The blunder that saw 16,000 positive Covid-19 tests go missing was caused by failing to replace software an astonishing 13 years out of date, experts believe.
It could handle only about 65,000 rows of data, rather than the one million-plus that the newer software XLSX is capable of – which meant any additional test results were lopped off.
The revelation will pile further pressure on Matt Hancock, the health secretary, who has refused to discuss details of the fiasco, or explain why modern software was not used.
Christina Pagel, a research professor at University College London, said it was understandable that the system was put together at huge speed when the pandemic struck in the spring.
But she added: “For that still to be the case in September seems a little unforgiveable to me.”
One expert told the BBC that even a high-school computing student would have known that better alternatives to XLS existed.
“Excel was always meant for people mucking around with a bunch of data for their small company to see what it looked like,” said Professor Jon Crowcroft from the University of Cambridge.
“And then when you need to do something more serious, you build something bespoke that works. There’s dozens of other things you could do – but you wouldn’t use XLS, nobody would start with that.”
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Downing Street has launched an inquiry into the mistake, which has left tens of thousands of contacts of positive Covid cases still untraced.
Although all the infected people were informed, their details were not passed onto the tracing system – so they were not asked for their close contacts, for that work to begin.
And the true infection rate was vastly underestimated, particularly in the North West, which now faces a tougher lockdown – prompting a revolt by city council leaders.
Infection rates in Manchester (529 cases per 100,000 people), Knowsley (499), Liverpool (487) and Newcastle (434) are higher than EU infection hotspots such as Madrid and Paris.
Public Health England is now believed to be breaking down the test result data into smaller batches to create a larger number of Excel templates – and ensure none hit their limit.
Mr Hancock told MPs the weakness in the “PHE legacy system” had been recognised in the summer, with contracts for an upgraded system awarded in August, but failed to say why it had not been replaced.
One third of the 15,841 unreported positive tests – more than 5,000 people – had not been reached by Tuesday, in order for their contacts to be informed.
