August 27, 2020

expert reaction to the latest NHS Test & Trace stats for the week 13 – 19 August

Te government have released the latest statistics from the NHS Test and Trace service.

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“It’s encouraging to see the decline in new positive tests in the Test and Trace data for the latest week (ending 19 August), compared to the previous week’s number. We do have to be careful in interpreting these numbers, though. The number of positive tests depends on several things, including the numbers of tests that were carried out, the reasons why people are being tested, and what actions are being taken to encourage people to be tested. If any of those change from one week to another, that might have an effect on the number of positive tests that is not entirely connected with the number of actual new infections that occurred. The number of tests carried out in the latest available week was, however, up slightly on the previous week, so the reduction in positive tests certainly can’t be because fewer tests were done – but it just might still have something to do with the reasons why people are coming forward for testing.

“But the Test and Trace data aren’t the only source of information on new infections in England. The Office for National Statistics (ONS) infection survey tests a representative sample of people, not because they have symptoms or have been a contact of someone who was infected, but simply to get a measure of the overall level of infections in the country. The numbers tested for the ONS survey each week are very much smaller than the number tested under Test and Trace, but the trends in numbers of new cases are much the same. The ONS survey found that the lowest rate of new infections was at the end of June, and that was followed by a slow rise – but their last three weekly reports have said that there is evidence that this rise is levelling off, and I think that was particularly clear in their data reported last week (21 August, but based on data up to 13 August). Because this is a survey and there is inevitable statistical uncertainty in the estimates, we can’t be absolutely sure that the slow rise has levelled off. But it’s certainly reassuring to see the same sort of pattern in the Test and Trace new positive tests as in the ONS infection survey results – new infections lowest at the end of June, a slow rise after that, but then levelling off. We’ll know a bit more when this week’s ONS infection survey results are released tomorrow.”

Latest Test & Trace statistics: https://www.gov.uk/government/publications/nhs-test-and-trace-england-and-coronavirus-testing-uk-statistics-13-august-to-19-august-2020

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 member of the SMC Advisory Committee, but my quote above is in my capacity as a professional statistician.”