This spontaneous case was reported by a consumer and describes the occurrence of INFLAMMATORY CARCINOMA OF THE BREAST (Inflammatory Breast Cancer/Severe swelling of her breast) in a female patient of an unknown age who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 prophylaxis. No Medical History information was reported. In February 2021, the patient 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 received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On an unknown date, the patient experienced INFLAMMATORY CARCINOMA OF THE BREAST (Inflammatory Breast Cancer/Severe swelling of her breast) (seriousness criterion medically significant). At the time of the report, INFLAMMATORY CARCINOMA OF THE BREAST (Inflammatory Breast Cancer/Severe swelling of her breast) outcome was unknown. DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Biopsy: Inflammatory Breast Cancer which showed a triple-positive result (oestrogen-receptor-positive, progesterone-receptor-positive and HER2-positive ('ER/PR/HER2') in her lymph node). No concomitant medications were reported. Second dose administered in early February 2021. She suffered a severe swelling of her breast which a course of antibiotics did not clear up. This case was identified as part of a retrospective clean-up activity where certain emails received in the Moderna mailbox were missed to be booked-in. Company Comment: This spontaneous case concerns a female patient, of unknown age, with no medical history reported, who experienced the unexpected, serious (medically significant) event of inflammatory carcinoma of the breast. The event occurred unknown number of days after receiving the second dose of the mRNA-1273 vaccine. Of note, the vaccination date was reported as early Feb2021 and the event onset date was not provided, hence, the onset latency could not be assessed. It was reported that soon after vaccination, the patient experienced severe swelling of her breast (not specified which side). A course of unspecified antibiotics afforded no relief of symptoms. On an unknown date, the patient underwent biopsy and the result confirmed Inflammatory Breast cancer which showed a triple-positive result (estrogen receptor-positive, progesterone receptor-positive and HER2-positive [ER/PR/HER2]) in her lymph node. Treatment information was not provided. The patient's gender remains a confounder since females are at an increased risk of Inflammatory Breast cancer. The benefit-risk relationship of the mRNA-1273 vaccine is not affected by this report. Sender's Comments: This spontaneous case concerns a female patient, of unknown age, with no medical history reported, who experienced the unexpected, serious (medically significant) event of inflammatory carcinoma of the breast. The event occurred unknown number of days after receiving the second dose of the mRNA-1273 vaccine. Of note, the vaccination date was reported as early Feb2021 and the event onset date was not provided, hence, the onset latency could not be assessed. It was reported that soon after vaccination, the patient experienced severe swelling of her breast (not specified which side). A course of unspecified antibiotics afforded no relief of symptoms. On an unknown date, the patient underwent biopsy and the result confirmed Inflammatory Breast cancer which showed a triple-positive result (estrogen receptor-positive, progesterone receptor-positive and HER2-positive [ER/PR/HER2]) in her lymph node. Treatment information was not provided. The patient's gender remains a confounder since females are at an increased risk of Inflammatory Breast cancer. The benefit-risk relationship of the mRNA-1273 vaccine is not affected by this report.