A 28-year-old man who died from a vaccine-related blood clot was wrongly offered the jab due to a clinical error by a GP surgery 17 years earlier.
When he was 11, Alex Reid's weight was wrongly recorded as 22st 11lb (145kg) - suggesting he had a BMI of almost 69.
His parents, who say his death has "destroyed" them, said this put him in the "vulnerable" category eligible for an early AstraZeneca shot.
The GP surgery said it sent "sincere sympathies" to the Reid family.
Mr Reid received his first dose of the vaccine in March 2021, but weeks later an official recommendation said under 30s should be offered an alternative due to an extremely rare chance of blood clots.
Those who had already received the AstraZeneca jab as their first vaccination were advised to receive it as their second, with Mr Reid having his in May 2021.
He died on 29 June 2021, with the medical cause of death being Covid-19 vaccine-induced immune thrombotic thrombocytopenia, a prevention of future deaths report said.
His family said he had called Moorfield House Surgery in Garforth, Leeds, to query why he was being invited for an early vaccine.
His mother, Halina Reid, told the BBC: "Surely this one call from Alex should have raised alarm bells since most of the calls the surgery was getting were from people asking why they had not been called.
"The person who took the call said she did not know, made no attempt to find out and just said, 'count yourself lucky'."
She continued: "After Alex's death, we knew something was not right - it has taken two years and five months to get answers."
Mr Reid's weight had been incorrectly entered in 2004 during a routine appointment for asthma when he was 11, his family said.
At no point over the next 17 years was it flagged up as being unusual that a 4ft 9in (145cm) boy had a BMI of nearly 69, with a healthy BMI being between 18 and 25, they added.
"The error was so serious that a safeguarding concern should have picked it up," his mother said.
"It would be almost impossible for a child to weigh that much - they had opportunities to rectify this mistake over the course of his life."
The AstraZeneca vaccine, which is being withdrawn due to shifting demands, was estimated to have saved millions of lives during the pandemic but also caused rare, and sometimes fatal, blood clots.
The prevention of future deaths report, prepared by coroner Oliver Longstaff, called for "validation rules in general practice IT systems" for when data is entered or a function that "challenges" potential errors.
He wrote: "If the obviously erroneous BMI had not been recorded or had been challenged at the point of entry by the relevant IT system, Alex would not have been classed as vulnerable... and would not have died when he did."
His report was sent to NHS England, the British Medical Council and the Royal College of General Practitioners as well as EMIS and TPP - healthcare technology companies used by GPs.
All have been contacted for comment by the BBC.
An NHS England spokesperson said: "We extend our deepest sympathies to the family and friends of Alex Reid.
"We are carefully considering the prevention of future deaths report sent to us by HM Coroner and will respond in due course."
Mrs Reid said she was alerted to the BMI error in July 2021 during a phone call with one of the surgery doctors, who suggested her son's weight may have been incorrectly inputted.
But the surgery was "not forthcoming with the truth" and failed to disclose the error when replying to two coroner's letters about the matter, she said.
In response to a further letter, the surgery stated in February 2023 a raised BMI on its system could have triggered the early vaccine invitation, she added.
'My best friend'
The surgery has since apologised to Mr Reid's parents, but his mother called it an "insult".
"It is gobsmacking they were so blatant in trying to blame others, when it was one fatal mistake they made," she said.
Alex loved aviation and music, his family said, and described him as "their world".
His father, Antony Reid, said: "We feel we have been destroyed. He was my best friend.
"It's a nightmare we are living every single day - we've followed every avenue to get the truth and some justice for him but it will not bring him back."
He added: "The last three years have been horrendous, not only grieving him but fighting for justice he deserves."
Mrs Reid said she hoped the coroner's report would cause "real change" in how medical records were kept.
In a statement, the surgery said: "We are very sorry about the death of Mr Reid and send our sincere sympathies to his family and friends.
"The practice gave evidence at the inquest hearing, however we are unable to comment further because of our duty of patient confidentiality."
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