In May 2021, Israeli health officials reported a possible link between the Pfizer COVID-19 vaccine and myocarditis after reporting that 62 cases had been reported following vaccination. Of these cases, 56 had occurred following the second shot, and most had involved persons 30 years of age and younger. A Pfizer spokesperson, however, stated that a causal link had not been established and that they had not observed a higher rate of myocarditis post-vaccination than what would have been expected in the overall population.
On May 17, 2021, the ACIP’s COVID-19 Vaccine Safety Technical (VaST) Work Group met and reviewed information on myocarditis following mRNA vaccines. VaST reported that most of the cases had occurred in teens and young adults, and more cases had occurred in males. Additionally, there were more cases reported after the second vaccine dose, and most occurred on average within four days of vaccination. Members of VaST reported that few cases had been reported but that information on myocarditis following COVID-19 vaccination should be given to vaccine providers.
Reports of myocarditis and pericarditis following mRNA vaccines continued to increase and on June 11, 2021, the CDC scheduled an emergency ACIP meeting for June 18, 2021 to discuss the higher-than-expected number of cases. A total of 301 cases following Moderna vaccination and 488 cases following Pfizer vaccination had been reported at the time the meeting was scheduled.
The meeting, however, was postponed due to the newly created Juneteenth National Independence Day holiday, with the CDC announcing that it would discuss concerns the following week at the regularly scheduled June ACIP meeting. By the June 23, 2021 meeting, CDC officials reported that through June 11, 2021, 1,226 cases of myocarditis/pericarditis had been reported to VAERS, with 791 occurring after Pfizer vaccination and 435 after Moderna vaccination. Most cases had been reported in males, and most had occurred following the second dose.
In the data presented during the June 23, 2021 ACIP meeting, the CDC reported that in females between the age of 12 and 17 years, after the second dose, the case rate of myocarditis/ pericarditis was 9.1 per million doses administered. In males 12 to 17 years of age, however, the reported rate after the second vaccine dose was 66.7 per million doses. Cases among females 18 to 24 years old after dose two were reported at 5.5 per million, while after dose two, males of the same age range were affected at a rate of 56.3 per million doses. Most cases of myocarditis/pericarditis resulted in hospitalization, and while most were reported as being resolved, the long-term health outcomes were reported to be unknown.
The CDC, however, declined to pause use or make changes to the vaccine recommendations, as they reported the benefits to vaccination outweighed the risk. Additionally, they stated that persons with a history of myocarditis and pericarditis could still receive an mRNA vaccine and persons who developed pericarditis after the first mRNA vaccine dose could receive the second dose after symptoms resolved. The CDC also advised that individuals who developed myocarditis after the first dose could consider receiving a second dose under certain circumstances. No data to support this recommendation was provided.
A study of myocarditis after mRNA vaccines on members of the military found a higher-than-expected number of cases following vaccination. Additional studies have also associated mRNA vaccines with heart inflammation, with researchers reporting the need for further investigation.
A population-based cohort study published in the British Medical Journal in December 2021 found that the Moderna COVID-19 vaccine was four times more likely to cause heart inflammation than the Pfizer vaccine. A study published in Nature Medicine in December 2021 reported that men under the age of 40 were more likely to develop myocarditis following Moderna vaccination than following SARS-CoV-2 infection.
In late February 2022, the CDC stated that the interval time between the first and second dose of mRNA vaccine could be extended to eight weeks. The increase in interval time was based on data suggesting that an increase in interval time might decrease the risk of heart inflammation, especially among males 12 through 39 years.
