July 2, 2020

expert reaction to the latest statistics from NHS Test and Trace in England, 18 – 24 June

The government have released the latest statistics for the NHS Test and Trace programme in England, covering the 18th – 24th June.

Prof Sheila Bird, former Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:

“Congratulations to the Department of Health & Social Care statisticians for the depth of new analysis presented in their week 4 report on Test & Trace (T&T).

“First, there were notable improvements by week 4 (18-24 June) in the timeliness of receiving Pillar 2 test results back from: Regional Test Sites (72% back within 24 hours of 76,340 tests taken versus 35% in the preceding week); and Mobile Testing Units (61% back within 24 hours of 37,365 tests taken versus 14% in the preceding week). Timeliness at England’s Pillar 2 Satellite Test Centres has some way to go (22% back within 24 hours of 59,251 tests taken versus 14% in the preceding week).

“Overall, 24-hour timeliness was achieved for 52% (90,190) of the above 172,956 Pillar 2 tests taken in week 4 and 48-hour timeliness for 96% (165,701). In addition, 48-hour timeliness from the time of using a Pillar 2 home test kit to receiving test results was achieved for 41% of the 58,500 home-test-kit users.

“Three-quarters (4,639 out of 6,183) of people transferred to the contact tracing system in week 4 were reached and asked to provide contact details. This performance has held up well over four weeks.

“Third, as week 4 ended, there were 10,395 infected people who had been reached in weeks 1+2 and whose household members were quarantined for 14 days. These household members are likely to number at least 10,000: when will T&T report on how many of these quarantined at-risk household members were swab-test positive? At 5%, we’d expect 500; at 1%, we’d expect 100 positives. High time to find out?

“Moreover, in weeks 1+2, approximately 90,000 recent close contacts were reached and asked to self-isolate. How many of them were swab-test positive during their quarantine period – because they had developed symptoms. The expected number should be greater than ONS’s currently estimated rate of new infections (1 in 2,000 per week) and so at least 90 swab-positives. How many have there been? So much has been invested in T&T that these answers are critically important.

“Last, the bad news from good statistics: During T&T’s first 4 weeks, there were over 110,000 identified recent close contacts of complex cases Only 920 (0.8%) were not reached by local public health endeavours. By contrast, of nearly 31,900 identified recent close contacts of non-complex cases, T&T tracers failed to reach over 9,700 (30%). In addition, there were a further 10,000 identified close contacts of non-complex cases for whom there were no communication details. That puts the non-reach rate for identified close contacts up to 47% (19,997/42,1164).”

Dr Daniel Lawson, Lecturer in Statistical Science, University of Bristol, said:

“This data shows that the NHS testing system is improving, with testing times often less than 24 hours and usually less than 48 hours. There is a delay of usually up to 24 hours to identify and follow up contacts.

“The system is leaky due to the difficulty of following contacts. For the most recent data, 75% of people were reached to provide close contact details, and in non-complex about two thirds of contacts are ultimately reached.  There are complex cases too. Therefore perhaps 40-50% of close contacts of people testing positive are actually asked, and we don’t know the proportion of infected people who never take a test.

“Many of those identified may be isolating anyway, as household members. The number of contacts includes people within the same household and so may simply reflect household demographics. The definition of ‘Contact’ means a person who has been in close contact with someone who has tested positive for coronavirus and who may or may not live with them”(https://www.gov.uk/guidance/nhs-test-and-trace-how-it-works). The statistics on ‘People who were not able to give any recent close contacts’ may simply reflect the demographics of who lives alone.

“Of non-complex cases which are handled remotely, in the week up to 24th June, 4,229 people were reached to provide contacts and 6,348 people were reached and asked to self-isolate; close to how many we would expect if contact-tracing only worked within-households.

“This is therefore encouraging news that the track and trace system is operational, but highlights the need for automatic contact tracing via mobile apps, as well as more appropriate reporting.”

NHS test and trace statistics (England): 18 June to 24 June 2020
https://www.gov.uk/government/publications/nhs-test-and-trace-statistics-england-18-june-to-24-june-2020?utm_source=62cc69db-114a-493f-ab2c-2796c1e4d419&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

Declared interests

Prof Sheila Bird: SMB chairs a panel of the RSS COVID-19 Taskforce which interacts with DHSC’s statisticians on testing statistics.

None others received.