cytokine release syndrome in a patient who received the Pfizer/BNT vaccine; This is a Literature-Spontaneous report from Journal of Nature Medicine, 2021; entitled "Cytokine release syndrome in a patient with colorectal cancer after vaccination with BNT162b2."   Patients with cancer are currently prioritized in coronavirus disease 2019 (COVID-19) vaccination programs globally, which includes administration of mRNA vaccines. Cytokine release syndrome (CRS) has not been reported with mRNA vaccines and is an extremely rare immune-related adverse event of immune checkpoint inhibitors. We present a case of CRS that occurred 5 d after vaccination with BTN162b2 (tozinameran)-the Pfizer-BioNTech mRNA COVID-19 vaccine-in a patient with colorectal cancer on long-standing anti-PD-1 monotherapy. The CRS was evidenced by raised inflammatory markers, thrombocytopenia, elevated cytokine levels (IFN-y/IL-2R/IL-18/IL-16/IL-10) and steroid responsiveness. The close temporal association of vaccination and diagnosis of CRS in this case suggests that CRS was a vaccine-related adverse event; with anti-PD1 blockade as a potential contributor. Overall, further prospective pharmacovigillence data are needed in patients with cancer, but the benefit-risk profile remains strongly in favor of COVID-19 vaccination in this population.  CRS/cytokine storm is a systemic inflammatory response, characterized by excessive cytokine release (that is, elevated INF-y, IL-6, IL-10 and IL-2R) #. CRS might develop after infection (including COVID-19) or due to iatrogenic causes, most notably chimeric antigen receptor T cell (CAR-T) therapy and, less frequently, cytotoxic chemotherapy or stem cell transplantation#. Extremely rarely, it occurs after immune checkpoint inhibitor (ICI) therapy#, and, to our knowledge, it has not been reported after administration of any vaccine.   Here the authors report a case of CRS after vaccination with BNT162b2 (tozinameran), an mRNA COVID-19 vaccine.  A 58-year-old male commenced anti-PD-1 monotherapy (an investigational ICI within an ongoing interventional clinical trial; NCT02715284)) in Feb2019 for the treatment of mismatch repair-deficient colorectal cancer (MMRd CRC) metastatic to mesentry and rectus muscle. Two months after treatment initiation, he experienced a neurological immune-related adverse event (irAE) with worsening ataxia (grade 1 to grade 2, and magnetic resonance imaging changes in pons, medulla and cerebellum) on the background of pre-existing spinocerebellar ataxia of unknown etiology. ICI was suspended, and he was commenced on 1 mg kg-1 prednisolone (tapered over 1 month), and ataxia returned to grade 1 (baseline). Anti-PD-1 therapy was re-started in Jun2019, with stable disease as per immune-related Response Evaluation Criteria in Solid Tumors. In Mar2020 (13 months after commencing ICI), he developed an endocrine irAE (grade 1 hypocortisolemia from adrenocorticotropic hormone deficiency) and was commenced on physiological corticosteroid replacement (prednisolone, 3 mg daily). Disease control was maintained, and the last ICI dose was administered in 27Dec2020 d before BNT162b2. The patient had no history of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and had negative SARS-CoV-2 serological tests in Jun2020 and Oct2020. He received the first dose of BNT162b2 vaccine on 29Dec2020 without immediate adverse events, except for grade 1 inflammation at the vaccination site. Five days later (32 d after the last anti-PD-1 dose), he presented with myalgia, 2-d history of diarrhea (grade 1) and 1-d history of fever (38.4 degree C) despite anti-pyretics (ibuprofen) use. On admission to the hospital, his vital signs were as follows: oxygen saturation, 100% on room air; respiratory rate, 18 breaths per minute; blood pressure, 111/71 mmHg; heart rate, 86 beats per minute (Normal); and temperature, 36.7 degree C.  Laboratory investigations revealed elevated inflammatory markers (C-reactive protein (CRP)), 125 mg L-1 (normal, <6 mg L-1); serum lactate dehydrogenase (LDH), 184 U L-1 (normal range, 120-246 U L-1); and thrombocytopenia (68 x 109 cells per liter (normal range, 150-410 cells per liter)), confirmed on microscopy. Empirical treatment with broad-spectrum intravenous antibiotics was commenced; however, blood and urine cultures were negative, as was SARS-CoV-2 RT-PCR of serial nasopharyngeal swabs. There were no clinical signs or symptoms during admission or follow-up to suspect a thrombotic event in this patient. Computed tomography of thorax, abdomen and pelvis revealed no nidus of infection or thrombosis and showed stable disease with respect to cancer. The patient was not heparinized. Over the next 5 d, fevers up to 39.8 degree C continued, with worsening thrombocytopenia (28 x 109 cells per liter) and increasing inflammatory markers (CRP, 317 mg L-1; LDH, 849 U L-1), including significantly elevated ferritin (6,010 ug L-1 (normal range, 18-464 ug L-1)). At this point (5 d after admission), CRS was suspected (grade 3), and he was commenced on 1 mg kg-1 of intravenous methylprednisolone (IVMP), and antibiotics were ceased 3 d later. Biochemical and hematological indices normalized within 7 d of IVMP initiation, and the patient was afebrile and asymptomatic upon discharge home with a weaning corticosteroid regimen. He remained well and was re-challenged with anti-PD-1 on 8Feb2021 (36 d after initial presentation) without any adverse events. He did not receive the second dose of BNT162b2. Other lab data included MCP-1: Elevated (Test: Macrophage, Results: increased).   No follow-up attempts are possible; information about lot/batch number cannot be obtained.; Sender's Comments: Based on the compatible temporal association, there was a reasonable possibility that the vaccination with BNT162B2 played a contributory role in triggering the onset of the reported cytokine release syndrome.  The impact of this report on the benefit/risk profile of the Pfizer product is evaluated as part of Pfizer procedures for safety evaluation, including the review and analysis of aggregate data for adverse events. Any safety concern identified as part of this review, as well as any appropriate action in response, will be promptly notified to Regulatory Authorities, Ethics Committees and Investigators, as appropriate.