The Incidence of Myocarditis Following an Influenza Vaccination: A Population-Based Observational Study.

Background And Objective: Recently, studies have pointed to a link between coronavirus disease 2019 vaccinations and myocarditis. Myocarditis following an influenza vaccine has been sporadically reported. However, it is not known whether this adverse event occurs among elderly individuals who have received influenza vaccines. We used a population-based database and a self-controlled case-series design to estimate the incidence of myocarditis following an influenza vaccination.

Methods: Data were extracted from Taiwan's National Health Insurance Research Database. The study population consisted of elderly people aged ≥ 65 years who had de novo myocarditis, which required hospitalization, within 6 months after receiving an influenza vaccination between 2003 and 2017. The first 1-7, 1-14, and 1-42 days after vaccination were defined as risk intervals, and the other periods were defined as control intervals. Poisson regression was used to calculate the incidence rate ratio for myocarditis between the risk and control periods.

Results: Within 180 days following a vaccination, 191 people were hospitalized for myocarditis among 19,678,904 people. In comparison with control intervals, the incidence rate ratios of an admission for myocarditis for days 1-7, 1-14, and 1-42 were 0.80 (95% confidence interval 0.36-1.81), 0.72 (95% confidence interval 0.39-1.32), and 0.73 (95% confidence interval 0.50-1.05), respectively. Subgroup analyses by sex, age, Charlson Comorbidity Index scores, and comorbidities did not yield significant differences in the incidence rate ratio.

Conclusions: Regardless of the post-vaccination time and underlying baseline characteristics, the incidence risk of myocarditis is not significantly increased in the elderly following an influenza vaccination.