Increasing access to allotransplants in the United States: the impact of race, geography, and socioeconomics

Allogeneic hematopoietic cell transplantation (HCT) is particularly susceptible to racial, socioeconomic, and geographic disparities in access and outcomes given its specialized nature and its availability in select centers in the United States. Nearly all patients who need HCT have a potential donor in the current era, but racial minority populations are less likely to have an optimal donor and often rely on alternative donor sources. Furthermore, prevalent health care disparity factors are further accentuated and can be barriers to access and referral to a transplant center. Research has primarily focused on defining and quantifying a variety of social determinants of health and their association with access to allogeneic HCT, with a focus on race/ethnicity and socioeconomic status. However, research on interventions is lacking and is an urgent unmet need. We discuss the role of racial, socioeconomic, and geographic disparities in access to allogeneic HCT, along with policy changes to address and mitigate them and opportunities for future research.

High dose valacyclovir for cytomegalovirus prophylaxis following allogeneic hematopoietic cell transplantation

Introduction Cytomegalovirus (CMV) is one of the most common and clinically significant viral infections following allogeneic hematopoietic cell transplantation (HCT). Currently available options for CMV prophylaxis and treatment present challenges related to side effects and cost. Methods In this retrospective medical record review, the incidence of clinically significant CMV infection (CMV disease or reactivation requiring preemptive treatment) following allogeneic HCT was compared in patients receiving valacyclovir 1 g three times daily versus acyclovir 400 mg every 12 h for viral prophylaxis. Results Forty-five patients who received valacyclovir were matched based on propensity scoring to 35 patients who received acyclovir. All patients received reduced-intensity conditioning regimens containing anti-thymocyte globulin. Clinically significant CMV infection by day + 180 was lower in the valacyclovir group compared to the acyclovir group (18% vs. 57%, p = 0.0004). Patients receiving valacyclovir prophylaxis also had less severe infection evidenced by a reduction in CMV disease, lower peak CMV titers, delayed CMV reactivation, and less secondary neutropenia. Conclusion Prospective evaluation of valacyclovir 1 g three times daily for viral prophylaxis following allogeneic HCT is warranted. Due to valacyclovir's favorable toxicity profile and affordable cost, it has the potential to benefit patients on a broad scale as an option for CMV prophylaxis.

Safety of Live-Attenuated Measles, Mumps, and Rubella Vaccine Administered Within 2 Years of Hematopoietic Cell Transplant

Background Measles, mumps, and rubella (MMR) vaccine is a live-attenuated vaccine usually contraindicated within the first 2 years of hematopoietic cell transplant (HCT). The objective of this study was to assess the safety of MMR vaccine when administered within 2 years of HCT. Methods We conducted a retrospective review of patients who received MMR vaccination within 2 years of an autologous or allogeneic HCT, mostly in the context of the 2019 measles outbreak. Adverse reactions were collected for 42 days postvaccination, and all hospitalizations and deaths following vaccination were reviewed. Results A total of 129 patients (75 autologous and 54 allogeneic HCT) were vaccinated 300–729 days after HCT (median, 718 days), and 39 (30%) of these were vaccinated earlier than 23 months post-transplant. Ten adverse reactions in 7 patients (5%) were identified within 42 days of vaccination: 6 respiratory tract infections (3 with fever) and 1 rash. The rash was seen in a 37-year-old female who had an allogeneic HCT 542 days before vaccination. She presented with a centrifugal maculopapular rash, confirmed to be caused by the vaccine strain rubella virus. She fully recovered. No other vaccine-associated illness was identified in the cohort after a median follow-up of 676 days. Conclusions MMR vaccine appears to be well tolerated in select HCT recipients when given between 300 and 729 days after transplant. An uncomplicated case of vaccine-associated rubella illness was seen after vaccination. Assessment of potential risks and benefits of MMR vaccination given within 2 years of HCT remains important.

