Why Covid cases soared by 23,000 in a day amid track-and-trace data chaos

The total number of coronavirus cases recorded in the UK jumped by almost 23,000 on Sunday as a result of a glitch in the NHS Track-and-Trace app.

A technical issue with the app resulted in thousands of cases initially reported over the previous week being left out of the reported daily coronavirus figures, according to Public Health England (PHE).

The government confirmed today that the figures reported over the weekend had been “artificially high for England and the UK” - an admission that has fuelled widespread criticism of what Labour shadow health secretary Jonathan Ashworth described as the “shambolic” track-and-trace system.

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What are the most recent figures?

PHE says that between 25 September and 2 October, around 15,841 new infections reported to the app were omitted from the official tally of daily coronavirus cases. After these cases were added to the official figures, the daily total for the UK rose by a record 12,872 on Saturday, followed by an even bigger spike of 22,961 on Sunday.

By contrast, on Friday the total stood at 6,968 positive cases, which was on a par with the levels reported previously in the week.

A further 33 deaths - the recording of which were not impacted by the technical issue - were also confirmed on Monday.

The surge in recorded new cases takes the total for the UK to more than 500,000, according to latest figures.

What went wrong?

The technical issue was caused by the names of people reporting positive test results exceeding the maximum file size of the Microsoft Excel sheet in which they were recorded. This prevented new names being added by an automated process.

PHE says the files have now been split into smaller batches to prevent the error from being repeated.

But health officials are still struggling to correct the faulty data caused by the fault, which “also means that daily totals reported on the UK government’s coronavirus dashboard over the last week have been lower than the true number”, The National.

For example, 4,786 cases that should have been reported on 2 October were not included in the total for that day, when the figure was given as 6,968.

Boris Johnson admitted this morning that “some of the data got truncated and it was lost”, but added that health authorities had “contacted all the people who were identified as having the disease” and “are now working through all the contacts as well”.

“The key thing, I would say, and it goes for everybody, is that if you are contacted by NHS Test and Trace then you must self-isolate, if you are told you have been in contact with somebody who has the virus,” Johnson told reporters.

The glitch has also further thrown into doubt the ability of government modellers to accurately predict future outbreaks.

The reporting delays “play havoc with data streams and make them very difficult to analyse in real time”, tweeted Professor Graham Medley, an outbreak modeller at the London School of Hygiene and Tropical Medicine and a member of the Scientific Advisory Group for Emergencies (Sage).

“If the delays change or vary by group then they can distort a lot. Wonder what these will do to the R [value] estimates next week,” Medley added.

Those concerns were echoed by Jonathan Read, a biostatistician at Lancaster University who sits on the Sage outbreak modelling subgroup.

Speaking to The Guardian, Read said: “An apparent surge in cases due to a change in reporting delays, unless accounted for, will look like the epidemic is growing and so the reproduction number could be overestimated.”