similar symptoms as his Myasthenia Gravis/It may have made the Myasthenia Gravis symptoms to come back; sore left arm injection site; This spontaneous case was reported by a patient and describes the occurrence of MYASTHENIC SYNDROME (similar symptoms as his Myasthenia Gravis/It may have made the Myasthenia Gravis symptoms to come back) in an 82-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 013L20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     Concurrent medical conditions included Myasthenia gravis (under control) since 1996.    On 01-Nov-2021, the patient received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 01-Nov-2021,  after starting mRNA-1273 (Moderna COVID-19 Vaccine), the patient experienced VACCINATION SITE PAIN (sore left arm injection site). On 02-Nov-2021, the patient experienced MYASTHENIC SYNDROME (similar symptoms as his Myasthenia Gravis/It may have made the Myasthenia Gravis symptoms to come back) (seriousness criterion medically significant). On 03-Nov-2021, VACCINATION SITE PAIN (sore left arm injection site) had resolved. On 04-Nov-2021, MYASTHENIC SYNDROME (similar symptoms as his Myasthenia Gravis/It may have made the Myasthenia Gravis symptoms to come back) had resolved.      Company comment: This is a spontaneous case, reported by a patient, concerning a 82-year-old male patient with relevant medical condition of Myasthenia gravis, who experienced the unexpected serious (medically significant) adverse event of special interest Myasthenic syndrome, reported as Myasthenia gravis-like symptoms, which occurred 1 day after receiving the third dose of mRNA-1273 vaccine as the first booster dose of COVID-19 vaccine. Patient had also received 2-dose primary series of mRNA-1273 vaccine with adverse event reported as little weakness for a couple of days, given at an interval of 28 days which falls within the recommended vaccine dosing schedule. It was reported that 24 Hours after the first monovalent booster, patient noted muscle weakness, loss of bladder and muscle control. Patient also had a sore left arm injection site. No further details about the diagnostic procedures and treatments were provided. All symptoms last for 2 days and eventually resolved. The medical condition of Myasthenia gravis remains as a confounder for the event Myasthenic syndrome. The benefit-risk relationship of mRNA-1273 is not affected by this report.      No concomitant medications were reported.  Patient had received the First Monovalent Booster dose.  It was reported that 24 Hours after the first monovalent booster, patient had really bad muscle weakness, and lost his bladder control. Patient also had a sore left arm injection site. All symptoms Last for 2 days.  Patient states that the vaccine gave him similar symptoms as his Myasthenia Gravis. It may have made the Myasthenia Gravis symptoms to come back. He called the doctor after recovery, but they did not do anything about it.  No treatment details were reported.  Patient had really bad muscle weakness, lost his bladder control and Lost his muscle control after the first monovalent booster subsume them in the event/diagnosis of Myasthenic syndrome, since there are common symptoms seen in Myasthenic syndrome.   This case was linked to MOD-2021-016707, MOD-2022-667356 (Patient Link).; Sender's Comments: This is a spontaneous case, reported by a patient, concerning a 82-year-old male patient with relevant medical condition of Myasthenia gravis, who experienced the unexpected serious (medically significant) adverse event of special interest Myasthenic syndrome, reported as Myasthenia gravis-like symptoms, which occurred 1 day after receiving the third dose of mRNA-1273 vaccine as the first booster dose of COVID-19 vaccine. Patient had also received 2-dose primary series of mRNA-1273 vaccine with adverse event reported as little weakness for a couple of days, given at an interval of 28 days which falls within the recommended vaccine dosing schedule. It was reported that 24 Hours after the first monovalent booster, patient noted muscle weakness, loss of bladder and muscle control. Patient also had a sore left arm injection site. No further details about the diagnostic procedures and treatments were provided. All symptoms last for 2 days and eventually resolved. The medical condition of Myasthenia gravis remains as a confounder for the event Myasthenic syndrome. The benefit-risk relationship of mRNA-1273 is not affected by this report.