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expert reaction to latest weekly statistics for NHS Test and Trace (England): 1 April to 7 April 2021

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:

“The people involved in the parts of Test and Trace that actually do tests, or record and collate test results, have had to be pretty busy in recent weeks because of all the extra rapid lateral flow tests that have been done in people without symptoms, including students and staff in schools.  But we know from all data sources that the number of confirmed infections has fallen hugely since the peak level at the start of the year.  So you might think that things would have become easier for the people that follow up contacts of new cases, to tell them to self-isolate.  The performance of that part of the system was actually pretty consistently good during the height of the latest wave.  However, I have a concern now that some aspects have been getting worse, despite the reductions in the number of new positive cases that have to be dealt with.

“The latest data are for the week from 1 to 7 April – that does include the Easter holiday weekend and perhaps that affected things, but on some measures, things began to change well before that.  As always, the great majority of the contact tracing across the country is done outside the local health protection teams (HPTs) that deal with outbreaks in some specific settings.  Of the recent contacts of confirmed positive cases dealt with outside local HPTs, the great majority, 88%, were reached by the contact tracers in the most recent week, but that’s a decrease on the previous week when 90% were reached, and the percentage being reached at the height of the latest wave was usually around 94%.  That’s not a huge decrease, but other aspects have seen larger changes.  The proportion of contacts dealt with outside HPTs that were reached within 24 hours of the original case being identified fell in the most recent week to 56%, not all that much more than half.  The previous week it was two-thirds (67%), and during the height of the recent peak it was generally over 70%.  During January and February, around 90% of contacts dealt with outside HPTs were being contacted within 3 days of the original case being tested.  In the most recent week, only 70% were contacted within three days – the week before it was 80%, and the percentage has been falling since the end of February.  One thing that had improved, compared to the position at the peak of the recent wave in early January, was the median time between the original case first reporting symptoms and the time the contact was reached (again in cases not managed by local HPTs).  That’s the time within which half of the contacts have been reached.  In early January that time was over 100 hours, but during February and the first part of March it was below 80 hours and falling.  Since mid-March it rose again, but only slowly, to 79 hours in the week ending 31 March – but in the most recent week there was a huge increase to 95 hours.

“So, overall, there’s been a small decrease in the proportion of contacts that have been traced at all, outside local HPTs, and more worryingly quite a substantial increase in the time it takes to reach these contacts and tell them to self-isolate.  If it takes too long to reach contacts, then that increases the chance that they might be going round and infecting others before they even know that that’s a risk.  This increase in time might well have something to do with the fact that, in recent weeks, far more of the contacts don’t come from the same household as the case that they were identified from.  In the most recent week, almost half (47%) of the contacts came from a different household than the original case that they were in contact with.  In January and February, only around 1 in 7 contacts came from a different household, but that number has risen quite rapidly since then, with a particularly big rise in the most recent week.  The previous week, just over a third of contacts (35%) came from a different household, but that rose to almost half in the most recent week.  This general increase in contacts from other households may well be because, as lockdown restrictions are removed, people generally do have many more contacts outside their own home.  That makes the task of contact tracing more difficult – but we really do need to keep on top of this as we move out of lockdown.”



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



Declared interests

Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee.  I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic.  My quote above is in my capacity as an independent professional statistician.”

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