Feasibility and efficacy of salvage allogeneic stem cell transplantation in AML patients relapsing after autologous stem cell transplantation

Autologous hematopoietic cell transplantation (HCT) is suitable for consolidation of favorable-/intermediate-risk AML patients in CR1. However, ~50% of AML patients relapse after autologous HCT, and efficacy of subsequent salvage strategies including allogeneic HCT remains unclear. We studied 123 consecutive patients with newly diagnosed AML undergoing high-dose chemotherapy (HDCT)/autologous HCT in CR1. In relapsing patients afterwards, we analyzed salvage treatments and outcomes focusing particularly on salvage allogeneic HCT. Of 123 patients, 64 (52%) relapsed after autologous HCT. Subsequently, 13 (21%) received palliative therapy, whereas 51 (79%) proceeded to salvage therapy with a curative intent. Of the 47 patients with a curative intent and who did not proceed directly to allogeneic HCT, 23 (49%) achieved CR2 or had ongoing hematologic CR1 despite molecular relapse. Finally, 30 patients (47%) received allogeneic HCT with estimated 3-year leukemia-free and overall survival rates of 33% and 43%. Hematologic remission at allogeneic HCT and lack of acute GvHD had a positive impact on OS and LFS (p < 0.05). Our study suggests that almost 80% of AML patients can undergo salvage therapy following relapse after front-line HDCT/autologous HCT. Allogeneic HCT can provide cure in one third of patients relapsing after front-line HDCT/autologous HCT.

Internal and External Validation of the High-2-Low Model: A Predictive Score for Venous Thromboembolism after Allogeneic Transplant

