Association between seasonal influenza vaccines and the increased risk of acute disseminated encephalomyelitis, estimated using the Vaccine Adverse Event Reporting System.

Acute disseminated encephalomyelitis (ADEM) is a rare and immune-mediated inflammatory disorder of the central nervous system (CNS) that can be triggered by infections and vaccinations. To date, only anecdotal case studies have reported the association between ADEM incidence and seasonal influenza vaccines, and multiple studies have found no association. This study aimed to investigate the association between the incidence of ADEM and seasonal influenza vaccines in a real-world setting using data from the United States Vaccine Adverse Event Reporting System (VAERS). Further, propensity score matching and disproportionality analysis was performed by calculating the adjusted reporting odds ratio (ROR) of reported ADEM cases associated with seasonal influenza vaccines using multiple logistic regression. Additionally, we analysed the time-to-onset using Weibull shape parameters (WSPs). The VAERS database contained 390,352 adverse events reported from January 2011 to December 2020. The ROR of seasonal influenza vaccines for ADEM was 3.02 (95% confidence interval: 1.72-5.33). The median duration (interquartile range) of ADEM was 11.0 (5.0-33.0) days. The median duration of ADEM induced by egg culture-based influenza vaccine (Egg-based vaccine) and cell culture-based influenza vaccine (Cell-based vaccine) was 10.0 (5.0-24.0) and 91.0 (79.0-125.0) days (P < 0.001), respectively. Only Cell-based cases had WSP β > 1, indicating a wear-out failure type. The incidence of ADEM within 30 days after administration of egg- and Cell-based vaccines was 78.6% and 0.0%, respectively. Our findings indicate that ADEM incidence is associated with seasonal influenza vaccines; thus, careful monitoring of ADEM is required within the first month of Egg-based vaccination and after two months of Cell-based vaccination. Neurologists and general practitioners should exercise caution, as the timing for careful monitoring varies depending on the vaccine type.