blood clots; Revaccination with different COVID-19 vaccine; Interchange of vaccine products; This spontaneous case was reported by a patient and describes the occurrence of THROMBOSIS (blood clots) in a  patient of an unknown age and gender who received mRNA-1273 (Spikevax) for COVID-19 prophylaxis. The occurrence of additional non-serious events is detailed below.     Previously administered products included for Product used for unknown indication: Pfizer covid booster, Pfizer covid booster, Pfizer covid booster and Pfizer covid booster. Past adverse reactions to the above products included No adverse event with Pfizer covid booster, Pfizer covid booster, Pfizer covid booster and Pfizer covid booster.    On an unknown date, the patient received dose of mRNA-1273 (Spikevax) (unknown route) 1 dosage form. On an unknown date, the patient experienced THROMBOSIS (blood clots) (seriousness criterion medically significant), COVID-19 IMMUNISATION (Revaccination with different COVID-19 vaccine) and INTERCHANGE OF VACCINE PRODUCTS (Interchange of vaccine products). At the time of the report, THROMBOSIS (blood clots), COVID-19 IMMUNISATION (Revaccination with different COVID-19 vaccine) and INTERCHANGE OF VACCINE PRODUCTS (Interchange of vaccine products) outcome was unknown.        The action taken with mRNA-1273 (Spikevax) (Unknown) was unknown.       No concomitant medications were reported. The patient have had four additional Pfizer covid boosters. It was reported that the patient had blood clots only with Moderna vaccine. No treatment medications were reported.  Company comment-This spontaneous case concerns a patient of unspecified gender and age with no medical history reported, who experienced the unexpected serious (Medically significant) AESI of Thrombosis (reported as (clot blood) on unknown date after unspecified dose of mRNA-1273 vaccination in covid -19 vaccination series. Latency cannot be assessed as event onset date nor vaccination date was provided. Additionally Interchange of vaccine products and Covid-19 immunisation was noted as patient had received 4 doses of TOZINAMERAN on unknown dates. No further information on risk factors, detailed clinical course, investigations, treatment of the events was available in the report. Outcome of the events was not known at the time of report. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.   Reporter did not allow further contact; Sender's Comments: This spontaneous case concerns a patient of unspecified gender and age with no medical history reported, who experienced the unexpected serious (Medically significant) AESI of Thrombosis (reported as (clot blood) on unknown date after unspecified dose of mRNA-1273 vaccination in covid -19 vaccination series. Latency cannot be assessed as event onset date nor vaccination date was provided. Additionally Interchange of vaccine products and Covid-19 immunisation was noted as patient had received 4 doses of TOZINAMERAN on unknown dates. No further information on risk factors, detailed clinical course, investigations, treatment of the events was available in the report. Outcome of the events was not known at the time of report. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.