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Use of mRNA COVID-19 Vaccine After Reports of Myocarditis Among Vaccine Recipients: Update from the Advisory Committee on Immunization Practices – United States, June 2021

Our take —

The Advisory Committee on Immunization Practices (ACIP) issued two reports that the benefits of mRNA vaccination against COVID-19 far outweigh the potential risks of myocarditis for all ages and sexes. Even among males 18-29 years old, who had the highest reported prevalence of myocarditis (24.3 cases per million second doses of vaccines administered vs. 3.5 cases/million second doses administered overall), the individual-level risk-benefit favored vaccination. Specific to this age/sex group, one million second vaccine doses could prevent up to 11,000 cases of COVID-19 compared to ~45 cases of myocarditis. These reports used data provided to United States’ Vaccine Adverse Event Reporting System (VAERS) between December 2020 and June 2021. During this time, 1,226 people received an mRNA vaccine and reported myocarditis within three days of their shot. Of these, 58% were among people < 30 years and two thirds occurred among males.  ACIP selected a subset of 484 young people for an assessment by a CDC physician, who found that only 67% met the criteria for myocarditis.  This would imply that the prevalence of myocarditis was overestimated. However, the prevalence may have also been underestimated since VAERS relies on passive ascertainment, i.e., voluntary submission from the public who often decide not to report adverse reactions like myocarditis to the CDC. Regardless, to support timely treatment of myocarditis, ACIP recommends increased awareness of vaccine-associated myocarditis among both providers and potential vaccine recipients. This analysis did not consider other benefits that COVID vaccines may confer besides preventing severe disease, such as preventing mild cases that progress to long COVID. As the pandemic evolves, continuous updates to these risk-benefit estimations will be required to account for new viral variants and potential changes in vaccine effectiveness.

Study design

Case Series

Study population and setting

This is a summary of two reports: (1) “Use of mRNA COVID-19 Vaccine After Reports of Myocarditis Among Vaccine Recipients” by Gargano et al. and (2) “Use of COVID-19 Vaccines After Reports of Adverse Events Among Adult Recipients of Janssen and mRNA COVID-19 Vaccines” by Rosenblum et al. These reports from the Advisory Committee on Immunization Practices (ACIP) review the risks of myocardial or pericardial inflammation (myocarditis) after mRNA COVID-19 vaccines in the United States, particularly among adolescents and adults below the age of 30 years. The reports also review the risk/benefit ratio from immunization across different age and gender groups.

Summary of Main Findings

A total of 296 million doses of mRNA COVID-19 vaccines was given as of June 11, 2021 in the United States.  Fifty-two million doses (30 million first doses and 22 million second doses) were given to individuals between the ages of 12-29 years old.  Between December 29, 2020 and June 11, 2021, 1,226 cases of myocarditis associated with mRNA vaccines were reported through the Vaccine Adverse Event Reporting System (VAERS).  The median age of cases was 26 years and the median time from vaccination to symptoms was 3 days. About 58% of the cases occurred in individuals below the age of 30 years (76% males).  Since the majority of myocarditis cases were among children, adolescents and young adults, sample of 484 cases reported between May 1-June 2021 and occurring in people below the age of 30 years was selected for review by CDC physicians. Of these, 67% (median age, 19 years old; median time from vaccination to symptom onset, 2 days) met CDC criteria for myocarditis and were reviewed further.

A benefit-risk ratio for mRNA COVID-19 vaccines across different age and sex groups was calculated by assuming a 95% vaccine effectiveness in preventing severe COVID-19, hospitalization, and death over a period of 3 months, based on the rate of cases during the week of June 13-19, 2021, the rate of hospitalization during the week of June 19, 2021, and the rate of ICU admission or deaths attributed to COVID-19. COVID-19-related myocarditis, defined as myocarditis that occurred within 7 days of the 2nd dose of an mRNA COVID-19 vaccine, represented the risk. Men between the ages of 18-29 years had the highest reported prevalence at 24.3 cases of myocarditis per million mRNA COVID-19 2nd doses administered (compared to 3.5 cases/million overall).  However, even among this relatively high risk for myocarditis subgroup, 1 million second doses of COVID-19 vaccine could prevent 9600 cases of COVID-19, 300 hospitalizations, 60 ICU admissions, and 3 deaths.  The benefit is significantly greater among all other subgroups who are at lower risk for myocarditis, e.g. men over 30 years of age in whom 1 million second doses of COVID-19 vaccine could prevent 15,300 cases of COVID-19, 4598 hospitalizations, 1242 ICU admissions, and 700 deaths compared to risk of 3-4 expected myocarditis cases.  This suggests a highly favorable benefit to risk ratio even among those who have the highest risk for myocarditis due to mRNA vaccines, including males between the ages of 12-29 years of age.

Furthermore, at the population level, the reports conclude that vaccinating adolescents and young adults is critical to ensuring their return to normal pre-pandemic social and educational activities, while also limiting spread within the community and the emergence of new virus strains.  Excluding young adults and adolescents from vaccination could disproportionally increase viral spread among populations particularly at risk for COVID-19 including racial and ethnic minority groups, exacerbating the widespread disparity in COVID-19 severity and death among these subgroups.

Study Strengths

A sample of cases of myocarditis reported nationally and occurring among those younger than 30 years of age were actively reviewed and validated by CDC physicians to determine the prevalence, severity and the short-term outcomes of this adverse event.  Also, risk benefit analysis was conducted across different age and sex subgroups.


The report does not consider new strains of SARS-CoV-2, which could affect some of the assumptions that were used to determine vaccine effectiveness in reducing rates of cases, hospitalization, or death (benefit). Relying on passive ascertainment of COVID-19-related myocarditis (VAERS) may underestimate the number of COVID-19-related myocarditis.

Value added

The paper addresses the safety concern of mRNA vaccine among adolescents and young adults in the United States and provides quantitative benefit/risk ratio analyses with further subgroup analysis by age and sex.

This review was posted on: 8 October 2021