Study: myocarditis remains rare after 3rd booster dose of Covid-19 vaccine

The possibility of myocarditis - inflammation of the heart muscle - after the third booster dose of Pfizer-BioNTech anti-Covid-19 vaccine remains rare. The relatively higher risk - a probability of about one in 15,000 - is for young men 16 to 19 years old, but in any case the incidents are mild in addition to rare, a new Israeli scientific study shows.

The researchers, led by Professor of Medicine Dr. Dr. Mevorah, chairman of the Israeli Ministry of Health's special committee to identify cases of myocarditis as a side effect of mRNA vaccines, published in the American Heart Association's cardiology journal Circulation.

Several previous studies and reports from public health agencies around the world, including from the US Centers for Disease Control and Prevention (CDC), have indicated a possibility of myocarditis following coronavirus mRNA vaccines. Typically myocarditis caused by viral infection is rare and can lead to transient 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, and in rare cases can cause lasting damage to the heart. In the general population before the pandemic, the frequency of diagnosis was about 10 to 20 cases per 100 000 people per year.

Conducting booster doses - third or fourth - has raised the question of whether the chance of myocarditis increases more than after the first two doses. The new study of nearly 4 million adults who had Pfizer-BioNTech booster doses shows that this is not the case and that related incidents remain rare.

The analysis found only 91 cases of myocarditis, of which 35 had occurred within 30 days of the third dose of the vaccine, and 18 within the next seven days. All 28 cases were clinically mild and patients recovered after an average hospital stay of 3.5 days.

Overall across the age range, the risk of myocarditis was almost nine times greater in men than in women. The highest risk was among young men aged 16-19 years (6 cases per 100,000 vaccinated), followed by men aged 20-24 years (5.2 cases per 100,000 vaccinated), 30-39 years (1.8 per 100,000) and 25-29 years (0.8 per 100,000).

The researchers stressed that further study is needed to investigate what accounts for the relatively greater predisposition of young adults to myocarditis after mRNA vaccine, in order to identify possible pathophysiological mechanisms.