select search filters
briefings
roundups & rapid reactions
Fiona fox's blog
The government have released the latest Test & Trace figures for the week 17-23 September.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
Also comments on latest interim data from the REACT-1 study
“Though the Test and Trace statistics do provide potentially useful data on how the system is working – how long it takes to get a result after the test is done, and so on – they are difficult to interpret in relation to the level of SARS-CoV-2 infection in the country, or in specific regions. That’s because people who are tested are not typical of the population of England. They are tested for a reason – for example because they have symptoms, or are in hospital, or work in health and care. If the balance between these kinds of reasons changes, or if the ease of getting a test changes, the number of positive tests might well change too, without that change necessarily corresponding to changes in infections in the country. That said, the number of positive tests for the latest week (ending 23 September) is 61% higher than the previous week, despite the total number of tests carried out having hardly changed. With no real doubt, that must reflect a real and considerable rise in infections in England.
“This increase may look like particularly bad news compared to the interim results from the REACT-1 study that have also just been published. The latest REACT-1 results seemed to be giving some good news, in that the conclusion was that the number of infections was growing more slowly than it had been just a few weeks before. While that is indeed a good sign (assuming it holds up when the full analysis is published next week), we must remember that they were not concluding that the number of infections is falling, just that it may well be rising more slowly than it had been at the time of their previous report. Roughly speaking, the latest REACT-1 report shows the number of infected people in the population being about 4 times as high in their latest round of tests so far (Round 5) compared to their previous round. For the nearest available weeks to the dates of those rounds, the number of people testing positive in Test and Trace didn’t go up so fast over that period – in fact the number roughly doubled. Given that Test and Trace figures can be affected by the availability of tests to different parts of the population, there therefore isn’t a major contradiction between these Test and Trace figures and the REACT-1 results. One could almost argue that the Test and Trace results are slightly less worrying in terms of changes during late August and September than are the REACT-1 results. However, REACT-1, and the ONS infection survey, test representative samples of the population. So they are not affected by changes in the availability of the sort of tests used in Test and Trace, and can therefore give a much clearer indication of the level in infection in the English population. We’ll know more about that after the weekly data from the ONS survey are published tomorrow, and the full results from REACT-1 are published next week.
“The pattern of increase is different in Pillar 1 tests (for people with a clinical need, and workers in health and care) and Pillar 2 tests (for everyone else). Two-thirds of the people tested in these groups are in Pillar 2. The number of people testing positive in Pillar 1 went up by 34% between the most recent week and the week before, while in Pillar 2 the increase was greater, at 67%. I’m not sure that we can conclude much about the reasons for this difference from the data in this report. My strong suspicion is that it may have something to do with issues in availability of community testing. (In the previous week, the people testing positive in Pillar 2 actually fell compared to the week before that, even though people testing positive in Pillar 1 increased and other data sources also indicated an increase in infections.) A much smaller percentage, 2.5%, of people tested in Pillar 1 had a positive result than for those tested in Pillar 2, where the percentage positive was 6.7% in the latest week. This reflects the differences in reasons for testing in the two pillars.”
Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:
“The sharp rise in newly diagnosed COVID-19 cases referred to Test & Trace (T&T) that I had expected to see last week is now evident with referrals (to nearest 100) in the past 5 weeks having been: 7700, 8900, 15500, 21300, 29000 (week ended 23 September). Doubling per 2-weeks is evident.
“Sadly for T&T, increased referrals in the week to 23 September have been associated with a sharp drop in the percentage reached: down from 84% for referrals during 20 August to 9 September to 81% for referral during 10-16 September to 71% for referrals in the week ended 23 September.
“Of those reached in the week to 23 September, 85% provided contact-details –just two-thirds of non-complex cases were reached within 24 hours of being referred to Test & Trace.
“I could go on but I shall not. Why? Because the time-frame of relevance for infectious disease control is the percentage of non-complex cases referred to T&T who were reached within 48 hours of symptom-onset; and, indeed, the distribution of these times for all reached cases.
“Similarly, what we need to know about identified external close contacts is “how many days of their intended 14-day-quaratine period had already elapsed before T&T asked them to “self-isolate”. The answer is all 14 days for identified close contacts whom T&T never reaches.
