select search filters
briefings
roundups & rapid reactions
before the headlines
Fiona fox's blog
The government have released the latest NHS Test and Trace statistics for England between the 30th July and 5th August 2020.
Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:
“Week on week, it is a pleasure to read the DHSC’s illuminating statistical reports on Test & Trace. It is due to these reports that we, as the public, press or fellow-scientists, have come to understand the limitations of Test & Trace; and called constructively for reforms to be made.
“In particular, the Royal Statistical Society’s COVID-19 Taskforce (of which I am a member) made three recommendations. Please see https://rss.org.uk/RSS/media/File-library/Policy/RSS-COVID-19-Task-Force-Statement-on-TTI-final.pdf.
Here, I shall deal only with the first of those recommendations, namely: apply record linkage methods within T&T’s own data to tell us how many of those in the two high-risk groups quarantined by T&T have developed symptoms and tested positive during their quarantine or soon thereafter. The two high-risk groups are:
“DHSC statisticians have revealed that, astonishingly, under 60% of persons in high-risk group a) are reached by T&T. For non-complex cases, I estimate below that even identified external close contacts in high-risk group b) stay at home for only 50% of their intended quarantine period. Even if the index case has already told his/her household members [high-risk group a)] to quarantine themselves, T&T still needs to know basic information about each household-member so that T&T can keep track of how many of develop symptoms and are swab-test positive. This matters as members of some high-risk households (eg multi-generational) may be at too high a risk. We need to learn this if we are to offer support and risk-reducing interventions.
“52,735 symptomatic index cases were transferred to T&T of whom 41,254 (78%) were contacted, of whom 29,723 (72%) identified one of more close contacts. Of 263,515 identified close contacts, 214,890 (81.5%) were reached: that is 4.1 reached contacts per symptomatic index case transferred to T&T.
“Let’s focus now on just the 35,029 non-complex symptomatic cases reached by T&T: they had just over 72,000 household contacts [high-risk group a)], of whom only 38,506 (53.5%) were reached and asked to self-isolate. Test & Trace, tell us please urgently: how many of these 72,000 household contacts developed symptoms & tested positive for COVID-19 during their intended quarantine or soon after? Did the positive rate differ between the 38,500 in high-risk group a) who were reached versus the 33,500 in high-risk group a) who were not reached by T&T? And which households (in terms of size or composition) were most at risk? Otherwise, please explain why Test & Trace was designed in such a manner that these obvious epidemiological questions are unanswerable.
“Non-complex index cases handled by T&T, identified external close contacts: High-risk group b) comprised just over 32,500 identified external close contacts of non-complex index cases, of whom 20,800 (64%) were reached by T&T. Test & Trace: tell us please urgently: how many of these 32,500 external close contacts developed symptoms & tested positive for COVID-19 during their intended quarantine or soon after. Did the positive rate differ between the 20,800 in high-risk group b) who were reached versus the 11,700 in high-risk group b) who were not reached by T&T? Which demographic or occupational attributes (if any) characterized external close contacts who were most at risk? Otherwise, please explain why any world-beating Test & Trace would be designed in such a manner that these obvious epidemiological questions are unanswerable.
“Test & Trace: tell us please urgently also, how many of the 14-day intended quarantine was spent “at home” the 32,500 identified external close contacts of non-complex cases? I shall assume zero for 11,700 who were not reached and a generous 11 days in quarantine for the 20,800 who were reached but with delay, that is: 11 * 20,800 days/ [intended 14 * 32,500] = 228,800/455,000 (50%).
“My back-of-envelope assumptions, which are conservative, lead me to suspect that – due to delays in reaching them or because never-reached – identified external close contacts of non-complex cases spend only half of their intended quarantine period “at home”.”
Dr Daniel Lawson, Lecturer in Statistical Science, University of Bristol, said:
“The data show that cases continue to gently rise. Whilst this is against a background of rising numbers of tests taken, the number of tests is controlled by two factors: people having perceived symptoms, and the width of testing policy. It seems unlikely that the rise is due only to better identification of cases, especially in light of the European increases in prevalence.
“There was an IT failure in a test centre causing a setback measured in days in the length of time taken for test results to be received. Any delay increases onward infection. Such failures need to be planned for by keeping the overall R sufficiently under 1.
“The track-and-trace system itself is stable in performance, though modelling has shown that the effectiveness will have been dramatically impacted by the testing time delays.”
Prof James Naismith, Director of the Rosalind Franklin Institute, and Professor of Structural Biology, University of Oxford, said:
“There is some good news in the latest release, the number of tests is growing faster than the number of cases. We do not have long before the winter is here, without widespread testing we face the risk of a repeat of February where we were blind to spread of COVID-19.
“There are some concerning aspects of the release. In a virus that moves as rapidly as COVID-19, the increase in the turn-around time of pillar 2, is disappointing. I think we need to do better. For reasons that escape me, we are not routinely testing all identified close contacts in this system. For this week, this would have been an extra 20,638 tests maximum. Even allowing for the risk of false negatives, I believe immediate isolation of contacts, followed by testing to decide whether to end isolation or not is more sustainable system. Further, such a follow up system will provide hard data that will allow us to plan risk-based system for contacts (pub vs restaurant vs beach vs train vs home).
“I am anxious to see a plan that supports the isolation of those who are infectious with financial payments and / or offers of free hotel accommodation. Effective isolation of people who are infectious is the only way we can reduce deaths. Yet for some people, for example young people who are well, those with families to support or the self-employed, isolation by staying at home may be a significant burden for little personal gain. Since over 60 % of close contacts share the household, allowing infectious people to isolate out of the home may reduce transmission.”
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 is a member of the Royal Statistical Society’s COVID-19 Taskforce which made three key recommendations – not yet implemented – on how efficient statistical methods can remedy gaps in Test & Trace. Please see https://rss.org.uk/RSS/media/File-library/Policy/RSS-COVID-19-Task-Force-Statement-on-TTI-final.pdf. SMB also chairs an RSS COVID-19 Panel which liaises with DHSC statisticians on testing issues.
None others received.