Myocarditis: A rare and mostly mild side effect of the Pfizer vaccine

By Te Ao - Māori News

The Whole Truth: Covid-19 Vaccination | By Stuff reporter Hannah Martin


As the number of Covid-19 vaccine doses given around the world climbs daily, regulators are keeping an eye out for reports of any new potential side effects.

One such side effect seen in New Zealand and several countries overseas is myocarditis - inflammation of the heart muscle - after vaccination with the Pfizer or Moderna vaccine.

On July 21, New Zealand's medicines regulator Medsafe concluded, alongside other national regulators such as the FDA in the United States, that myocarditis is a rare side effect of the vaccine. 

It’s sometimes hard to tell the difference between myocarditis and a similar condition called pericarditis (inflammation of the bag that surrounds the heart), so the warning will relate to both conditions, and will be added to the data sheet for Cominarty (the Pfizer vaccine).

Symptoms of myocarditis can include chest pain or palpitations (a feeling that your heart is beating rapidly or irregularly) but they can also be non-specific, such as a cough, or feeling constantly tired or weak.

There are many possible causes of myocarditis - the most common is viral infection - and more than 100 people get hospital treatment for it in New Zealand every year. 

As of July 19, New Zealands’ Centre for Adverse Reactions Monitoring (CARM) had received nine reports of myocarditis, five cases of pericarditis, and four cases of myopericarditis (inflammation of both the heart muscle and the surrounding bag) following vaccination with Comirnaty, out of more than 1.5 million doses given.

Eight of the reports were in males, and 13 reports were after the second dose. 

Around the world, most of the myocarditis cases reported after the Pfizer vaccine happen within a week of vaccination, appear to be mild and people affected have recovered without needing treatment.

Myocarditis has affected less than one in a million people who have received the vaccine in the European Union. 

The group most of the reports have come from are teenagers and young adults - most commonly boys and young men.

To June 11, there had been 1200 reports to the Vaccine Adverse Events Reporting System (VAERS) in the US of myocarditis and pericarditis after Covid-19 vaccination. The reports included both the Pfizer and Moderna vaccines. 

These reports are rare, given the hundreds of millions of vaccine doses administered in the US.

The Centres for Disease Control and Prevention (CDC) in the United States said there was a higher than expected number of reports of heart inflammation among those aged 16 to 24. However, it is continuing to recommend Covid-19 vaccination for everyone 12 years and older, saying the benefits of vaccination still outweigh the known and potential risks.

Medsafe has granted provisional approval for the Pfizer vaccine to be given to 12 to 15-year-olds, but young people here are not expected to start receiving the vaccine until after October.

While there is a link between mRNA vaccines (Pfizer and Moderna) and myocarditis, it is rare and the Pfizer vaccine remains highly effective in protecting people from Covid-19.

Reporting disclosure statement: This post was reviewed by The Whole Truth: Covid-19 Vaccination expert panel members Dr Maia Brewerton, a clinical immunologist, allergist and immunopathologist; and Professor David Murdoch, clinical microbiologist and infectious diseases expert, and Dr Rawiri Jansen (GP and clinical director for a primary healthcare organisation).