system more than once

People who are transferred to contract tracing more than once will be contacted more than once

Currently there will be attempts to contact them following each report to contact tracing.

NHS Test and Trace has 2 principal ways to handles cases, based on their complexity

NHS Test and Trace has 2 principal ways to handle cases:

• local health protection teams to manage complex cases linked to outbreaks (referred to as complex cases here). Examples of complex cases would be if someone works in or has recently visited a health or care setting such as a hospital or care home, a prison or other secure setting, a school for people with special needs, or critical national infrastructure or areas vital for national security
• wider online and other call centre capacity for less complex cases (referred to as non-complex cases here)

For those cases that are less complex, the NHS Test and Trace service will get in contact via a text, email alert or phone call. People are asked to share details of other people with whom they have had close, recent contact and places they have visited. They can respond online via a secure website or by telephone with a contract tracer. Once contacts have been identified, they will be contacted in turn by the NHS Test and Trace service and advised to self-isolate for a period of 14 days.

On occasion, a person’s case may be handled by the NHS Test and Trace service and later escalated to a local health protection team. Contacts managed as part of outbreaks by local health protection teams have been de-duplicated where possible to only count a person once.

There are several points in the tracing process when a case can be identified as complex

After a positive case is transferred to the contract tracing system, NHS Test and Trace makes an initial decision on how to handle the case based on its complexity. There are a few points when this can happen:

• when a case is first reported there is a check against care home postcodes
• when the data provided by people responding online via the secure website is queried to look for certain exposure settings
• if call handlers identify a complex exposure setting while speaking to the individual

Why the percentage of contacts who are reached and asked to self-isolate has decreased since the start of contract tracing

In complex cases the number of close contacts identified has decreased since the start of contract tracing. In contrast, the change in the number of close contacts identified in non-complex cases has been less notable. As non-complex cases have a higher proportion of contacts who are unable to be reached, this can partially explain why the overall percentage of contracts who were reached and asked to self-isolate has decreased.

Why we cannot currently say how many people who get contact traced then go on to have a positive test result

We do not currently have figures for how many people who are reached by contract tracers and advised to self-isolate, go on to have a positive test result. We are currently exploring the feasibility of reporting this information.

The time taken to reach close contacts is not provided for complex cases because these are managed at a situation level

Time taken to reach close contacts is not recorded for complex cases as these cases are often managed at a situation rather than individual level, with advice being issued to the contact institution (for example in a care home or prison). In these cases, close contacts are not directly reached by the programme so recording a time taken is not possible.