Very rare reports received worldwide of myocarditis with a possible link to the use of mRNA vaccines against COVID-19 are currently under investigation. Swissmedic has only received a relatively small number of ADR reports of myocarditis to date. Nevertheless, Swissmedic believes that it would be appropriate to inform healthcare professionals about the latest findings.

Myocarditis is an inflammatory heart disease caused primarily by viruses. Other infectious pathogens can also trigger the condition, however, as can toxic substances, medicines or immune-mediated diseases.

The incidence of myocarditis is around 22 in 100,000 (for comparison: the incidence of acute myocardial infarction in Switzerland is ten times higher, at 227 in 100,000). The typical symptoms of acute myocarditis include chest pain, dyspnoea, exhaustion and palpitations, progressing to syncope and cardiogenic shock in severe cases.

The clinical manifestation of myocarditis can vary considerably, even resembling an acute myocardial infarction. The possibility of myocarditis should always be considered in patients with elevated cardiac biomarkers such as troponin, or ECG changes that are compatible with myocardial damage or document a previously unknown cardiac arrhythmia.

Changes in left ventricular morphology and function on an echocardiogram or cardiac MRI (magnetic resonance imaging) scan can be caused by myocarditis. Measurements of the inflammatory reaction and its distribution pattern in the heart by cardiac MRI enable myocarditis to be diagnosed with a high degree of certainty. To sum up: the tentative clinical diagnosis of myocarditis is based on a combination of clinical parameters and findings.

Against a backdrop of almost 5 million administered vaccine doses (as at the start of June 2021), the following cases have been reported to Swissmedic up to 27 May 2021: myocarditis (n=2), perimyocarditis (n=4) and pericarditis (n=6), with overlaps between these clinical presentations. The reporting rate for these reactions therefore equates to around 1 per 400,000 vaccine doses. The reports concern 3 women, 8 men and one person with unspecified gender. The average age is 47 (range: 18-70). Four reports concern Comirnaty and seven the vaccine from Moderna, while no specific vaccine is stated in the remaining case.

The symptoms appeared after the first or after the second vaccination in nine and three cases, respectively. The average time between the last vaccine dose and the reaction was 8.75 days (range: 1 to 28 days). Five of the 12 patients had a history of relevant illnesses, including chronic kidney disease, kidney transplant, myelodysplastic syndrome, recurrent pericarditis (now with reported pericarditis after vaccination). One male patient (aged 67) with pre-existing heart disease and renal failure requiring dialysis developed cardiogenic shock and died. Insofar as can be ascertained from the documentation, most of the other patients experienced a fairly mild episode, or else the final details on the outcome of the illness are not yet available.

Very rare cases of myocarditis/pericarditis after vaccinations with mRNA vaccines against COVID-19 are currently also being investigated internationally. A committee on vaccine safety at the US Center for Disease Control and Prevention (CDC) recently stated that only a relatively small number of cases of myocarditis have been reported to date. The reported cases of myocarditis under suspicion appear to have occurred primarily in adolescents and young adults, more frequently in vaccinated men and after the second dose of vaccine. Typically, the symptoms appear during the first four days after vaccination, and most of the reported cases showed a mild progression.

Whether a causal link actually exists between the mRNA vaccines and these reactions is currently classed as unclear internationally in view of the low reporting rate, the low background incidence of the disease and the clinical complexity of the reported cases.

In any case, healthcare professionals should consider this tentative diagnosis when symptoms that are compatible with a myocarditis/pericarditis, but were not caused by other heart diseases, occur in individuals shortly after a vaccination.

Suspected cases should be reported to Swissmedic with the corresponding investigation results. The positive benefit-risk profile for the mRNA vaccines used in Switzerland is not currently affected by these reports.

Swissmedic will continue to monitor this issue closely in cooperation with the regional pharmacovigilance centres while also sharing information with foreign authorities. It will provide further information or introduce risk minimisation measures without delay if any new aspects come to light.