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expert comment on Test and Trace data

A comment from Professor Sheila Bird on the NHS Test and Trace statistics.


Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:

“We have excellent statistics about operational details for Test & Trace but – after more than 6 months – we still lack any statistics on what Test & Trace achieves in terms of infection control.

“After more than 6 months of Test & Trace, we still do not know:

* what percentage of other members of the household of index cases were already SARS-CoV-2 infected when the household went into self-isolation,

* what percentage of those other members of the household developed symptoms, booked a test and tested RT-PCR positive during quarantine,

* what their risk-factors were for being thus affected.

“This absence of evidence is a public health disgrace. Nor do we know objectively (ie by random home-visits) about adherence to self-isolation within the household of index cases.

“Until very recently, the quarantine period for external close contacts was 14 days from the date of their last close contact with an index case (exposure:  from 2 days prior to case’s symptom-onset date). For the Prime Minister as external close contact, three days of his intended 14-day quarantine period had already elapsed before he was pinged and asked to self-isolate: that is 3/14 days of intended-quarantine (21%) were missed out on.

“What is the missed-out on percentage for all external contacts reached by T&T?

“Of course, for those external close contacts who are never identified or are identified but never reached, the entirety of their intended quarantine period is missed.

“Are the infection control data about Test & Trace so depressing that the public is not allowed to see them?

“At the end of July 2020, there were 1.6 identified within-household contacts for every identified external close contact. This ratio rose to 1.9 at the end of September; 2.9 at the end of October; and  5.6 at the end of November (26 to 2 December) before lockdown was relaxed.

“Increasingly, therefore, Test & Trace has been concerned primarily with the quarantining of members of the household of an index case, either through folk’s reluctance to identify external close contacts or because the number of them has genuinely diminished as the public became increasingly precautionary.

“None of this lets Test & Trace off the hook: either for failure to report robustly its contribution to infection control or for its failure to glean intelligence about the level of, and risk-factors for, asymptomatic SARS-CoV-2 infections early in the quarantine period for an affected household. Precisely this information (as signalled by the Royal Statistical Society’s COVID-19 Taskforce statement on 23 July) was needed to provide an evidence-base for decisions on the required duration of self-isolation.”



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



Declared interests

Prof Sheila Bird: SMB serves on the Royal Statistical Society’s COVID-19 Taskforce.

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