Introduction: In patients undergoing allogeneic hematopoietic cell transplantation (HCT), venous thromboembolism (VTE) remains a serious complication that lacks validated risk assessment models to guide optimal timing and implementation of thromboprophylaxis. We recently derived the HIGH-2-LOW score that incorporated 7 simple clinical predictors assessed at day 30 post-transplant (Table 1, PMID 33570631). In this present study, we performed validation in two independent datasets. Methods: We selected consecutive patients undergoing first allogeneic HCT from Fred Hutchinson Cancer Research Center (FHCRC, 2015-2019) and MD Anderson Cancer Center (MDACC, 2016-2020). Patients who died, received therapeutic anticoagulation, or did not engraft platelets at day 30 were excluded (Table 2). Day 30 was chosen as the index date because most patients would be transfusion-dependent before that time. We used a combination of ICD9/10 codes and natural language processing (NLP) algorithms to identify possible cases of VTE, followed by confirmation via individual chart review. VTE was defined as radiology-confirmed pulmonary embolism (PE), lower-extremity deep venous thromboembolism (LE-DVT), or catheter-related DVT (CR-DVT). Covariates were captured and weighted according to the original model, except that grade 2-4 was used instead of grade 3-4 GVHD. Discrimination was assessed in each cohort by fitting logistic regression models with VTE and PE/LE-DVT outcomes at 100 days to estimate the c statistic, where a higher c statistic is desirable. Both continuous scores and categorical models were assessed. Kaplan Meier failure curves were plotted to compare the final risk stratification with high- vs. low/intermediate-risk groups. Results: The two cohorts (n=772 in FHCRC, n=1109 in MDACC) had similar characteristics in age, sex, race, weight, disease, and conditioning intensity. Key differences between the two cohorts included a higher number of umbilical cord transplants at FHCRC (vs. haploidentical at MDACC), higher numbers of acute GVHD at FHCRC (due to differences in grading criteria), fewer historical CR-DVT at FHCRC (4.0% vs. 6.8%), and more anticoagulation treatment at day 30 at FHCRC (9.0% vs. 3.5% - excluded). Incident VTE was 2.5% by 100 days and 7.8% by 365 days at FHCRC; incident VTE was 5.4% by 100 days and 9.4% by 365 days at MDACC (Table 2). Incident PE or LE-DVTs were similar in the two cohorts. When treated as a continuous score, every 1-point increase in the HIGH-2-LOW score was associated with odds ratio (OR) of 1.55 for VTE (1.06-2.27, c=0.64) and 2.50 (1.40-4.44, c=0.84) for PE/LE-DVT in the FHCRC cohort. The same increase was associated with OR of 1.93 (1.55-2.39, c=0.64) for VTE and 2.46 (1.61-3.76, c=0.79) for PE/LE-DVT in the MDACC cohort (Table 3). A total of 24% and 19% of patients were classified as high-risk (2+ points), respectively. High vs. low/intermediate-risk was associated with OR of 2.99 (1.20-7.49, c=0.62) for VTE and 19.93 (2.38-166.67, c=0.81) for PE/LE-DVT in the internal validation cohort. High vs. low/intermediate-risk was associated with OR of 4.58 (2.69-7.79, c=0.66) for VTE and 12.05 (3.17-45.81, c=0.77) for PE-LE-DVT in the external validation cohort. The VTE risk stratification separated early and persisted beyond 100 days (Figure 1). Conclusion: Despite differences in HCT and patient characteristics, the HIGH-2-LOW score identified ~20% of allogeneic HCT recipients at high-risk for VTE, particularly that of PE or LE-DVT, in both independent validation cohorts. The lower-than-expected absolute VTE incidence in the FHCRC cohort was likely driven by the increasing use of anticoagulation immediately post-transplant (exclusion criteria); however, the model retained similar OR with modest discrimination in both cohorts. In patients with prior history of PE/LE-DVT off anticoagulation, or those with prolonged admission and at least 1 additional risk factors from the HIGH-2-LOW score (2+ points), VTE prophylaxis should be considered upon platelet engraftment. Figure 1 Figure 1. Disclosures Lee: Kadmon: Research Funding; Syndax: Research Funding; Takeda: Research Funding; Pfizer: Research Funding; Novartis: Other: clinical trials, Research Funding; JANSSEN: Other; Incyte: Research Funding; AstraZeneca: Research Funding; Amgen: Research Funding; National Marrow Donor Program: Membership on an entity's Board of Directors or advisory committees. Shpall: Bayer HealthCare Pharmaceuticals: Honoraria; Novartis: Consultancy; Takeda: Patents & Royalties; Affimed: Patents & Royalties; Magenta: Honoraria; Magenta: Consultancy; Navan: Consultancy; Novartis: Honoraria; Axio: Consultancy; Adaptimmune: Consultancy.

Superior Survival Outcome of Blinatumomab Compared to Mitoxantrone-Etoposide-Cytarabine Salvage for Adult Patients with Relapsed or Refractory B-Cell Precursor Acute Lymphoblastic Leukemia: A Propensity Score-Matched Cohort Analysis

