The extent to which Covid-19 was spreading in hospitals across England, which could have helped trusts reduce infections on wards, was not shared with NHS trusts for weeks, ITV News can reveal.
But NHS England didn't inform individual trusts of how they were performing until weeks later.
Allowing trusts to compare their performance in controlling the spread of Covid-19 between patients and staff could have saved lives.
"There's always lessons we could learn from other trusts," said Professor Michael Weekes, a consultant in infectious diseases at Addenbrookes Hospital in Cambridge.
They've implemented a hospital-wide testing programme for all staff as well as compulsory mask-wearing in all clinical areas of the hospital.
"If we found there was another trust that had a surprisingly low number of infections and there was something else that we could be doing then we'd do it straight away," he added.
Professor Callum Semple co-leads a consortium of doctors compiling hospital admission data on coronavirus called the Coronavirus Clinical Information Network of CO-CIN.
As cases of Covid-19 began to reach the peak, they noticed something unusual in their data.
"We thought people were entering data incorrectly, because they were telling us that people where having their [Covid-19] symptom onset after the date of admission, but it turns out they weren't," he said.
What the researchers saw was that people being admitted to hospital for non-Covid-19 problems like stroke, heart attacks or broken legs, were getting infected in hospital.
"Across England the typical rate was around 10% of people acquiring Covid actually in hospital," said Prof Semple.
Prof Semple, who sits on the government's scientific advisory committee (Sage) said that he shared this information with NHS England and Public Health England as soon as he had it.
However he wasn't prepared to go into more detail about the data.
And according to the Sage sub-committee minutes published online, reports based on his CO-CIN data were classified by the government, "as to their sensitive nature".
But we've learned from a publicly available list of documents discussed by Sage that NHS England was aware of the CO-CIN data no later than March 26.
However they didn't share that information with individual trusts until May 5.
In a statement NHS England told ITV News: "Quite rightly, this emerging and evolving data had to be verified and checked for accuracy before it was sent to hospitals."
It added: "Hospitals across the country have established effective mechanisms for infection prevention and control and in the first week of April, the NHS asked all trusts to assess and report on how they were implementing Public Health England’s guidance, including how they were preventing transmission within hospital, to help local clinicians to limit the spread of infection."
There's plenty of anecdotal evidence of Covid-19 spreading in hospitals during the outbreak, and individual hospitals' infection control teams would have been tasked with preventing that spread.
A former nurse's son told ITV News of his family's shock after his mum died alone after contracting Covid-19 from an elderly dementia patient she was sharing a ward with in hospital.
Callum Strudwick said: "You're always a bit worried but you think with them being professionals and they would know these sorts of things, so you think she's in the best place possible in the sense because at the end of the day being at home being that ill it's not going to do her any good so she needs to be in a place where they can manage it and take care.
"I think that's the shocking part that the one place where you believe she'll be the safest is the one place where she ends up catching it, it kind of stings a bit for the whole family," he added.
His mum, Amanda Strudwick, had survived lung cancer last year but was admitted to a hospital in Lincolnshire in March with pneumonia.
The 52-year-old was told that her cancer had returned and was admitted to Pilgrim Hospital.
Mr Strudwick also paid tribute to his mum saying: "She was the brightest person in the crowd, I can tell you that, my mum was one to have party, have a laugh, she'd be the loudest person in the room, everyone loved her for that.
"She was so caring in the sense that she always put people before herself, even right up to her death when she helped that old woman with the virus, if we'd have asked her if she'd do it again, she would've.
"She was just such an amazing person and it's left such a big hole for the family," he added.
Protocols about managing the flow of newly admitted patients, staff shift patterns and PPE requirements can all make a difference to hospital outbreaks.
Allowing trusts to know who was performing well at controlling Covid-19 on wards could have helped share the most successful strategies.
"Knowing what it was like and the way we were trying to increase capacity at the time I think it's very difficult to say we would have done anything differently," said Alison Pittard, Dean of the Faculty of Intensive Care Medicine.
"But what we can do is learn from that because what we mustn't do is allow that to continue because we want patients to feel safe that they can come into hospital and get the services that they need without being infected with Covid," she added.
Rates of coronavirus are now at their lowest levels in hospitals since the start of the outbreak.
The NHS is urging patients not to avoid hospital if they need treatment.
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