SERIOUSNESS CRITERIA: IMPORTANT MEDICAL EVENT. Initial unsolicited report received from literature on 30 May 2018. This case was linked to 2018SA150554, 2018SA150652, 2018SA150690, 2018SA150703, 2018SA150719, 2018SA150760, 2018SA150738, 2018SA150628, 2018SA150659, 2018SA150674, 2018SA150695, 2018SA150724 and 2018SA150746 (same literature). Abstract: Background: It is unknown if vaccines cause non-specific immune activation in patients with neuromyelitis optica spectrum disorder and no consensus on the use of vaccines exists for this population. We investigated the temporal association of vaccinations with relapses in patients with neuromyelitis optica spectrum disorder. Methods: This is a multi-center retrospective analysis of patients with neuromyelitis optica spectrum disorder for whom immunization history and clinical records from disease onset were available. Ninety patients who met 2015 diagnostic criteria received a total of 211 vaccinations and experienced 340 relapses over a median disease course of 6.6 years. The likelihood of a relapse occurring within 30, 60, and 90 days of a vaccine was compared to the likelihood of a relapse occurring within each time point of a randomly generated date. We also compared the relapse rate between patients who received any vaccination(s) after disease onset to those who did not. Results: We identified seven patients with neuromyelitis optica spectrum disorder who relapsed within 30 days of a vaccination, six between 31 and 60 days, and four who relapsed between 61 and 90 days. The rate of vaccine-associated relapses within 30, 60, and 90 days was significantly higher than the likelihood of a relapse spontaneously occurring within each of the given time frames (p=0.034, 0.01, 0.016, respectively) among patients who were not on preventive immunotherapy only. Among those who were on immunotherapy to prevent relapses, there was no significant association of relapse with vaccines. Additionally, among patients on immunotherapy, the annualized relapse rate of those who received routine vaccinations was significantly lower than in unvaccinated patients. Conclusion: The evidence suggests that there may be a risk of vaccination-associated relapses among untreated neuromyelitis optica spectrum disorder patients, however immunosuppressive therapy at time of vaccine may abort the risk; this suggests that the patients who are treated with preventive immune suppression and receive routine vaccinations for common infections may fare better. Further prospective studies are necessary to verify these findings. This case involves patient (age, gender not reported) who was vaccinated with a dose of Tdap (Batch number, expiration date, dose, route and site of administration was not reported) on 24 DEC 2012. The patient had a medical history of neuromyelitis optica spectrum disorder (ongoing). Concomitant medications was not reported. On an unspecified date, 26 days following the vaccination, the patient experienced myelitis. It was reported that, the background immunosuppression was none. Lab data includes MRI (result unknown), aquaporin-4 antibody (AQP-4) serostatus negative and Relapse severity was characterized by net change in the Expanded Disability Status Scale (EDSS) from baseline to nadir was 5.5. Corrective treatment was not reported. The outcome of events was not reported. Upon internal review, the company decided to consider the event serious because of important medical event myelitis. List of documents held by sender: none. Sender's Comments: This case concerns a patient of unknown age with ongoing Neuromyelitis Optica Spectrum Disorder who presented with myelitis 26 days after vaccination with DIPHTHERIA, TETANUS AND ACELLULAR PERTUSSIS VACCINE (unknown manufacturer). The time to onset is compatible. Patient with NOSD are likely to develop recurrent myelitis. Other possible causes of relapse in such patients, including old age, immune status, hormonal imbalance and others need to be ruled out before a causal association can be made between the vaccine and the event. Additionally, patient was not on any background immunosuppression therapy which increases the risk of relapse. Based upon the reported information, the role of the vaccine cannot be assessed.