First Paediatric Case of mRNA COVID-19 Vaccine Associated New Onset Systemic Myasthenic Crisis.

Myasthenia gravis (MG) is an autoimmune disorder characterized by abnormal neuromuscular conduction. The thymus is believed to play a key role in the pathogenesis of MG; therefore, thymectomy is an important treatment option for the disease. As MG exacerbations and new-onset MG cases following Coronavirus Disease 2019 (COVID-19) vaccinations have been previously reported in the literature, various complications highly related to autoimmunity, such as Guillain-Barré syndrome, have also been described after vaccination. A previously healthy girl developed her first life-threatening systemic MG attack following administration of the messenger RNA (mRNA) BNT162b2 COVID-19 vaccine. Despite receiving treatment targeting MG, her clinical status did not show significant improvement, which led to the decision to perform thymectomy via the video-assisted thoracoscopic surgery (VATS) approach. After the procedure, a significant improvement was observed in her clinical condition. We aimed to contribute to the literature on this rarely encountered condition by reporting the first paediatric case in our country presenting with BNT162b2 vaccination-associated generalized weakness and severe respiratory distress, who was diagnosed with MG and thymoma during follow-up, in the light of contemporary literature. This case also serves as a warning for other mRNA vaccines.