By Helen Petousis Harris 30/08/2021

After hundreds of millions of doses away, the most rare side effects have revealed themselves. The one of concern involves inflammation of the heart.

What is myocarditis?

Myocarditis is an inflammation of the heart muscle; pericarditis is inflammation of the sac around the heart. Each year NZ has around 100 cases of myocarditis. A key trigger is a viral infection, including COVID-19. Symptoms include an abnormal heartbeat, new chest pain, and difficulty breathing. The condition can be severe and life threatening. Most cases will be admitted to hospital.

Myocarditis and pericarditis following mRNA vaccines
Q.  Is this a thing?
A.  Yes.

Q.  Should I be worried?
A.  Not unduly, this is very rare, and most cases recover. You are much more likely to get myocarditis if you get COVID-19. In fact, struck by lightning is more likely so while you should be for-warned, you should not loose sleep over it either.

Since rolling out the mRNA vaccines (Pfizer and Moderna) a rare safety signal has been identified. A few months ago, safety systems alerted us to an unexpected number of cases of myocarditis as well as pericarditis. After extensive investigations we know the following:

  • Myocarditis cases are typically occurring in males under 30-years of age within days of receipt of the second dose.
  • In these cases, the course of disease in generally mild and resolved with rest and anti-inflammatory medication.
  • Long term follow-up of cases in ongoing.
  • When hospitalised the mean stay is 2-days.

Summaries from the United States and the European Economic are are available.

From the United States CDC

  • 133 million second doses administered and 636 reported myocarditis cases to Jul 11 2021 (about 6 per million 2nd doses)
  • In young males 12-17 years old this is 63 cases, 18-24 years old 51 cases, and 25-29 it is 16 cases per million dose twos’
  • In females the risk is about 8 times lower

From the European Medicines Agency 

  • 138 cases after 177 million doses of Pfizer
  • 19 cases after 20 million doses of Moderna
  • Cases usually occurred within 14 days after dose 2
  • There were five cases in older persons or with underlying health conditions who died

Benefits and risks for Pfizer vaccine

The table below is based on contemporary US data and reflects the disease risks for these younger age groups when the Delta variant is circulating in the community.

  Cases prevented vs. myocarditis cases for every million second doses over 120 days
  COVID-19 symptomatic infections COVID-19 hospitalisations Intensive care admissions COVID-19 deaths Vaccine related myocarditis COVID-19 related myocarditis
Females 12-17 years 8500 183 38 1 8-10
Males 12-17 years 5700 215 71 2 56-69 450 per million infections*
Females 18-24 years 14,000 1,127 93 13 4-5
Males 18-24 years 12,000 530 127 3 45-56
Females 24-29 years 15,000 1,459 87 4 2
Males 24-29 years 15,000 936 215 13 15-18

Based on US data for week May 22, 2021
Adapted from Wallace and Oliver, ACIP meeting June 23, 2021.
*New Scientist

The benefits of receiving the Pfizer vaccine in these younger age groups far outweigh the risks associated with it. While NZ has very little COVID-19 in the community at the moment due to lockdown, once we ease the restrictions and open the boarders then community transmission of this infection is inevitable.


Each year we give infants a vaccine against rotavirus, nasty little virus that is a major cause of serious diarrhoea in infants, lots of hospitalisations occur without a vaccine. However, the vaccine comes with a rare and potentially serious side effect. In around 1-3 per 100,000 doses babies can develop a telescoping of the bowel called intussusception, a medical emergency. This risk should be communicated to every parent before the vaccine is given and advice given. Despite this small risk, most babies receive their vaccine and now hospitalisation for this disease in NZ is uncommon.

The myocarditis risk associated with mRNA vaccines is not dissimilar to the intussusception risk with rotavirus vaccine. Vaccinees should be informed about the risk and what to do if symptoms occur.


Useful links

MMWR ACIP update July 2021

COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS): updated guidance regarding myocarditis and pericarditis reported with COVID-19 mRNA vaccines