LOW VITAMIN D LEVEL; FELT VERY SHORT OF BREATH, THINKS IT MAY BE DUE TO BLOOD CLOT; This spontaneous self-report was received from a patient and concerned a 32 year old female. Initial information received on 11-APR-2021 was processed with additional information obtained from telephone follow up with the patient on 15-APR-2021. The patient's weight was 200 pounds and height was 65 inches. The patient's past medical history included COVID-19 infection last year with minimal symptoms and not hospitalized, and concurrent conditions included protein C and protein S deficiencies with "many previous clots" with most recent in 2019, alpha 1 antitrypsin deficiency, low iron, low ferritin, low vitamin D, and contrast dye allergy. The patient was previously treated with warfarin for anticoagulation  for 13 years, and 8 months ago was switched to XARELTO.  The patient received COVID-19 VACCINE AD26.COV2.S (suspension for injection, route of administration, dose, vaccination site, and batch number were not reported) administered on 31-MAR-2021 at 10:30 for prophylactic vaccination. The batch number would be requested in follow up. Concomitant medications included rivaroxaban for anticoagulation.   On 31-MAR-2021 around 19:30, the patient felt very short of breath, had a hard time breathing and "thought it may be due to a blood clot" and felt "like it maybe  getting worse". She saw hematologist on 14-APR-2021, who, at her request, stopped XARELTO effective15-APR-2021, and started a 2 week course of Lovenox, with plan to start warfarin after that. This was not medically indicated, but requested by patient, and physician agreed. Her oxygen saturation at that visit was normal. On 14-APR-2021, laboratory data included: platelet count (NR: not provided) 470 (units unspecified), and vitamin D (NR: not provided) low at 19 (units unspecified); as treatment "mega dose" was ordered to her pharmacy. Her hematologist scheduled a VQ scan for 19-APR-2021 to evaluate for pulmonary embolism. The patient reported the event felt very short of breath, thinks it may be due to blood clot related to COVID-19 VACCINE AD26.COV2.S. The action taken with COVID-19 VACCINE AD26.COV2.S was not applicable. The patient had not recovered from the events. This report was serious (Other Medically Important Condition).; Sender's Comments: V0: This 32 year-old female with medical history significant for obesity (BMI 33), Protein C & S deficiencies with "many previous clots" (most recent in 2019) began to feel very short of breath approximately 9 hours after receiving Janssen Covid-19 vaccination. Other medical history included alpha 1 anti-trypsin deficiency, low ferritin, low vitamin D, allergy to contrast dye, and mild Covid-19 infection last year which did not require hospitalization. Concomitant medications included rivaroxaban which was started 8 months ago; warfarin had been used for 13 years prior to rivaroxaban. She felt her difficulty breathing could be a blood clot and went to see her hematologist 2 weeks after vaccination. Rivaroxaban was stopped at her request, not because it was medically indicated, and enoxaparin was started with a plan to start warfarin 2 weeks later. Platelet count was 470, vitamin D level was low, and oxygen saturation was normal at that time. A VQ scan is scheduled 5 days later since she is allergic to dye. While the cause of the dyspnea has not yet been determined, a new blood clot would most likely be related to the patient's underlying hypercoagulable state and not the vaccine. Based on insufficient information, the causal association of serious shortness of breath to immunization is assessed to be indeterminate per the WHO causality classification for adverse events following immunization.