To investigate the association between influenza vaccination and the risk of developing myopathy in the United States.

Influenza vaccinations may be associated with adverse neurological events. Therefore, their safety must be continually monitored.

The Vaccine Adverse Event Reporting System database from 1990-2018 was reviewed. Cases of myopathy, myositis, and rhabdomyolysis were identified and characterized by their temporal relationship with influenza vaccination. Case-centered analysis using a logistic regression model was utilized. The initial 6 weeks after vaccination were defined as the risk period for possible cause-effect between vaccination and myopathy.

224 cases of myopathy (mean age 41.87 years, 67.86% female) following influenza vaccination with a known onset time were identified. There was an unbalanced distribution of post-vaccination myopathy cases: 219 patients (97.78%) developed myopathy within 6 weeks of vaccination, of which 204 (93.15%) developed myopathy within 2 weeks. 2 patients (0.89%) developed myopathy between 6 to 10 weeks, and 3 (1.34%) developed myopathy after 10+ weeks post-vaccination. 62 patients (27.68%) reported muscle weakness, 112 (50.00%) reported muscle pain, 98 (43.75%) visited the ER or doctor’s office, 66 (29.46%) were hospitalized, and 18 (8.04%) reported disability. No deaths occurred. The annual reporting rate of myopathy after vaccination was approximately 0.861 cases per 10 million vaccinations given, lower than the occurrence rate in the general population. Case-centered analysis found that an increased relative risk of myopathy in the 42 days following vaccination (relative risk: 5.210, 95% confidence interval: 3.975-6.964).

This study reveals that there is an increased risk of developing myopathy within the first 6 weeks following influenza vaccination than outside of this period. Though the incidence of myopathy post-vaccination is not increased as compared to the general population, the unbalanced distribution of cases within the 6 weeks following vaccination suggests the need for careful monitoring of recently vaccinated patients for myopathy.