Vaccines and the Risk of Hospitalization for Multiple Sclerosis Flare-Ups

IMPORTANCE Scientific literature is sparse about the association of vaccination with the onset of multiple sclerosis (MS) flare-ups. Immunization by vaccines of the entire population is crucially important for public health. OBJECTIVE To evaluate the risk of hospitalization for severe MS flare-ups after vaccination in patients with MS. DESIGN, SETTING, PARTICIPANTS This cohort study included patients diagnosed with MS between January 1, 2007, and December 31, 2017, who were included in the System of National Health Databases, a national health claims database in France. In a nested case-crossover analysis, cases were defined by vaccine exposure prior to the onset of hospitalization due to an MS flare-up, and flare-up rates were compared with those that occurred prior to vaccine exposure in up to 4 control time windows immediately preceding the at-risk time window (ie, the MS flare-up) for each patient. Data were analyzed from January 2022 to December 2022. EXPOSURE Receipt of at least 1 vaccination, including the diphtheria, tetanus, poliomyelitis, pertussis, or Haemophilus influenzae (DTPPHi) vaccine, influenza vaccine, and pneumococcal vaccine, during follow-up. MAIN OUTCOMES AND MEASURES The primary outcome was the risk of hospitalization for an MS flare-up after receipt of a vaccine. Adjusted odds ratios (AORs) and 95% CIs were derived using conditional logistic regression to measure the risk of hospitalization for an MS flare-up associated with vaccination. RESULTS A total of 106 523 patients constituted the MS cohort (mean [SD] age, 43.9 [13.8] years; 76 471 females [71.8%]; 33 864 patients [31.8%] had incident MS and 72 659 patients [68.2%] had prevalent MS) and were followed up for a mean (SD) of 8.8 (3.1) years. Of these patients, 35 265 (33.1%) were hospitalized for MS flare-ups during the follow-up period for a total of 54 036 MS-related hospitalizations. The AORs of hospitalization for an MS flare-up and vaccine exposure in the 60 days prior to the flare-up were 1.00 (95% CI, 0.92-1.09) for all vaccines, 0.95 (95% CI, 0.82-1.11) for the DTPPHi, 0.98 (95% CI, 0.88-1.09) for the influenza vaccine, and 1.20 (95% CI, 0.94-1.55) for the pneumococcal vaccine. CONCLUSIONS AND RELEVANCE A nationwide study of the French population found no association between vaccination and the risk of hospitalization due to MS flare-ups. However, considering the number of vaccine subtypes available, further studies are needed to confirm these results.