Miller Fisher syndrome; This is a literature report for the following literature source(s): "Miller Fisher syndrome after Pfizer BioNTech vaccine booster responsive to intravenous Ig treatment".  A woman in her 70s received the Pfizer BioNTech (BNT162b2) COVID-19 vaccine booster. Two weeks later, she developed progressive gait imbalance, diplopia and headache without antecedent constitutional or infectious symptoms. Her medical history was significant for hypertension and breast cancer in remission. On presentation, the patient was hypertensive, with blood pressure fluctuations between 133/61 mmHg and 191/79 mmHg over the 2 days after admission, perhaps reflecting a degree of autonomic dysfunction. The initial examination demonstrated partial global ophthalmoparesis, bilateral ptosis, slowed saccades in restricted range, minimally responsive pupils with a degree of light near dissociation, bilateral limb ataxia with preserved reflexes, and preserved sensation to light touch, pinprick, vibration, proprioception and temperature in all four limbs. Lumbar puncture showed 0 nucleated cells, glucose of 87 mg/dL and protein of 29 mg/dL. Bacterial and fungal cultures were negative. MRI of the brain with and without contrast showed no abnormalities. Nerve conduction study (NCS) and electromyography showed acute left facial neuropathy, reduced fibular and tibial motor amplitudes, and a mildly prolonged ulnar F wave relative to the F estimate, but no additional abnormalities and no electrophysiological evidence of a neuromuscular junction disorder. A 5-day course of intravenous Ig 0.4 g/kg resulted in symptom improvement. Serum GQ1b IgG antibodies titres were elevated at 1:12800 (normal <1:100), supporting the diagnosis of Miller Fisher syndrome (MFS). As for other serum labs, the patient had unremarkable comprehensive metabolic panel, complete blood count with differential, sedimentation rate, C reactive protein, rapid plasma reagin, aquaporin-4- IgG, myelin oligodendrocyte glycoprotein antibody and myasthenia gravis panel. At the 2-month follow-up visit, the patient had further improvement in symptoms. She continued to have mild ophthalmoplegia and mild ataxia but was able to ambulate independently and complete her activities of daily living. MFS is a subtype of Guillain-Barre Syndrome (GBS), a group of immune-mediated acute neuropathies. MFS can present after infections or following vaccinations. MFS had been reported in patients after COVID-19 infection and following the first and second doses of the Pfizer vaccine. There also had been multiple cases of GBS reported in patients after receiving COVID-19 vaccination. A causal link cannot be established with the very low number of cases compared with the number of vaccines administered, but the timing is supportive. The classic triad of MFS is ophthalmoplegia, ataxia and areflexia; however, many patients present with incomplete forms. In authors' case, the patient had ophthalmoplegia and bilateral ataxia with preserved reflexes. In conclusion, authors describe a case of MFS after the Pfizer COVID-19 vaccine booster. Patient's perspective: "This is a timeline of events that led to my Miller Fisher diagnosis. A. Received Pfizer COVID-19 vaccines: 1. First dose (date hidden) 2. Second dose (date hidden) B. Booster (date hidden) (3 weeks after the booster vaccine) I noticed my pupils were dilated and walking was difficult and getting worse each day. I decided to go to the local hospital emergency room, and I was admitted into the hospital that day. The hospital ran a series of tests on me to determine the cause of my condition. Meanwhile they were in contact with (hospital name hidden) because they were baffled by my symptoms. Unable to determine what was causing my symptoms, I was transferred to the (hospital name hidden). The physicians there determined that I had Miller Fisher. The next day I began 5 days of intravenous Immunoglobulin (IVIG). As of today, (date hidden), my eyes aren't normal, but they are better. My legs are weaker than before but seem to be getting stronger."; Sender's Comments: Based on the available information in the case, the causal association between the events and the suspect drug BNT162B2 cannot be excluded. The impact of this report on the benefit/risk profile of the Pfizer product is evaluated as part of Pfizer procedures for safety evaluation, including the review and analysis of aggregate data for adverse events. Any safety concern identified as part of this review, as well as any appropriate action in response, will be promptly notified to regulatory authorities, Ethics Committees, and Investigators, as appropriate.