Research: What it shows about myocarditis after a booster dose of vaccine

The possibility of myocarditis – inflammation of the heart muscle – after the third booster (booster) dose of Pfizer-BioNTech vaccine against Covid-19 remains rare. The relatively greater risk – a chance of about one in 15.000 – is among young men aged 16 to 19, but in any case cases are mild if not rare, new Israeli research shows.

The researchers, led by medical professor Dror Mevorach, chairman of the special committee of the Israeli Ministry of Health to identify cases of myocarditis as a side effect of mRNA vaccines, made the relevant publication in the cardiology journal "Circulation" of the American Heart Association.

Several previous studies and reports from public health agencies around the world, including the US Centers for Disease Control and Prevention (CDC), have pointed to a possibility of myocarditis after mRNA vaccines against the coronavirus. Typically caused by a viral infection, myocarditis is rare and can lead to temporary or permanent weakening of the myocardium and the electrical system that allows the heart to beat normally.

Myocarditis usually resolves on its own or with treatment, but in rare cases it can cause permanent damage to the heart. In the general population before the pandemic, the frequency of its diagnosis was about 10 to 20 cases per 100.000 people per year.

Carrying out booster doses – third or fourth – has raised the question of whether the likelihood of myocarditis increases more than after the first two doses. The new study of nearly 4 million adults who had received a Pfizer-BioNTech booster dose shows that this is not the case and that such cases remain rare.

The analysis found only 91 cases of myocarditis, of which 35 occurred within 30 days of the third dose of vaccine, and 18 within seven days. All 28 cases were clinically mild and patients recovered after a mean hospital stay of 3,5 days.

Overall across the age spectrum, the risk of developing myocarditis was almost nine times greater in men than in women. The highest risk was for young men aged 16-19 (6 cases per 100.000 vaccinated), followed by men aged 20-24 (5,2 cases per 100.000 vaccinated), 30-39 years (1,8 per 100.000) and 25-29 years old (0,8 per 100.000).

The researchers emphasized that further study is needed to account for the relatively greater predisposition of young adults to myocarditis after mRNA vaccine, so that potential pathophysiological mechanisms can be identified.