“Time for T&T to reveal its infectious disease control credentials – good or bad (not just its operational statistics, informative as these are).
“On 23 July 2020, the Royal Statistical Society (RSS) made recommendations – thus far ignored – to remedy the failures of Test & Trace (T&T) to glean intelligence.
“First, we need information about the symptomatic test-positive rate for T&T’s two high-risk quarantined groups:
1. Members of the household of symptomatic index cases
2. External close contacts of symptomatic index cases.
“Second, robust intelligence is needed – requiring home-visits on randomly sampled days – about asymptomatic swab-positive rates in the first five days of quarantine and in the next nine days for T&T’s two high-risk groups.
“Third, those random home-visits allow adherence to “stay at home” to be properly audited. Without ever having measured robustly adherence to “stay at home”, as recommended by RSS, we appear now to leap into punishing those who fail to adhere. Punishment means that the door may remain closed to any T&T survey team who knocks on 2 random days [in first 5 days; next 9 days] of a) index-case household’s or b) external contact’s quarantine to offer swab-testing. Amnesty from punishment, please, for household members subject to these random visits in order that co-operation is not destroyed before we ever start! Scottish Universities’ student amnesty is well-done.
“After 16 weeks of T&T’s reporting on operations and 9 weeks after the RSS’s recommendations, we are still failing to learn about asymptomatic positivity, especially how soon those quarantined – either a) in the household of symptomatic index cases or b) as external close contact – become swab-test positive, which could have economic implications if shorter quarantine were justifiable.
“To their credit, both Test & Trace in England and Test & Protect in Scotland are trying to use record-linkage to find out at least how many of those who have developed symptoms and tested positive had been in quarantined. The “quarantined symptomatic positives” are the “successes” of T&T/T&P because taken out of circulation while incubating and infectious. Both systems’ efforts have not got answers yet. In case they never succeed, a new approach is needed now which takes account of UK’s new contact tracing APPs.
New approach: when a positive swab-test-result is reported back, ask the person who receives the positive notification (and record answers centrally), what was their status when swab-tested:
1 = in quarantine or just-out of quarantine as member of household with a symptomatic index case
2 = in quarantine or just-out of quarantine as an external contact of a symptomatic positive case
3 = in quarantine or just-out of quarantine as having returned to UK from COVID-high-risk country
4 = alerted by APP to take a swab-test because of recent at-risk contact
5 = tested as part of routine active surveillance as patient-facing social care or healthcare worker
6 = none of the above but I developed at least one of three key COVID symptoms and got tested
7 = other reason (please specify).
The positives that take us by surprise (those coded 6 and 7) are the ones we need to count.”
Prof James Naismith FRS FRSE FMedSci, Director of the Rosalind Franklin Institute, and University of Oxford, said:
“As expected from the daily figures, there has a been a large (61 %) increase in people testing positive up to the 23rd of September. The results of the REACT study released this morning would indicate that the rate of increase may be slowing as social restrictions have their expected effect. It is too early be sure.
“Although more people were put into the track and trace this week, (29, 000 vs 21, 000), the percentage of people reached and asked to identify their contacts fell for the second week in a row. The percentage of the contacts reached and asked to isolate also fell a little. Each person identifies roughly four people, the majority of contacts are in the same household.
“Although the 6 % increase in swab test processing is welcome, it does not match the increase in cases. The number of pillar 2 tests has remained almost static, yet this is where the most new cases are identified. There is some improvement in the turnaround times for pillar 2, but they exceed 24 hrs. The turnaround times for home and satellite testing both reduced but the median is still over 2 days.
“It is worth, taking a step back from the mesmerising updating of numbers and ask what has been the value of the Test, Trace and Isolate system in reducing viral spread?
“The Royal Society has previously modelled the effectiveness under various scenarios (https://rs-delve.github.io/reports/2020/05/27/test-trace-isolate.html).
“It would seem highly desirable to evaluate the current system against these predictions.”
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
Declared interests
Prof Kevin McConway: “I am a Trustee of the SMC and a member of the Advisory Committee, but my quote above is in my capacity as a professional statistician.”
Prof Sheila Bird: “SMB is a member of the Royal Statistical Society’s COVID-19 Taskforce”
None others received.