In May 2021, the CDC issued a statement regarding a possible risk for myocarditis linked to both the Pfizer and Moderna mRNA COVID-19 vaccines. This risk was recently assessed in a large retrospective cohort study conducted in Israel. Using the database from the main national health care organization, the investigators evaluated the electronic health records of all persons aged 16 years and older who received at least one dose of the Pfizer BNT162b2 vaccine between December 2020 and May 2021. Each case of myocarditis occurring within 42 days after the first dose of vaccine was adjudicated based on criteria from the CDC.

The cohort included more than 2.5 million persons with a median age of 44 years, 94 percent of whom received two doses of the vaccine. Overall, 54 patients had myocarditis, resulting in an estimated incidence of 2.13 cases per 100,000 persons. The majority (76 percent) of cases of myocarditis were classified as being of mild severity, whereas 22 percent were of intermediate severity and two percent were fulminant. Male patients had a strikingly higher risk of developing myocarditis, with an incidence of 4.12 cases per 100,000 persons, compared with only 0.23 cases per 100,000 female patients. Moreover, the risk was significantly increased in adolescents and young men, with an incidence of 10.7 per 100,000 persons 16 to 29 years old, compared to 2.11 per 100,000 men 30 years and older. The incidence of myocarditis among women was also age-dependent, although to a lesser extent, with 0.34 cases per 100,000 persons 16 to 29 years old and 0.2 cases per 100,000 women 30 years and older.

At the heart of it, the risk of myocarditis from the COVID vaccine appears lower than the risk of myocarditis from COVID-19, which is one more reason to get vaccinated.

Regarding the clinical presentation, 83 percent of the 54 patients who developed myocarditis had no coexisting medical conditions. In 38 patients with available electrocardiogram data, 53 percent exhibited ST-segment elevation, 26 percent had minor abnormalities and 21 percent had normal findings. Among 48 patients with echocardiographic findings, 14 had left ventricular dysfunction which persisted in 10 patients at the time of discharge. Sixty-five percent of affected patients were discharged from the hospital without any ongoing medical treatment. After a median follow-up of 83 days after the onset of the disease, one patient had to be readmitted and one patient with preexisting cardiac disease died from an unspecified cause.

This large cohort study estimates the incidence of myocarditis after receiving the Pfizer COVID-19 mRNA vaccine at 2.13 cases per 100,000 persons. By way of comparison, the risk of developing myocarditis from COVID-19 infection in otherwise healthy individuals may be as high as three percent. This study is not without limitations, mostly based on study design and use of medical records for diagnosis. Moreover, cases may have been missed due to a lack of specific data or if the diagnosis was made in an out-of-network hospital and was therefore unreported. At the heart of it, the risk of myocarditis from the COVID vaccine appears lower than the risk of myocarditis from COVID-19, which is one more reason to get vaccinated.

For more information, see the topic COVID-19 (Novel coronavirus) in DynaMed®.