Background: Complete remission (CR) rate and long-term overall survival (OS) is very poor in patients with relapsed or refractory B-cell precursor acute lymphoblastic leukemia (R/R BCP-ALL). Previous clinical trials revealed a good remission rate and a safe bridging role to allogeneic-HCT compared to various standard treatments, and several real-world data have been reported. Aim: Here, we analyzed outcome of blinatumomab versus our conventional intensive chemotherapy by propensity score-matched analysis. Methods: From 2009 to 2020, 197 consecutive R/R BCP-ALL were treated with mitoxantrone-etoposide-cytarabine (MEC, n=113) or blinatumomab (n=84), and allogeneic-HCT was planned in patients with CR. For propensity score-matching, the MEC and blinatumomab cohorts were matched in a 1:1 ratio using 5 criteria of age &lt; 35 years old, short CR duration &lt; 12 months, adverse-risk karyotype including Ph-chromosome at relapse, previous allogeneic-HCT, and first-line salvage or not. We compared CR rate, regiment related mortality, and OS with cumulative incidence of relapse (CIR) and non-relapse mortality (NRM). Results: Matched cohort consisted of 52 patients for each salvage group and matched criteria were the same exactly. Median age was 42 years and 34 (65.4%) patients were &gt; 35 years old. There were 22 (42.3%) primary refractoriness, 9 (17.3%) post-chemotherapy relapse and 21 (40.4%) post-HCT relapse. Among 30 relapsed patients, 19 (63.3%) were early relapse with CR duration less than 12 months. There were 25 (48.1%) with poor-risk karyotype at relapse of which 10 were Philadelphia-chromosome. Extramedullary relapse was observed in 14 (26.9%) and 8 (15.4%) were advanced-line salvage. CR rate was significantly higher in blinatumomab (78.8% vs. 53.8%, p=0.012) and more patients proceeded to allo-HCT (80.8% vs. 46.2%, p&lt;0.001). Regimen-related mortality was significantly higher in MEC (40.4% vs. 1.9%, p&lt;0.001). After median follow-up of 32.5 months (range 6.2 to 131.2), 3-year OS of blinatumomab and MEC was 33.2% and 15.4% (p&lt;0.001), and 3-year NRM was 30.3% and 51.9% (p=0.004), respectively. After CR achievement, CIR was not significantly different (46.9% vs. 60.0%). In multivariate analysis, CR duration &lt; 12 months was related with poor OS with high CIR. MEC regimen showed poor OS with high NRM. High NRM was also related with advanced-line salvage. Conclusion: Our matched cohort analysis clearly showed that blinatumomab is superior to MEC salvage regimen. However, we are observing relapse in up to 50% after blinatumomab and following allo-HCT still shows high NRM. Further combinatorial using novel therapeutics are needed for R/R BCP-ALL. Figure 1 Figure 1. Disclosures Kim: AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; AIMS Biosciense: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; AML-Hub: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Astellas: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BL & H: Research Funding; BMS & Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Boryung Pharm Co.: Consultancy; Daiichi Sankyo: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Handok: Consultancy, Honoraria; LG Chem: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Honoraria; Pintherapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Sanofi Genzyme: Honoraria, Speakers Bureau; SL VaxiGen: Consultancy, Honoraria; VigenCell: Consultancy, Honoraria. Lee: Alexion, AstraZeneca Rare Disease: Honoraria, Membership on an entity's Board of Directors or advisory committees.

