It is impossible to tell if a 13-year-old boy died from his Covid-19 vaccine or a virus, a coroner has ruled.

The teenager had his second Pfizer vaccine 10 days before his sudden death at home in October 2021 - he had gone to bed in the evening and never woke up.

Coroner Robin Kay ruled the teenager died after suffering cardiac arrhythmia causing acute myocarditis - but could not conclude how that myocarditis was caused.

Reporting restrictions mean that identifying details of the person - such as their name - cannot be reported on.

On the afternoon of 27 October, the teenager had a normal afternoon doing chores, playing cards and watching a movie.

He went to bed around 8.30pm saying he felt warm. He was found in his bed on the morning of 28 October and ambulance staff pronounced him dead at his home.

Coroner Kay noted the teenager was fit and active, without any significant medial issues.

He had received the Covid-19 Pfizer vaccine 10 days' before his death, and had not displayed any adverse reaction to it in the days following.

This is one of four deaths which had been reported to the Covid-19 Vaccine Independent Safety Monitoring Board as being possibly linked to the vaccine as of 2024.

Overall, more than 13 million vaccines have been administered to people in New Zealand, and 3923 people have died as a result of the virus.

Myocarditis is inflammation of the heart muscle. It can be caused by different things, most commonly from viral infections. In rare cases, it is a side effect of Covid-19 vaccines.

Dunedin man Rory Nairn's death was one that was ruled as being caused by the Covid-19 vaccine - the 26 year old experienced heart flutters in the days after receiving the vaccine.

In this case, Coroner Kay considered reports from two forensic pathologists and from the Covid-19 Independent Monitoring Board in coming to his conclusions.

He found that the cause of the teenager's death was probable cardiac arrhythmia due to myocarditis, but that the cause of the myocarditis could not be "satisfactorily determined".

He referred to one of the pathologist's evaluations that this was a complicated case, where death was caused by a tiny amount of inflammation that occurred in a "crucial part" of the heart which helps it beat.

"Had that myocarditis been present somewhere else in the heart, the expert evidence is that it would not have caused his death."

He noted that this pathologist found a low presence of two viruses in the heart, which could have pointed to a viral cause of the myocarditis, instead of the vaccine.

The pathologist said the microscopic appearance of myocarditis was quite different from other deaths caused by the Covid-19 vaccine in New Zealand and elsewhere.

However, Coroner Kay could not rule out the vaccine as a possible cause of the myocarditis, quoting a part of the second pathologist's report.

"Because death occurred 10 days' following a Covid-19 vaccination, and the vaccination is known to cause myocarditis in rare cases with young males at greatest risk, the possible role of the vaccination in either causing or helping to cause the myocarditis cannot be completely excluded".

Coroner Kay provided several possibilities for the teenager's death. Including that -

• One or both of the detected viruses caused the myocarditis, and the timing of the death 10 days after the Covid-19 vaccine was just a coincidence.
• The vaccination caused the myocarditis and the viruses just happened to be in the heart tissue and were not causing a problem.
• The vaccination somehow interacted with the viruses or the immune system to allow the viruses to cause the myocarditis, when they would otherwise not have made the teenager unwell.
• Neither vaccination nor the viruses caused the myocarditis and it was caused by something entirely different which cannot be identified with current medical knowledge.

University of Otago Professor of biochemistry and infectious disease physician Kurt Krause says contracting myocarditis as a result of the vaccine is "very rare".

Myocarditis is most often caused by viruses, he says, and it's much more common to get myocarditis as a result of the Covid-19 virus, rather than the vaccine.

He cites a recent paper in the journal Biomedicines, which found the incidence rate of myocarditis and pericarditis was calculated to be 5.98 instances per million Covid-19 vaccine doses delivered.

The rare risk is heightened in young males, he said, and if people got it, the disease can be "very serious".

He says generally these people respond well to treatment if the myocarditis is caught - but that the risks need to be consistently monitored into the future.

-Ellen O'Dwyer/RNZ.