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Following over 300 cases of what the Centers for Disease Control (CDC) defines as myocarditis, the FDA has decided to place a warning about rare cases of heart inflammation on its fact sheets for both the Pfizer and the Moderna Covid-19 vaccines.

The warnings are specifically addressed to adolescents and young adults, as it is primarily among these groups (between the ages of 12 and 39) that the increased risk has been documented, mainly among males.

The CDC now estimates the risk at 19.8 cases per million within three weeks of the second dose of Moderna’s regimen, and 8 cases per million for Pfizer’s regimen, with symptoms typically emerging within a week of vaccination.

In the wake of the announcement, Moderna shares closed down 4.2%, while Pfizer fell 1.4%.

Dr. Tim Shimabukoro of the CDC used data from the Vaccine Adverse Event Reporting System (VAERS) to demonstrate the risks. 323 reports on the VAERS website met the CDC’s working definition for heart inflammation (myocarditis or pericarditis). 309 of them were hospitalized, and nine of them are still in hospital, two of whom are in intensive care. No deaths were described in the data, and 79% of cases have apparently recovered.

Put differently, Dr. Shimabukuro showed, the incidence of heart inflammation among adolescent boys specifically (aged between 12 and 17) was 32 times higher among the vaccinated population (128 cases observed where 4 would otherwise be expected). In young men (aged between 18 and 24), the rate was more than 27 times higher (219 cases where 8 would otherwise have been expected).

Doctors and hospitals have already been warned by the CDC to be on the watch for heart inflammation symptoms, but Dr. Sarah Long of the Advisory Committee on Immunization Practices (ACIP) stressed that the post-vaccine myocarditis doctors are seeing presents very differently from what is usually found among teenagers.

"Traditional myocarditis is not [multi-inflammatory syndrome in children], and it's not this," she said, noting the more rapid onset of symptoms that are more severe in nature, but tend to resolve more quickly (when they do). The FDA is now warning anyone experiencing symptoms to “seek medical attention” and notes that information on long-term effects due to the condition is limited.

No deaths were noted in the CDC’s data, but the author of a letter published by the BMJ last month and based on official government data from the MHRA (Britain’s equivalent of the FDA) noted that data from “up to 2nd June 2021” included “118 deaths recorded with AZ [AstraZeneca’s vaccine] and 63 for Pfizer in the broader category of cardiovascular events.”

For cases specifically defined as either myocarditis or pericarditis, only two deaths were recorded, the former with AZ’s vaccine and the latter following Pfizer’s vaccine. In addition, 79 fatal myocardial infarctions were reported following Covid-19 vaccination (55 linked to AZ and 24 linked to Pfizer).

In Israel, an investigative report submitted to the Health Ministry noted a myocarditis rate of approximately 1 in 4500 among young people aged between 16 and 24. Most cases were mild and resolved within a few weeks, but at least one fatality has been confirmed: 22-year-old Shirel Halel, who passed away shortly after receiving the second dose of Pfizer’s vaccine.