COVID-19 Vaccine Safety and Efficacy

Myocarditis Risk

The American College of Cardiology notes that myocarditis risk is higher with mRNA vaccines, particularly after the second dose and in young males aged 12-29 years 1, 2
The risk appears higher with the mRNA-1273 (Moderna) vaccine compared to BNT162b2 (Pfizer-BioNTech) 2
A small but significant risk was also observed after the first dose of the adenoviral vector vaccine ChAdOx1 (Oxford/AstraZeneca), but not after the second dose 1
Most cases of vaccine-associated myocarditis are mild and self-limiting 3

For every 1 million males aged 12-29 years receiving a second dose of COVID-19 mRNA vaccine, 39-47 cases of myocarditis would be expected, while 560 hospitalizations, 138 ICU admissions, and 6 deaths would be prevented 1, 3, 4
The overall incidence of adverse cardiovascular effects in clinical trials has been low (<0.05%), with no reported cases of myocarditis in the initial trials 4
Rates of hypertension, bradycardia, atrial fibrillation, acute coronary syndrome, cerebrovascular events, and heart failure have been similar between vaccine and placebo arms 4

In patients with cancer, COVID-19 vaccination significantly reduces the risk of hospitalization and death within 30 days (odds ratio 0.44) 5
Most adverse events after COVID-19 vaccination in cancer patients are mild to moderate (grade 1 or 2), with the most common side effects being injection site pain, fatigue, myalgia, headache, and fever 5

The exact mechanism is not fully understood, but hypothesized mechanisms include molecular mimicry between the spike protein and self-antigens, autoantibody formation, triggered immune dysregulation with activation of natural killer cells, and dysregulated cytokine and immune response to mRNA 1, 2, 4
Male predominance may be related to sex hormone differences in immune response 2

Individuals presenting with chest pain early after receiving COVID-19 mRNA vaccines should be evaluated for possible myocarditis 3
Initial testing should include ECG, cardiac troponin measurement, and echocardiogram 3
If myocarditis is suspected, cardiology consultation and further testing with cardiac MRI should be performed 3