Paraspinal and prox leg myositis/paraspinal and proximal muscle myositis triggered by Fluad Quadrivalent vaccine; This spontaneous case was received on 28-Feb-2023 by physician (practicing rheumatologist) (reference number: SEQW23-00781) and concerned a 77-year-old, male patient.   The patient's medical history was not reported. The patient was using unspecified concomitant medication.   On an unspecified date in Oct-2022, reported as probably middle of October, the patient was vaccinated with Fluad Quadrivalent (influenza vaccine inact sag 4v; dose 0.5 ml, route of administration, and anatomical location: not reported) for active immunization of persons 65 years of age and older against influenza caused by influenza virus subtypes A and types B contained in the vaccine. The batch number was not reported.  On an unspecified date in Oct-2022, reported as one day after receiving Fluad Quadrivalent, the patient experienced leg weakness that led him to be bedbound and progressed over the following four weeks.  On 15-Nov-2022, the patient was admitted to the hospital. The patient had an extensive workup which showed paraspinal and proximal leg myositis and a muscle biopsy showed non-specific muscle inflammation with differential including toxic, metabolic, ischemic, or drug induced (statin) myositis. The patient, therefore, had an extensive autoimmune workup with antinuclear antibodies (ANA), Anti Jo1 antibody (Ab), anti-hydroxy-methyl-glutaryl-coenzyme A reductase Ab (HMG-co-A) and an extensive myomarker panel for polymyositis, dermatomyositis, and the anti-synthetase syndrome, and all were negative. Interestingly, the patient's blood creatine phosphokinase (CK) was normal, but red blood cell sedimentation rate (ESR) and C-reactive protein (CRP) were elevated. The patient had no infections identified as an etiology. The patient responded to a prednisone taper and had not required immunosuppression.  On 21-Nov-2022, the patient was discharged from the hospital.  On an unspecified date, the patient recovered from the event. The patient was completely functional present at his baseline and had no return of symptoms.  The reporter assessed the event as related to Fluad Quadrivalent. After discussion with neurology, it was assumed that was a paraspinal and proximal muscle myositis triggered by Fluad Quadrivalent vaccine, and as the unspecified concomitant medication did not change at the time of the adverse event (AE), they ruled out the possibility that the AE was caused by another medication besides the Fluad vaccine.  The event was assessed as serious due to seriousness criterion of hospitalization.  Company comment: A 77-year-old patient developed myositis, reportedly one day after receiving the suspect product Fluad Quadrivalent. More information regarding patient's medical history concomitant drugs is needed. Nevertheless, based on compatible temporal relationship, causality for the reported event is assessed as possibly related.; Reporter's Comments: After discussion with neurology, it was assumed that was a paraspinal and proximal muscle myositis triggered by Fluad Quadrivalent vaccine, and as the unspecified concomitant medication did not change at the time of the adverse event (AE), they ruled out the possibility that the AE was caused by another medication besides the Fluad vaccine.; Sender's Comments: A 77-year-old patient developed myositis, reportedly one day after receiving the suspect product Fluad Quadrivalent. More information regarding patient's medical history concomitant drugs is needed. Nevertheless, based on compatible temporal relationship, causality for the reported event is assessed as possibly related.