Patient states that, on evening after receiving booster dose of Moderna vaccine, she developed bloody diarrhea. These symptoms persisted for weeks. Pt. presented to emergency department on 11/22/2021 with complaints of GI discomfort, blood in stool, straining, and constipation. She was diagnosed with acute colitis and discharged from the ED with a prescription for antibiotics.  Pt presented to her outpatient provider on 11/30 with the same chief complaint. Diagnosis of colitis remained unchanged, and patient was prescribed a new 10-day course of antibiotics.  On 12/7, patient presented to the emergency department again with worsening symptoms of colitis, diarrhea, blood in stool, and GI discomfort. She was subsequently admitted for the next 24 days for major surgery with significant co-morbid complications, including:  Hypercoagulable state (new) Ischemic colitis Cerebrovascular infarction, embolic (new) New cardiomyopathy, +troponin/NSTEMI- Acute blood loss anemia PUD/gastritis/duodenitis Hypervolemia Marked hypoalbuminemia Ascites Dysphagia Severe malnutrition AKI, resolved Deconditioning  On 12/14, the patient underwent a near total colectomy with ileostomy due to new-onset ischemic colitis. Pathological analysis was conducted by providers from both Medical Center and the Health System. This reporting is below:                                                                      Diagnosis:  Terminal ileum, cecum with appendix, and colon, subtotal colectomy: Segmental active colitis with multifocal ulceration, focal pseudomembrane formation, and areas of ischemic-type mucosal injury. Associated prominent and extensive microvascular thrombosis.  See comment. Comment: Sections demonstrate contiguous involvement of the colon by a process of active colitis involving both lamina propria and crypts, areas of exudative luminal inflammation, and submucosal abscesses. While some changes of chronicity are evident, this is less than would typically be present in an inflammatory bowel disease.  The most striking feature of the process is the presence of extensive microvascular thrombosis involving both larger vessels in the submucosa and extending into small vessels in the mucosa.  In a few sections, entire vascular trees appear to be involved.  These areas are associated with ischemic-type mucosal injury.  Sections of lymph nodes also demonstrate focal microvascular thrombosis.  Because this is a very unusual colitis, the case is discussed with the attending hospitalist caring for the patient who indicates that there is an unusual clinical setting as well. Because of this, the case will be forwarded to the University for consultative opinion.  When available, this will be reported in an addendum.  Procedures/Addenda: Addendum  Date Ordered:12/22/2021 Status:Signed Out   Date Complete:12/22/2021   Date Reported:12/23/2021 15:56  Addendum Diagnosis Following examination of the glass slides at Medical Center, this case was forwarded in consultation to Laboratories, whose report reads:  "DIAGNOSIS: Colon, subtotal colectomy (12/14/2021):  Severely active chronic colitis resembling ulcerative colitis with extensive venous thrombi affecting distal colon.  Terminal ileum and proximal colon spared.  Sixteen benign lymph nodes with scattered thrombi.  See comment.  The colonic mucosa demonstrates changes resembling ulcerative colitis.  This is characterized by diffuse involvement of mainly the distal aspect of the colon by inflammation, villous elongation with crypt architectural distortion, lamina propria lymphoplasmacytosis, and crypt abscesses.  There are some areas of marked hemorrhage; otherwise usual features of ischemia, such as withered crypts, lamina propria hemorrhage, followed by fibrosis, are not present.  Moreover, ischemia typically does not have so much active inflammation.  What is unusual is the striking presence of transmural organizing venous thrombi. Ulcerative colitis does have an association with hypercoagulability and I have seen ulcerative colitis cases with thrombi, but I cannot exclude the possibility of thrombi associated with the vaccine, which also appears to have isolated reports of hypercoagulability as a complication.  Finally, we have encountered gastrointestinal thrombi as a systemic complication of COVID-19 itself. As such, the possibilities in this case include hypercoagulability related to the chronic colitis, the vaccine, or, if the patient actually had COVID-19 during this time, SARS-CoV-2.  I shared this unusual case with Dr. as well as with my other GI pathology colleagues at our daily GI pathology consensus conference.  They agree that this case has features identical to ulcerative colitis and the etiology for the findings, including the thrombi, is unclear but still may be related to the vaccine. According to the clinical information that you kindly provided, this is a 70-year-old female with a history of acute abdominal symptoms beginning after her Moderna COVID-19 booster vaccination on 11/8/2021.   The patient had progressive diarrhea with blood/mucous and was admitted on 11/22.  CT scan at that time showed diverticulosis and diffuse moderate circumferential thickening in the transverse and descending colon.  She was placed on a course of antibiotics and discharged, which resulted in no improvement.  She was readmitted on 12/7 and CT scan showed progressive thickening of the colon from ileocecal valve through the sigmoid, etiologies included infectious versus inflammatory versus ischemic although the mesenteric arteries and veins were widely patent.  Stool studies including an ova and parasite exam and C. difficile were negative and her WBC count was within normal range.  She was found to be in a hypercoagulable state and a MRI of the head on 12/7 showed multiple infarctions secondary to emboli.  On 12/9, she underwent colonoscopy and biopsies of the terminal ileum, right colon, transverse, and left colon suggested ischemic colitis so she was started on steroids.  Prior colonoscopy in 2017 had showed diverticulosis.  She subsequently suffered an embolic stroke of the left middle cerebral artery on 12/11.  On 12/14, a colectomy was performed and the surgeon noted obvious hyperemia and induration with palpable thickening involving the colon from hepatic flexure through the rectosigmoid junction with sparing of the cecum, proximal ascending colon, and proximal rectum."