This case was reported in a literature article and described the occurrence of bilateral optic neuritis in a 6-year-old female patient who received Influenza vaccine Quadrivalent unspecified season. On an unknown date, the patient received Influenza vaccine Quadrivalent unspecified season (intramuscular). On an unknown date, 7 days after receiving Influenza vaccine Quadrivalent unspecified season, the patient experienced bilateral optic neuritis (serious criteria GSK medically significant), loss of vision (serious criteria GSK medically significant), optic nerve edema (serious criteria GSK medically significant), retinal bleeding (serious criteria GSK medically significant) and visual acuity lost. The patient was treated with prednisolone and methylprednisolone. On an unknown date, the outcome of the bilateral optic neuritis, loss of vision, optic nerve edema, retinal bleeding and visual acuity lost were recovering/resolving. The reporter considered the bilateral optic neuritis, loss of vision, optic nerve edema, retinal bleeding and visual acuity lost to be related to Influenza vaccine Quadrivalent unspecified season. Additional information was provided. This case was reported in a literature article and described the occurrence of reversible bilateral optic neuritis in a 6-year-old female patient who was vaccinated with unspecified quadrivalent influenza vaccine (manufacturer unknown). The patient was previously healthy. The patient's identical twin received the unspecified quadrivalent influenza vaccination without subsequent vision change. No information on patient's concurrent condition or concomitant medication was provided. On an unspecified date, the patient received unspecified quadrivalent influenza vaccine intramuscularly (administration site; dosages unknown; batch number not provided). Age of vaccination was not reported. On an unspecified date, 1 week after vaccination, the patient developed bilateral vision loss. At presentation, visual acuities were hand motion in the right eye and counting fingers in the left. Ophthalmoscopy revealed optic nerve swelling with vessel obscuration bilaterally and peripapillary haemorrhages on the left. Magnetic resonance imaging with intravenous (IV) contrast showed severe thickening and abnormal enhancement of both optic nerves but no white matter lesions in the brain or spinal cord. Cerebrospinal fluid analysis was unremarkable and negative for oligoclonal bands and Lyme antibodies. Serologic testing was negative for sarcoidosis, Lyme, and aquaporin 4-antibodies. After five days of IV methylprednisolone, visual acuities were 20/20 and colour vision (Ishihara) was 10/10 in each eye. Six days after her last dose of IV steroids, the patient developed vision loss to no light perception in the left eye with a relative afferent pupillary defect and left optic disc swelling. Oral prednisolone (2 mg/kg/day) was restarted and vision improved to 20/20 within one week. The author considered the event of reversible bilateral optic neuritis related to unspecified quadrivalent influenza vaccine. The author concluded, "The influenza vaccine is a rare (10 cases per one million vaccinations), yet significant, cause of bilateral optic neuritis. Vision loss, typically to hand motion or worse, occurs one to four weeks after vaccination and responds to steroid therapy. To the best of our knowledge, the present case is the only report of post-vaccination optic neuritis with a genetically-identical case-matched control in current literature."