Massive blood clots in both legs/blood clots in her right leg; platelet count is very low; This spontaneous case was reported by a pharmacist and describes the occurrence of THROMBOSIS (Massive blood clots in both legs/blood clots in her right leg) and PLATELET COUNT DECREASED (platelet count is very low) in a 74-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 026L20A and 002A21A) for COVID-19 vaccination.     The patient's past medical history included Radiation therapy in 2020, Chemotherapy in 2020 and Surgery (Over a year ago the patient had surgery for brain cancer) in 2020. Concurrent medical conditions included Brain tumor (Over a year ago the patient had surgery for brain cancer) since 2020.   On 02-Feb-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 02-Mar-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On an unknown date, the patient experienced THROMBOSIS (Massive blood clots in both legs/blood clots in her right leg) (seriousness criteria hospitalization and medically significant) and PLATELET COUNT DECREASED (platelet count is very low) (seriousness criterion medically significant). The patient was treated with HEPARIN at a dose of 1 dosage form; LEVOFLOXACIN (LOVENOX [LEVOFLOXACIN]) ongoing since an unknown date at a dose of 1 dosage form; FUROSEMIDE (LASIX [FUROSEMIDE]) ongoing since an unknown date at a dose of 40 mg twice a day and Surgery (IVC filter insertion) for Thrombosis. At the time of the report, THROMBOSIS (Massive blood clots in both legs/blood clots in her right leg) and PLATELET COUNT DECREASED (platelet count is very low) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Platelet count: very low (Low) The patient's platelet count is very low., abnormal (abnormal) 4 weeks ago the patient had a platelet count equal to 19,000. and abnormal (abnormal) 2 weeks ago the patient had a platelet count of 12,000..     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Concomitant product use was not provided by the reporter.  The patient suffered blood clots in her right leg starting in the summer of 2021. The patient went to the hospital and received Heparin and the patient was discharged on Lovenox. The clots spread to the left leg.  The patient had to stop taking Heparin because her platelet count was dropping. The caller did not have the exact dates of the Platelet lab values. The patient was on Heparin and the Heparin was discontinued because of a low platelet count. The patient had an IVC filter inserted in her leg in August 2021.  The patient was on Lasix 40mg twice daily and wears pressure stockings. Company comment: This case concerns a 74-year-old, female patient, with reported medical history of chemotherapy and radiotherapy due to brain cancer, who experienced the serious unexpected AESIs of Thrombosis and Platelet count decreased.  The events occurred on an unknown date after the administration of the first dose of the mRNA-1273 Vaccine.  The outcome of the events was reported as unknown.  It has been reported that the patient had an IVC filter inserted in her leg in August 2021 and was treated with Heparin, which had to be discontinued after her platelet count started to decrease.  Treatment medications included Levofloxacin and Furosemide.  No diagnostic tests performed are available on the report.  The rechallenge is unknown since no further information has been disclosed.  Reported medical history of chemotherapy and radiotherapy remains as a confounder for the events.   The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.  Conservatively and until further information is received the events have been captured as occurring after the first dose of the 1273 vaccine.; Sender's Comments: This case concerns a 74-year-old, female patient, with reported medical history of chemotherapy and radiotherapy due to brain cancer, who experienced the serious unexpected AESIs of Thrombosis and Platelet count decreased.  The events occurred on an unknown date after the administration of the first dose of the mRNA-1273 Vaccine.  The outcome of the events was reported as unknown.  It has been reported that the patient had an IVC filter inserted in her leg in August 2021 and was treated with Heparin, which had to be discontinued after her platelet count started to decrease.  Treatment medications included Levofloxacin and Furosemide.  No diagnostic tests performed are available on the report.  The rechallenge is unknown since no further information has been disclosed.  Reported medical history of chemotherapy and radiotherapy remains as a confounder for the events.   The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.  Conservatively and until further information is received the events have been captured as occurring after the first dose of the 1273 vaccine.