Arthritis/especially arthritis in her knees, where fluid accumulates; Allergy to polysorbate 80; This spontaneous case was received on 05-Oct-2021 from other non-health professional (patient's mother) via Regulatory authority (reference number: SEQW21-03357) and concerned a 15-year-old, female patient.  The patient's relevant concurrent condition included spina bifida. The patient's concomitant medications were not reported. As reported, the patient was involved in many sports.  On an unspecified date in 2009, the patient was vaccinated with H1N1 influenza vaccine (brand not specified; dose, route of administration and anatomical location: not reported) for influenza prophylaxis. The batch number was not reported.  On an unspecified date in 2009, within a week of receiving influenza virus vaccine monovalent, the patient experienced severe arthritis, especially in her knees, where fluid accumulates. After the arthritis developed, the patient had to give up any sports activities. The patient was tested by an allergist and it was determined that she had an allergy to polysorbate 80.   At the time of initial reporting, the patient had not recovered from arthritis.  The outcome of other events was not reported.  The reporter did not provide causality assessment. The patient's rheumatologist believed that the H1N1 vaccine contributed to the patient's arthritis.  The event of 'arthritis' was considered to be medically significant by a Physician within Seqirus' regulatory authority and Risk Management Department.  Company comment: A 15-year-old patient was vaccinated with influenza virus vaccine monovalent. Within a week of receiving vaccine, the patient experienced severe arthritis, especially in her knees, where fluid accumulates. The patient had to give up any sports activities. The patient was tested by an allergist and it was determined that she had an allergy to polysorbate 80. The patient's relevant concurrent condition included spina bifida. For the event of arthritis, causality was highly confounded by patient's medical history, since painful degenerative arthritis of the knee occurred in 24% of adults with spina bifida who walked. The patient's rheumatologist believed that the H1N1 vaccine contributed to the patient's arthritis. Causal role of the suspect vaccine cannot be excluded and is assessed as possibly related, due to biological and chronological plausibility.; Reporter's Comments: The reporter did not provide causality assessment. The patient's rheumatologist believed that the H1N1 vaccine contributed to the patient's arthritis.; Sender's Comments: A 15-year-old patient was vaccinated with influenza virus vaccine monovalent. Within a week of receiving vaccine, the patient experienced severe arthritis, especially in her knees, where fluid accumulates. The patient had to give up any sports activities. The patient was tested by an allergist and it was determined that she had an allergy to polysorbate 80. The patient's relevant concurrent condition included spina bifida. For the event of arthritis, causality was highly confounded by patient's medical history, since painful degenerative arthritis of the knee occurred in 24% of adults with spina bifida who walked. The patient's rheumatologist believed that the H1N1 vaccine contributed to the patient's arthritis. Causal role of the suspect vaccine cannot be excluded and is assessed as possibly related, due to biological and chronological plausibility.