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The government have released the latest statistics from the COVID-19 Test and Trace system.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“There’s some welcome news in this week’s Test and Trace statistics, which take the data up to the week ending 25 November. Average times taken to get test results have improved for almost all the different testing routes, and there haven’t been any marked deteriorations. The percentage of contacts who were reached has gone up a lot from the previous week, and the times taken to reach contacts and advise them to self-isolate have gone down. To some extent these improvements in efficiency may be because the system had to deal with fewer positive cases. That’s good news in itself, of course, and a reduction in cases is in line with data from more reliable sources like the ONS Infection Survey and the Imperial College/Ipsos MORI REACT-1 survey. The survey data are more reliable than Test and Trace results in indicating how numbers of infected people are changing, because they test representative samples of people, regardless of whether they are showing symptoms or the jobs they work in. The Test and Trace numbers of positive results depend on how many tests were done and on why people are being tested. This week, the number of people tested is down on the week before by 5%, so you might expect that there would be fewer positive results just because of that. But the reason that fewer people were tested is probably very largely because fewer people are being infected and so fewer people are showing symptoms. The percentage of people testing positive, out of all those tested, has also fallen this week – so putting these results together with the results from the surveys, there are many indications that infections are falling quite fast. They have much further to fall, though – the current weekly number of people testing positive in the Test and Trace system, over 110,000 in England, is still at about the same level as in mid-October, which is about ten times the number at the start of September.
“The main reason for the improvements in percentages of contacts reached, and in the time to reach them, is however not the reduction in the number of positive cases to be dealt with. Instead it comes from a procedural change in how household contacts aged under 18 are dealt with. From 18 November, these contacts are no longer contact-traced individually, as long as their parent or guardian in the household confirms that they have told their child to self-isolate. Since a very large proportion of contacts – 85% of them this week, in cases not managed by local health protection teams – are in the same household as the original cases, though of course these contacts will by no means all be under 18 – this substantially reduces the number of contacts that need to be individually contacted by the contact tracers. It does seem like a reasonably sensible idea to me, but I think it is important that there should be monitoring of the effect that this change might have on the number of contacts who actually do self-isolate as they ought to. Since monitoring of how closely people comply with the self-isolation instructions has so far been patchy to say the least, it’s not clear to me whether the impact of the change in procedure will in fact be properly monitored. From 27 November, there was a further change, whereby all adults in a household will have the option to be traced via a single phone call. Again, on the face of it this sounds reasonably sensible – it reduces the load on the contact tracers, and in many households the necessary information will be properly shared amongst household members anyway. But it’s even more important to monitor what effect this has on people’s compliance with the instruction to self-isolate, when adults are involved, given that adults do not have legal obligations to one another as is the case with parents or guardians and their children. I have no idea how, or indeed whether, this monitoring is going to take place.”
Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:
“Two weeks after lockdown, both the number of Pillar 2 tests and the percentage that are positive have decreased impressively so that 40,000 fewer cases were referred to Test & Trace in its most recent week.
“Test & Trace has also upped its game at last – by now reaching over 70% of identified other members in the same household of the symptomatic individual & asking them to self-isolate.”
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: “I am a member of the Royal Statistical Society’s COVID-19 Taskforce.”