COVID-19 in Pediatric Hematopoietic Cell Transplant Recipients: A CIBMTR Study

Background: Adult recipients of hematopoietic cell transplantation (HCT) are at a very high risk of adverse outcomes after COVID-19 (Sharma A, Bhatt NS, et al. Clinical characteristics and outcomes of COVID-19 in haematopoietic stem-cell transplantation recipients: an observational cohort study. The Lancet Haematology. 2021 Mar 1;8(3): e185-93). While children are known to have better outcomes after COVID-19 compared to adults in general, data on risk factors and outcomes of COVID-19 among pediatric recipients of HCT are lacking. Methods: Using the data reported to the Center for International Blood and Marrow Transplant Research (CIBMTR) between March 2020 and May 2021, we describe characteristics, severity, treatment approaches, and outcomes of pediatric HCT recipients who were ≤21 years of age at COVID-19 diagnosis. All diagnoses, donor choice/graft sources, and conditioning regimens were included. Patient, disease, and HCT-related factors were described as frequency for categorical variables and median, range, and interquartile range (IQR) for continuous variables. The probability of overall survival after COVID-19 was calculated using the Kaplan Meier estimator. Additionally, an analysis was performed in the subset of allogeneic HCT COVID-19 cases from the United States (US) to identify risk factors for developing COVID-19. COVID-19 cases were compared with a cohort of all pediatric allogeneic HCT recipients without COVID-19 matched by the transplant center. Impact of hematopoietic cell transplant comorbidity index (HCT-CI), HCT indication, donor type, conditioning intensity, graft vs. host disease (GVHD) prophylaxis, and occurrence of acute and chronic GVHD on development of COVID-19 was examined using Cox proportional hazards model. Hazard ratio (HR) and 95% confidence intervals (CI) were provided. Cumulative incidence of COVID-19 among the US centers reporting at least 1 COVID-19 infection was also calculated, using death from any cause as a competing risk. P value &lt;0.05 was considered statistically significant for the analyses. Results: A total of 167 pediatric HCT recipients (allogeneic, allo: 135 and autologous, auto: 32) met study inclusion criteria. Median age at COVID-19 diagnosis for allo and auto HCT recipients were 15 years (range &lt;1-21y) and 7 years (range 1-21y), respectively. Median time from HCT to COVID-19 diagnosis was 15 months (IQR 7-45) for allo recipients and 16 months (IQR 6-59) for auto HCT recipients. Forty-two percent (42%) of the patients had at least one comorbidity prior to HCT. Thirteen percent (13%) were receiving immunosuppression within six months prior to COVID-19 diagnosis. COVID-19 disease severity was mild in 87% of patients, while 4% of patients had severe disease requiring mechanical ventilation or supplemental oxygen. Only 36 HCT recipients (22%) received any COVID-19 directed therapy. Median follow-up from COVID-19 diagnosis was 53 days (range 1-270) and 37 days (range 1-179) for allo and auto HCT recipients, respectively. The overall probability of survival at 45 days was 95% (95% CI 90-99%) and 90% (95% CI 74-99%) for allo and auto HCT recipients, respectively (Figure 1). Forty-five (45) day survival was lower among recipients transplanted at the transplant centers outside the US [non-US recipients 85% (95% CI 71-95%) versus US recipients 98% (95% CI 93-99%)]. No deaths occurred in patients who had received a transplant between 2000-2013. The primary cause of death was COVID-19 in 54% of patients and primary disease in 38% of patients. In the subset analysis restricted to pediatric allogeneic HCT recipients transplanted at the US centers (n=34), the cumulative incidence of COVID-19 infection was noted to be 1.9% (95% CI 1.2-2.9%) at 6 months post-HCT and increased to 4.7% (95% CI 3.4-6.3%) by 1-year post-HCT. Cox regression analysis showed that compared to HCT-CI score of 0, patients with HCT-CI score of 1-2 were more likely to develop COVID-19 (HR 1.95; 95% CI 1.03-3.69, p=0.042). Underlying diagnosis, donor type, treatment exposures, or GVHD did not predict COVID-19 incidence. Conclusions: This is the largest series to date summarizing the cumulative incidence, risk factors, and outcomes of pediatric HCT recipients with COVID-19. Patients with pre-HCT comorbidities were more likely to develop COVID-19. However, the overall disease severity and mortality after COVID-19 were low in this patient cohort. Figure 1 Figure 1. Disclosures Bhatt: Rite Aid Corporation: Divested equity in a private or publicly-traded company in the past 24 months; Pfizer Inc.: Divested equity in a private or publicly-traded company in the past 24 months; Moderna, Inc.: Divested equity in a private or publicly-traded company in the past 24 months; Johnson & Johnson: Divested equity in a private or publicly-traded company in the past 24 months. Sharma: Medexus Inc: Consultancy; Spotlight Therapeutics: Consultancy; Vindico Medical Education: Honoraria; CRISPR Therapeutics: Other, Research Funding; Novartis: Other: Salary support paid to institution; Vertex Pharmaceuticals/CRISPR Therapeutics: Other: Salary support paid to institution. Riches: ATARA Biotherapeutics: Other: Payment; Jazz Pharmaceuticals: Other: Payment; BioIntelect: Membership on an entity's Board of Directors or advisory committees. Dandoy: Omeros: Other: Consulted and received Honorarium.

