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expert reaction to latest statistics from the NHS Test and Trace service in England

The government have published the latest figures for the NHS Test ad Trace service in England.


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

“There is greater clarity in week 3 of the NHS Test and Trace service in England but we look to week 4 for even greater transparency about complex cases.

“As the initial week may be different for a number of reasons, below I track all cases in weeks 2+3 from the point of their transfer to the contact tracing system. Only around half the transferred cases identify any close contacts.

“There is no information as yet about transfer-timeliness (from symptom-onset to swab-date to transfer to the Test & Trace system); nor about timeliness in managing locally complex cases. Coming shortly, I’m sure. And epidemiologically very important to know about.


** Also asked that index case and household members should isolate themselves at home.

“Report 3 on Test, Trace and Isolate now explains that complex cases are defined, not by their number of identified close contacts, but situationally. Examples include: having worked in or recently visited health or care setting, a school for people with special needs, a prison or other secure setting, or critical location in terms of national infrastructure or security.  Notable by their absence are mass gatherings or places of entertainment, both of which may qualify as lock-down is released.

“Complex cases are managed by local public health teams. By simple sums, I derived the table below which compares the weekly number of swab-positive persons transferred to the contact tracing system who were both reached and asked to provide contact details. In weeks 2+3, of those asked, 70% provided details for one or more contacts, up from 60% in week 1.

“As shown above, the bulk of the effort in tracing identified close contacts is expended locally in the ratio 9:1 in weeks 1 & 2 but reduced to 4:1 in week 3, essentially because there has been a 44% decrease between weeks 2 & 3 in the number of complex cases that have been reached and asked to provide details: down from 858 to 477.

“Next week’s update may shed valuable light on the “situations” reflected in the complex cases which local public health teams have been investigating.”


Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham, said:

“There are number of issues currently with the system – these can only be sorted by members of the public as it requires individuals to take the appropriate actions to reduce spread. 

“The number of people who are thought to have COVID-19 is much higher than the numbers testing positive – more people need to come forward for testing that is now much easier to access. 

“One in four positive people can’t be contacted – this is surprising and worrying – these people need to provide details to get the result of their test and they have a responsibility to be contactable, they should anyway be isolating at home so easy to get hold of.

“There has been a drop in the percentage of contacts who have been contacted from 90% to 80%, but this is still not a bad figure.

“Since launch, 128,566 people have been identified as recent close contacts. The bulletin states that 30,002 contacts are traced by wider online and other call centre capacity for non-complex cases. This means that more contacts have been identified by Public Health England (98,564, or 77%) compared to the national system (30,002, or 23%) from 21,106 COVID-19 cases.  This will partially reflect that there are likely to be many more contacts for an outbreak in a care home, which are under the PHE service, than for individual cases.  They are very much easier to identify and contact as they are residents and staff.  An individual cases may have 5 or less contacts whilst a care home easily have 50 to 100 or more.

“Every contact contacted is a possible chain of transmission stopped.”


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


Declared interests

Prof Shelia Bird: SMB chairs a sub-panel of the RSS COVID-19 Taskforce who liaise with statisticians in the Department of Health and Social Care to discuss a range of statistical reporting issues from definitions and data acquisition to broader epidemiological and statistical perspectives.

None others received.

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