A 15-year-old female patient presented to our paediatric emergency department with difficulty swallowing, followed by generalized weakness and respiratory distress, 36 hours after receiving the first dose of the BNT162b2 mRNA COVID-19 (Pfizer-BioNTech) vaccine. Her physical examination was remarkable for dysphagia, respiratory distress, and tachypnoea. She had normal deep tendon reflexes; her gag reflex was positive, but she had difficulty speaking. Her body temperature was 36.8 dereceC, blood pressure 116/72 mmHg, and pulse rate 103/minute. She had difficulty with swallowing and mastication. Her eye movements were normal, and there was no ptosis. Twenty-four hours after admission to the paediatric ward, her symptoms, most notably respiratory distress, worsened; thus, she was transferred to the paediatric intensive care unit. She was intubated shortly thereafter. A repeat neurological examination revealed positive DTRs and no ophthalmological abnormalities, including ptosis. Her blood tests (serum biochemistry, creatine kinase, CRP, sedimentation rate, free T4, and TSH) were normal. None of her smear or blood PCR tests, including the COVID-19 PCR, was positive. An electromyogram (EMG) performed using the polyneuropathy protocol revealed no abnormalities. A repetitive stimulation test produced a decremental response. After sending the anti-AChR and anti-MuSK antibody tests, she was started on intravenous immunoglobulin (0.4 mg/kg/dayx5 days), pyridostigmine, and methylprednisolone at a dose of 30 mg/kg/day for 3 days. The patient's complaints rapidly regressed after the initiation of treatment; she was extubated at the 18th hour and transferred to the regular ward 60 hours later. All of her symptoms resolved by the 72nd hour of therapy. The anti-AChR antibody level was 2.6 nmol/L (<0.25 nmol/L), while anti-MuSK antibodies were negative. Azathioprine treatment was added to the regimen, and steroid therapy was planned to continue for 6 months. However, her complaints recurred after tapering the steroid dose. As her symptoms did not abate despite azathioprine and pyridostigmine treatments, she underwent thymectomy via the video-assisted thoracoscopic surgery (VATS) technique. Examination of the pathology specimen revealed micronodular stage 1 thymoma (Fig. 1). She did not experience recurrent attacks during a 1-year follow-up after thymectomy.