Post-Transplantation Cyclophosphamide for Graft-Versus-Host Disease Prophylaxis in Allogeneic Hematopoietic Cell Transplantation for Higher-Risk Myelodysplastic Syndrome

Introduction: Allogeneic hematopoietic cell transplantation is an only potentially curative option for patients with higher-risk myelodysplastic syndrome (MDS). Owing to the advances in treatment strategies including reduced intensity conditioning, graft-versus-host disease (GVHD) prevention and supportive care, more elderly patients or those with comorbidities can proceed to allogeneic HCT. However, the long-term survival rate following allogeneic HCT is reported to be less than 50%, and non-relapse mortality (NRM) rate is still high reaching upto 30%. In this study, we aimed to evaluate the feasibility of using post-transplantation cyclophosphamide (PTCy) as a GVHD prophylaxis in allogeneic HCT for higher-risk MDS patients. We also compared the post-transplantation outcomes of PTCy group and those of historical control who received HCT using anti-thymocyte globulin (ATG). Methods: Patients with higher-risk MDS or MDS/myeloproliferative neoplasm (MPN) with bone marrow blast ≥ 5% were included in this study. Higher-risk MDS was defined by MDS with International Prognostic Scoring System &gt;1.0 or bone marrow blast ≥ 5% at any time points before HCT. Conditioning regimen consists of busulfan (4-days for patients aged &lt;55 years, 2-days for 55 years), fludarabine. For GVHD prophylaxis, PTCy (50 mg/kg on days 3 and 4), cyclosporine, and mycophenolate mofetil were administered. In historical group, patients received 2- or 4-days of busulfan, fludarabine, and ATG with short course of methotrexate and cyclosporine. Results: Ninety-two and 144 patients received allogeneic HCT using PTCy and ATG, respectively. The median overall survival were 47.9 and 44.0 months, respectively (P=.383). Cumulative incidence of total and grade II-IV acute GVHD in PTCy and ATG group were 19.6% vs. 37.5% (P=.002), and 2.6% vs. 21.7% (P&lt;.001), respectively. Incidence of total and extensive chronic GVHD (50.0% vs. 49.1%, P=.567; 32.5% vs. 33.4%, P=.581), 1-year NRM (20.8% vs. 22.9%, P=.702), and 2-year relapse (16.0% vs. 18.1%, P=.605) were not different between two groups. Neutrophil and platelet engraftment was significantly faster with ATG than PTCy (median 12 vs. 15 for neutrophil; median 15 vs. 24 days for platelet). Conclusion: Allogeneic HCT using PTCy as GVHD prophylaxis in higher-risk MDS seems feasible in terms of low rate of acute GVHD and relapse incidence. Disclosures Choi: Ingenium Therapeutics, Daejeon, Korea: Consultancy, Current holder of individual stocks in a privately-held company. Lee: Ingenium Therapeutics, Daejeon, Korea: Consultancy, Current holder of individual stocks in a privately-held company. Lee: Korean Society of Hematology: Membership on an entity's Board of Directors or advisory committees; Astellas Pharma, Inc.: Consultancy, Honoraria, Other: Advisory board; AbbVie: Honoraria, Other: Advisory board.

