2-3 days ago, his arm started swelling a little bit/later on, the swelling started, It became two sizes bigger than the other arm; Arm hurted,still hurting, still hurting; couldn't lift his arm,couldn't raise his arm up,couldn't lift anything heavy with that arm; Couldn't lift anything heavy with that arm; blood clot; This spontaneous case was reported by a consumer and describes the occurrence of THROMBOSIS (blood clot), VACCINATION SITE SWELLING (2-3 days ago, his arm started swelling a little bit/later on, the swelling started, It became two sizes bigger than the other arm) and PAIN IN EXTREMITY (Arm hurted,still hurting, still hurting) in an 18-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 051C21A and 022D21A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     No Medical History information was reported.   On 12-May-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 09-Jun-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. In September 2021, the patient experienced THROMBOSIS (blood clot) (seriousness criterion medically significant). On an unknown date, the patient experienced VACCINATION SITE SWELLING (2-3 days ago, his arm started swelling a little bit/later on, the swelling started, It became two sizes bigger than the other arm) (seriousness criterion medically significant), PAIN IN EXTREMITY (Arm hurted,still hurting, still hurting) (seriousness criterion medically significant), MOBILITY DECREASED (couldn't lift his arm,couldn't raise his arm up,couldn't lift anything heavy with that arm) and MUSCULAR WEAKNESS (Couldn't lift anything heavy with that arm). The patient was treated with RIVAROXABAN (XARELTO) for Adverse event, at an unspecified dose and frequency. At the time of the report, THROMBOSIS (blood clot), VACCINATION SITE SWELLING (2-3 days ago, his arm started swelling a little bit/later on, the swelling started, It became two sizes bigger than the other arm), PAIN IN EXTREMITY (Arm hurted,still hurting, still hurting), MOBILITY DECREASED (couldn't lift his arm,couldn't raise his arm up,couldn't lift anything heavy with that arm) and MUSCULAR WEAKNESS (Couldn't lift anything heavy with that arm) outcome was unknown.            No relevant concomitant medication information provided.  Patient visited hospital in month of September because of swelling.  Company Comment: This case concerns an 18-year-old male patient with no relevant medical history, who experienced the unexpected event of Thrombosis. The event occurred approximately 4 months after receiving the second dose of mRNA-1273 (Moderna COVID-19 Vaccine). The rechallenge was not applicable since only information about the second dose was disclosed. The benefit-risk relationship of mRNA-1273 (Moderna COVID-19 Vaccine) is not affected by this report.; Sender's Comments: This case concerns an 18-year-old male patient with no relevant medical history, who experienced the unexpected event of Thrombosis. The event occurred approximately 4 months after receiving the second dose of mRNA-1273 (Moderna COVID-19 Vaccine). The rechallenge was not applicable since only information about the second dose was disclosed. The benefit-risk relationship of mRNA-1273 (Moderna COVID-19 Vaccine) is not affected by this report.