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Neither U.S. nor European health regulators have yet confirmed a causal link between COVID-19 mRNA vaccines and myocarditis, but they continue to investigate reports of a potential relationship.

In early June, the CDC issued a raising awareness of myocarditis and pericarditis after vaccination, particularly in younger males. That guidance stated that, since April, there's been an increase in reports of myocarditis and pericarditis after getting the Pfizer or Moderna vaccines, but that there hasn't been a similar reporting pattern following the Johnson & Johnson vaccine.

The CDC's "clinical considerations" update followed a from Advisory Committee on Immunization Practices (ACIP)'s COVID-19 Vaccine Safety Technical (VaST) Work Group, which found "a higher number of observed than expected myocarditis/pericarditis cases in 16- to 24-year-olds" in Vaccine Adverse Event Reporting System (VAERS) data within 30 days of dose 2, though it didn't see the same pattern in data from Vaccine Safety Datalink. Nonetheless, "analyses suggest that these data need to be carefully followed as more persons in younger age groups are vaccinated," the work group wrote.

ACIP has scheduled a to present updated data on myocarditis and assess vaccine risks and benefits in adolescents and young adults. That meeting was announced during a Thursday FDA hearing on pediatric COVID vaccines, during which Tom Shimabukuro, MD, MPH, of the CDC's vaccine safety team, provided .

In VAERS data, there were 475 reports of the condition in people age 30 and under, 226 of which have met the CDC's working case definition. Preliminary data suggest that most patients (at least 81%) made a full recovery, Shimabukuro said.

Israeli health regulators that the small number of myocarditis cases seen mainly among men ages 16 to 30 were likely linked to Pfizer's vaccine.

The country saw 275 cases of myocarditis from December 2020 to May 2021 among more than 5 million vaccinated people, . Most of these patients spent no more than 4 days in the hospital, and 95% of cases were classified as mild. The association appeared strongest among men ages 16 to 19 and was more common after the second dose, regulators said.

Pfizer said in a statement to Reuters that no causal link between myocarditis and its vaccine has yet been established.

Israel still with authorizing the vaccine for 12- to 15-year-olds as soon as the myocarditis reports were made public.

In early May, the European Medicines Agency's Pharmacovigilance Risk Assessment Committee (PRAC) said it was among people who received Pfizer's vaccine. It requested data from both Pfizer and Moderna, and it is expected to finalize its review in July.

CDC's healthcare professional guidance notes that cases have occurred mainly in males ages 16 and up, and onset came within the first several days following vaccination, more often after the second dose.

Symptoms of myocarditis and pericarditis include chest pain, shortness of breath, or palpitations. In most cases, patients responded well to medications and rest, and their symptoms improved quickly, CDC said.

The agency has recommended an initial evaluation "considering an ECG, troponin level, and inflammatory markers such as C-reactive protein and erythrocyte sedimentation rate."

It also said that for suspected cases, physicians should consider consulting with cardiology for help with evaluation and management.

CDC has asked physicians to report all potential vaccine-related cases of myocarditis or pericarditis to VAERS. It continues to recommend vaccination for everyone age 12 and older, given the risk of severe illness and complications from infection with SARS-CoV-2.

The American Academy of Pediatrics has also published in its journal Pediatrics a "" series of seven male patients ages 14 to 19 who developed symptomatic myocarditis 2 to 4 days after the second dose of the Pfizer vaccine. All had rapid resolution of their symptoms.

In an , Sean O'Leary, MD, MPH, of the University of Colorado in Aurora, and Yvonne Maldonado, MD, of Stanford University, wrote that while the authors "are quick to point out that a causal relationship between vaccination and myocarditis has not been established, the temporal association of these cases with vaccination as well as the striking similarity in the clinical and laboratory presentations raise the possibility for such a relationship."

O'Leary and Maldonado acknowledged that the case series is limited by the potential for reporting bias and the fact that cases "mirror the seasonal prevalence, sex, and age profile of background cases of myocarditis, thereby complicating an assessment of a potential association with SARS-CoV-2 vaccines."

Still, they called for further investigation of a possible causal link, as several factors suggest one, including the consistent timing in development of symptoms, which indicates a uniform biological process; similarities in clinical findings and laboratory characteristics that point to a common etiology; and the fact that these cases occurred when other common respiratory viruses known to be tied to myocarditis weren't circulating widely.

They also emphasized that the benefits of vaccination outweigh risks at this point: "With over 4 million COVID-19 cases diagnosed in children under 18 in the U.S. that resulted in over 15,000 hospitalizations and between 300 and 600 deaths, the benefits of vaccination in this population far exceed the risks of rare adverse events."

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