Assessment of Serology after Sars-Cov-2 Vaccine in Allogeneic HCT Recipients

Introduction: Randomized trials demonstrated ~95% efficacy of SARS-CoV-2 spike messenger RNA (mRNA) vaccines. Patients (pts) after allogeneic hematopoietic cell transplant (HCT) have a variable period of immune deficiency and the impact of diagnosis, treatment regimens, GvHD, and immunosuppression on vaccine (vacc) immunogenicity is unknown. Methods: We performed a retrospective analysis of 149 consecutive pts (Table) who received a SARS-CoV-2 vacc between 12/17/2020, and 5/21/21, and were tested for anti-SARS-CoV-2 S1/S2 antibodies. Serology testing was performed with the Liaison® SARS-CoV-2 S1/S2 IgG assay (DiaSorin) with ≥15 AU/mL defined as a positive result. Pts with prior COVID-19 infection were excluded. Pts received mRNA-1273/Moderna (n= 46), BNT162b2/Pfizer-BioNTech (n= 100) or Jannsen vacc (n= 3). Reactogenicity was not investigated. Demographic and treatment variables were tested for prediction of vaccine response using Chi-square test or Fisher's exact test for categorical variables and two-sample t test for continuous variables. Univariate and multivariable logistic regression analyses with backward selection using Akaike information criterion (AIC) were used to examine interdependence of those variables and odds ratios (OR) with 95% confidence intervals (CI). Results: Pts underwent HCT from a related HLA matched sibling (n=36), related haploidentical (n= 23), or matched/mismatched unrelated donor (n= 89). 93% received fludarabine in the HCT conditioning regimen (data not shown). All pts received a calcineurin inhibitor (CNI) and 76 pts received ATG for GvHD prophylaxis. All pts achieved at least mixed donor lymphoid engraftment (data not shown). Median time from HCT to 1st vaccination was 26 months (range, 3-258 months). Median age at time of vaccination was 61 years (range, 24-78) and 75 (50%) were female. Serology was tested at a median of 37 days (range, 6-119 days) after the second vacc dose. Serology was tested &lt;14 days in 3 pts; all were seropositive. No pt developed COVID-19 during the period of observation. 101 pts (67%) tested positive for anti-SARS-CoV-2 S1/S2 antibodies (vacc responders). Of the responders, the median time from HCT to 1st vacc was 45.5 months (range, 3-258, SD 39.78). Among the 23 pts between 3-9 months after HCT, 26% (n=6) had a positive antibody response, but all were receiving ongoing immunosuppression at time of vaccination. 29% (n=29) vacc responders were receiving prednisone (pred) in the management of cGvHD at the time of vaccination. 48 pts did not mount an antibody response (vacc non-responders). Of the non-responders, 30 pts were receiving cGvHD treatment at the time of vacc, 31 pts were taking pred, and 20 pts were taking CNIs. In univariate analysis, we found a history of prolonged use of pred (&gt;8 weeks) and/or CNIs, on current treatment for cGvHD at time of vacc, and receipt of rituximab in the preceding 12 months predicted for lack of response (Table). Active use of pred and treatment with pred &gt;8 weeks in the preceding 12 months prior to vacc predicted vacc non-response [OR 0.221; 95% CI (0.106 - 0.456); p&lt;0.001] and [OR 0.408; 95% CI (0.197 - 0.844); p=0.016] in univariate analysis, respectively, however, active use of pred was predictive [OR 0.07; 95% CI (0.016-0.304; p&lt;0.001] while pred treatment &gt;8 weeks was not [OR 2.00; 95% CI (0.55-7.298; p=0.293] in multivariable analysis. Other significant predictors for non-response in the multivariable analysis include pt use of ruxolitinib [OR, 0.233, 95% CI, (0.067-0.808); p=0.022], and rituximab within 1 year [OR, 0.026, 95% CI, (0.007-0.099); p&lt;0.001]. Discussion: In this study, we found that 67% allogeneic HCT pts developed anti-SARS-CoV-2 S1/S2 antibodies after SARS-CoV-2 vaccination. Predictors of non-response after adjustment for potential confounders, were factors that are expected to suppress immune response including active use of immunosuppressive medications. Consistent with prior studies, anti-CD20 therapy likely impairs humoral response to vaccination. Ruxolitinib also appears to impair response. However, a proportion of pts being actively treated for cGvHD responded to vaccination and these pts should still be encouraged to receive vaccination in consideration of the COVID-19 mortality risk. Many questions remain including the protective benefit of immune response, the duration of response, and the potential value of booster vaccinations in non-responders. Figure 1 Figure 1. Disclosures Rowley: ReAlta Life Sciences: Consultancy.