In a national U.S. Veteran population–based report, researchers compiled data for all adults who were administered one or more COVID-19 vaccine(s) in the Veterans Health Association (VHA) between December 14, 2020, and October 9, 2022. Additionally, using an immunization database, COVID-19 vaccines administered at retail pharmacies outside of the Veterans' Affairs were also recorded in this system using vaccine product codes and vaccination dates.

The researchers noted that a team of pharmacists and expert clinicians conducted in-depth chart reviews and case validation for all potential cases of myocarditis/pericarditis identified in the database analysis.

The primary outcome was the incidence rate of confirmed myocarditis/pericarditis among vaccine recipients (both overall and among those aged 18-39 years) within 21 days of either a first, second, or booster dose of an mRNA COVID-19 vaccine. The researchers also calculated the ratio of observed events among COVID-19 vaccine recipients over projected events from historic vaccine recipient controls (2015-2020) in the VHA and employed confirmed cases to calculate incidence rates and 95% CIs.

Incidence rates were calculated by dividing the number of confirmed cases by the total number of vaccine doses administered during the corresponding time intervals matching the chart reviews.

The results revealed that across the VHA, through October 9, 2022, 8,098,850 doses of mRNA COVID-19 vaccines were administered as first or second doses and 2,168,721 booster doses were administered. Among all doses, the rapid cycle analysis detected 178 potential cases of myocarditis/pericarditis among those vaccinated of any age and 22 potential cases among those aged 18 to 39 years. Of these, 33 cases, including six cases among those aged 18 to 39 years, were confirmed after in-depth chart review and validation. Moreover, the results indicated that patients with confirmed cases began experiencing symptoms an average of 3 days after receiving a vaccine, and all symptoms resolved within 30 days. Additionally, individuals with confirmed cases were hospitalized for an average of 4.1 days (range 1-15), and the final disposition for 32/33 (97%) was discharge to home.

Based on their findings, the researchers indicated that the incidence of myocarditis/pericarditis among veterans was low across more than 10 million doses, including an estimated 2.2 million booster doses of mRNA COVID-19 vaccines as a primary or booster dose administered at the VHA.

The authors wrote, "We found that the incidence rate of confirmed cases were low and generally consistent, but slightly less, than previous reports or studies."

The authors concluded, "Although the rapid cycle analysis initially identified 178 potential cases of myocarditis/pericarditis, only 1 out of 5 cases were confirmed to be related to a COVID-19 vaccine after chart review. These findings highlight the paramount importance of both active surveillance and chart validation for rare but serious adverse events related to COVID-19 vaccines."

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