Myasthenia Gravis Presenting after Administration of the mRNA-1273 Vaccine

The mRNA-1273 SARS-CoV-2 vaccine received emergency use authorization in December 2021. We present a case of myasthenia gravis (MG) which became clinically apparent following vaccination against SARS-CoV-2. A 30-year-old man developed acute onset diplopia, 2 days after receiving his first mRNA-1273 vaccination against SARS-CoV-2. He reported blurred vision with horizontally displaced images, which worsened with increased eye strain. Diplopia resolved when one eye was covered. He also had fatigable arm weakness, but denied dysphagia, dysarthria, dysphonia or dyspnoea. On examination, he had left-sided ptosis and esotropia at rest which worsened with sustained upward gaze and prolonged focus. He also had fatigable weakness of neck flexion and extension (4+/5), and generalized, fatigable weakness (4/5). His single-breath count was 38. Cranial nerves, sensory examination and deep tendon reflexes were normal. A 2-min ice-pack test and neostigmine test temporarily improved his diplopia and ptosis. The acetylcholine receptor (AChR) antibody was borderline high and muscle-specific tyrosine kinase (MuSK) antibody was negative. Chest CT and brain MRI with contrast were unremarkable. The patient was diagnosed with MG and oral pyridostigmine and prednisone therapy were initiated. We present a case of newly diagnosed MG after administration of mRNA-1273 vaccination against SARS-CoV-2. Although there has been long-standing discussion regarding the potential for vaccines to exacerbate autoimmune conditions, data remain sparse and consensus has not been reached. Consequently, this case is important to make providers aware of potential side effects of a novel vaccine, and may also help guide the selection of vaccination candidates and monitoring parameters. LEARNING POINTS: We present a case of newly diagnosed myasthenia gravis after administration of the mRNA-1273 SARS-CoV-2 vaccine. mRNA-1273 vaccination against SARS-CoV-2 may exacerbate subclinical cases of myasthenia gravis. Recognition of new vaccine side effects may guide the selection of vulnerable patients.