Chart of the week: The knowns and unknowns of NHS Test and Trace

Each week, we'll be taking a look at a different aspect of the Covid-19 pandemic, presenting our analysis in chart form to illustrate some key issues and invite discussion. This week, Billy Palmer looks at the available data on the NHS's much-discussed Test and Trace system – and finds this essential and developing process to be lacking in efficiency and efficacy, with many questions remaining unanswered.

Data story

Published: 07/07/2020

In the government’s own words, the NHS Test and Trace service in England should play a “vital role” in controlling the pandemic.

The service’s role is to test key people (NHS and social care workers and care home residents) or anyone with symptoms quickly. Thereafter, it aims to trace close recent contacts of anyone who tests positive to notify them to self-isolate. Responsibility for the latter function is split, with complex cases (such as those in a health or care setting) handled by local health protection teams, while less complex cases are dealt with using online and call centre capacity.

However, piecing together the data on the first four weeks of the service, we can make the following observations about the Test and Trace system.

  • A large proportion of infected people have probably been missed in the first instance. While over 27,000 cases were ‘transferred’ to NHS Test and Trace, the number of new infections may have been in the region of 73,000, although this is not a precise estimate (95% confidence interval 35,000–145,000), and many infected people will be asymptomatic.
  • 1 in every 4 transferred cases could not be contacted. Out of the 27,125 cases transferred, over 6,245 could not be reached or refused to give recent contact details. Of the non-complex cases, a further 841 did not have communication details provided.
  • Nearly a third of infected people who were reached could not supply close contacts. Even after excluding those that refused to give details, only 13,554 out of 20,039 positive cases reached were actually able to give any recent close contacts.
  • There is no information on how many people, having been asked to self-isolate, have done so.

The system has only been running since the end of May, so some shortcomings are to be expected. But its performance is still opaque – for instance, without knowing what proportion of cases are ‘complex’, we cannot explore the individual effectiveness of the two (local and wider) tracing systems. Some other unknowns are more difficult to calculate, such as the comprehensiveness of the list of recent close contacts provided by each infected person – especially without technology (the long-awaited app) to help.

But where there is information, it does appear to be a leaky system. We need to do a lot better to achieve the “world beating” system that was promised.

The knowns and unknowns of NHS Test and Trace 07/07/2020



* Cases not reached includes those who declined to give details or, for non-complex cases, whose communications details were not given.

** Contacts not reached includes, for non-complex cases, known contacts without communication details.


Nuffield Trust analysis of Test and Trace data and ONS estimated infections data.

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