The UK’s already embattled Test and Trace system suffered what may yet prove to be its most consequential blow this weekend, after it was revealed an error with an Excel spreadsheet meant almost 16,000 cases of coronavirus recently went unrecorded.

The problem has led to a delay in efforts by contact tracers to find people who have been in the same vicinity as those who tested positive for the virus, in some cases by around a week.

Dr Duncan Robertson, lecturer in management sciences and analytics at Loughborough University and fellow of St Catherine’s College, Oxford, said the error was “an absolute scandal”.

What was the issue?

The problem was caused by an Excel spreadsheet reaching its maximum file size, which stopped new names being added in an automated process, PA Media reported.

For example, 4,786 cases which were due to be reported on October 2 were not included in the daily total on the dashboard that day, when the figure was given as 6,968. 

The files have now been split into smaller multiple files to prevent the issue happening again.

What’s the scale of the problem?

Public Health England (PHE) said a technical issue resulted in 15,841 cases between September 25 and October 2 being left out of the reported daily coronavirus cases.

Here’s what that means:

Reports last week that the pandemic was ‘slowing’ may have been wrong

A glimmer of hope was offered up last week by researchers looking at data from the end of September that said the number of new cases reported suggested the growth of the pandemic in the UK was “slowing”.

Professor Paul Elliott, director of the React study – the largest research of its kind in England – said although the numbers of people who now have the virus has “grown substantially”, there was reason for cautious optimism. 

He told BBC Radio 4’s Today programme: “In the very recent data, and we’re talking about people who did swabs last Saturday, it does seem that the rate of increase of the infection may have slowed a bit.”

But “last Saturday” was September 26, which is one of the dates affected by the glitch.

It has not yet been confirmed how much it affected the data used in the React study.

Regional cases have soared

The distribution of the unrecorded cases is not evenly spread across the country and means case numbers in cities such as Manchester, Leeds and Newcastle upon Tyne are far higher than previously thought.

Manchester now has the highest rate in England, with 2,740 cases recorded in the seven days to October 1 – the equivalent of 495.6 cases per 100,000 people, more than double the figure of 223.2 the previous week.

Liverpool has the second highest rate, up in a week from 287.1 to 456.4, with 2,273 new cases, PA Media reports.

Knowsley is in third place, up from 300.3 to 452.1, with 682 new cases.

People may have contracted coronavirus because of the glitch – but we have no idea how many

Prime minister Boris Johnson was unable to say on Monday morning how many contacts of positive coronavirus cases had been missed.

The problem with the delay means that he contacts of the cases that have only just been reported in cities such as Manchester have potentially been unknowingly spreading the virus for up to a week rather than self-isolating.

Professor Paul Hunter, an expert in health protection at the University of East Anglia, told BBC Radio 4′s Today programme: “If you’re going to do your contract tracing there’s a very short timeframe in which you can do it effectively, and the reason is we know now that this infection is at its most infectious at around the time when people develop symptoms. 

“So if you’re going to identify contacts and their contacts, it really needs to be done within a matter of a day or so if you’re going to have an effect.

“The big concern is there are a large number of people whose contacts haven’t been made aware that they’re at risk and who could have been spreading the infection last week.”

But Boris Johnson insists figures are ‘realistic’

Asked on Monday how many contacts of positive coronavirus cases had been missed as a result of the error, Johnson told reporters in central London: “I can’t give you those figures.

“What I can say is all those people are obviously being contacted and the key thing is that everybody, whether in this group or generally, should self-isolate.”

He said the updated figures meant that the prevalence of the virus was where experts had expected it to be and it would soon be apparent if extra restrictions were having the intended impact.

“The incidence that we are seeing in the cases corresponds to pretty much where we thought we were,” he said.

“And, to be frank, I think that the slightly lower numbers that we’d seen, you know, didn’t really reflect where we thought the disease was likely to go, so I think these numbers are realistic.”

Shadow health secretary Jonathan Ashworth said the error was “shambolic”, adding that “people across the country will be understandably alarmed.”

Health secretary Matt Hancock is to make a Commons statement on Monday afternoon on the issue.