Wilson posted a copy of a report to social media in which a pathologist who conducted the autopsy said: "It is my opinion that the cause of death is acute myocarditis, and in view of the history of symptoms since Covid-19 vaccination, and no other cause for myocarditis, is consistent with vaccine-related myocarditis.
"Vaccination with the first Pfizer dose had occurred 12 days earlier and myocarditis-related symptomatology was reported thereafter."
The pathologist said there were no other significant contributing factors linked to his death.
The Herald has verified the report is legitimate and has contacted the Coroner's office and the Ministry of Health for comment.
Myocarditis is an inflammation of the heart muscle wall which is often caused by a viral infection. It is a known but rare side effect of the Pfizer vaccine.
Vaccinologist Dr Helen Petousis-Harris said overall the likelihood of developing the condition is about 10 in a million overall, but it affects younger males at a higher rate.
But even in the highest-risk group, she said the possibility of having something serious happen is "way way way" lower than with the infection itself.
"Myocarditis, that is inflammation, heart inflammation. It can be very, very dangerous and it has a spectrum of severity. The myocarditis that is being seen after the vaccine tends to be very mild and some data from the US shows just an average of one day in hospital."
The average hospital stay for unvaccinated people who contract Covid-19 and get myocarditis is six days, she said.
"In terms of death being an outcome of this related to the vaccine, [it] is extremely rare. Very, very few cases."
She told the Herald anyone experiencing discomfort in the chest - whether they have been vaccinated or not - should see a doctor.
"The loss of a young person, a family member, is the worst thing that could ever happen to anyone. So this is awful. I think it's really important [to point out] that it's extremely rare."
According to Medsafe more than 7.3 million doses of the vaccination have been administered in New Zealand so far, with an average of 52 out of every 10,000 people reporting an adverse event afterwards.
While the majority are minor, there have been 117 deaths reported. Of those, only one has been deemed linked to the vaccination - a woman who also died from myocarditis.
Fifty-one of the other deaths were found to be "unlikely related to the Covid-19 vaccine", 54 could not be assessed due to insufficient information and 11 are still under investigation.
Nairn started experiencing "heart flutters" a few hours after his jab but put it down to the stress of buying a house and planning a wedding.
"We never, ever considered the events that followed," Wilson said.
At 3am on November 17, the couple decided to go to hospital for "reassurance" but within minutes Nairn collapsed.
"It was really, really traumatic," Wilson said.
"I watched him die and I could not get to him. We were about to leave for hospital and he was in the toilet and I heard a thud.
"He had fallen, his body was blocking the door, I could see that he was gone."
Wilson said she called 111 and paramedics tried for 40 minutes to resuscitate him but he was pronounced dead.
She wanted to share her fiancé's story to raise awareness about potential side effects from the vaccine.
"If you decide to get the vaccine and experience any side effects please seek medical advice immediately," Wilson said.
He also left behind his siblings and parents.
Medsafe advises people not to make any decisions about vaccination based on information contained in the weekly Adverse Events reports, saying the protective benefits of vaccination against Covid-19 far outweigh the potential risks of vaccination.
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