IMPORTANCEThe National COVID Cohort Collaborative (N3C) is a centralized, harmonized, highgranularity electronic health record repository that is the largest, most representative COVID-19 cohort to date. This multicenter data set can support robust evidence-based development of predictive and diagnostic tools and inform clinical care and policy.OBJECTIVES To evaluate COVID-19 severity and risk factors over time and assess the use of machine learning to predict clinical severity. DESIGN, SETTING, AND PARTICIPANTSIn a retrospective cohort study of 1 926 526 US adults with SARS-CoV-2 infection (polymerase chain reaction >99% or antigen <1%) and adult patients without SARS-CoV-2 infection who served as controls from 34 medical centers nationwide between January 1, 2020, and December 7, 2020, patients were stratified using a World Health Organization COVID-19 severity scale and demographic characteristics. Differences between groups over time were evaluated using multivariable logistic regression. Random forest and XGBoost models were used to predict severe clinical course (death, discharge to hospice, invasive ventilatory support, or extracorporeal membrane oxygenation). MAIN OUTCOMES AND MEASURESPatient demographic characteristics and COVID-19 severity using the World Health Organization COVID-19 severity scale and differences between groups over time using multivariable logistic regression. RESULTSThe cohort included 174 568 adults who tested positive for SARS-CoV-2 (mean [SD] age, 44.4 [18.6] years; 53.2% female) and 1 133 848 adult controls who tested negative for SARS-CoV-2 (mean [SD] age, 49.5 [19.2] years; 57.1% female). Of the 174 568 adults with SARS-CoV-2, 32 472(18.6%) were hospitalized, and 6565 (20.2%) of those had a severe clinical course (invasive ventilatory support, extracorporeal membrane oxygenation, death, or discharge to hospice). Of the hospitalized patients, mortality was 11.6% overall and decreased from 16.4% in March to April 2020 to 8.6% in September to October 2020 (P = .002 for monthly trend). Using 64 inputs available on the first hospital day, this study predicted a severe clinical course using random forest and XGBoost models (area under the receiver operating curve = 0.87 for both) that were stable over time. The factor most strongly associated with clinical severity was pH; this result was consistent across machine learning methods. In a separate multivariable logistic regression model built for inference, (continued) Key Points Question In a US data resource large enough to adjust for multiple confounders, what risk factors are associated with COVID-19 severity and severity trajectory over time, and can machine learning models predict clinical severity? Findings In this cohort study of 174 568 adults with SARS-CoV-2, 32 472 (18.6%) were hospitalized and 6565 (20.2%) were severely ill, and first-day machine learning models accurately predicted clinical severity. Mortality was 11.6%

This study investigated the impact of pre-transplant CMV serostatus and post-transplant CMV reactivation and disease on umbilical cord blood transplant (UCBT) outcomes. Between 1994 and 2007, 332 patients with hematologic malignancies underwent UCBT and 54% were CMV seropositive. Pre-transplant recipient CMV serostatus had no impact on acute or chronic GVHD, relapse, DFS or OS. There was a trend toward greater day 100 TRM in CMV seropositive recipients (p=0.07). CMV reactivation occurred in 51% (92/180) of patients with no difference in myeloablative vs. RIC recipients (p=0.33). Similarly, reactivation was not influenced by the number of UCB units transplanted, the degree of HLA disparity, the CD34+ or CD3+ cell dose or donor KIR gene haplotype. Rapid lymphocyte recovery was associated with CMV reactivation (p=0.02). CMV reactivation was not associated with acute (p=0.97) or chronic GVHD (p=0.65), nor did it impact TRM (p=0.88), relapse (p=0.62) or survival (p=0.78). CMV disease occurred in 13.8% of the CMV seropositive patients resulting in higher TRM (p=0.01) and lower OS (p=0.02). Thus, while recipient CMV serostatus and CMV reactivation have little demonstrable impact on UCB transplant outcomes, the development of CMV disease remains a risk, associated with inferior outcomes.

Key Points• UCB recipients have slower T-cell reconstitution but more robust NK and B-cell recovery after allo-HCT than MSD recipients.• Delayed CD4 1 total and naive T-cell reconstitution after allo-HCT increases the risk of infection, mortality, and chronic GVHD.Slow immune reconstitution is a major obstacle to the successful use of allogeneic hematopoietic cell transplantation (allo-HCT). As matched sibling donor (MSD) allo-HCT is regarded as the gold standard, we evaluated the pace of immune reconstitution in 157 adult recipients of reduced-intensity conditioning followed by MSD peripheral blood HCT (n 5 68)and compared these to recipients of umbilical cord blood (UCB; n 5 89). At day 28, UCB recipients had fewer natural killer (NK) cells than MSD recipients, but thereafter, NK cell numbers (and their subsets) were higher in UCB recipients. During the first 6 months to 1 year after transplant, UCB recipients had slower T-cell subset recovery, with lower numbers of CD3 1 , CD8 1 , CD8 1 naive, CD4 1 naive, CD4 1 effector memory T, regulatory T, and CD3 1 CD56 1 T cells than MSD recipients. Notably, B-cell numbers were higher in UCB recipients from day 60 to 1 year. Bacterial and viral infections were more frequent in UCBrecipients, yet donor type had no influence on treatment-related mortality or survival.Considering all patients at day 28, lower numbers of total CD4 1 T cells and naive CD4 1 T cells were significantly associated with increased infection risk, treatment-related mortality, and chronic graft-versus-host disease (GVHD). Patients with these characteristics may benefit from enhanced or prolonged infection surveillance and prophylaxis as well as immune reconstitution-accelerating strategies.

We have analyzed the long term outcome of 197 patients who were treated for grade II to IV acute graft-versus-host disease (GVHD) following histocompatible allogeneic bone marrow transplantation (BMT). Of 469 recipients of sibling donor allografts performed at our center between January, 1979 and October, 1987, 197 patients (42%) developed greater than or equal to grade II acute GVHD at a median of 38 days (range 9 to 98 days) post-BMT. After treatment with corticosteroids (n = 160) or other immunosuppressive therapies (n = 37), 72 patients (41% +/- 8%; 95% confidence interval [CI]) achieved complete and continuing resolution of acute GVHD after a median of 21 days of therapy. Sixty- one patients required additional immunosuppressive therapy with high dose methylprednisolone, antithymocyte globulin (ATG)/steroids, or other therapies because of refractory or progressive symptoms of acute GVHD. Seven of these 61 patients eventually obtained complete and continuing remission after 13 to 57 days (median 50) of secondary treatment. The overall rate of chronic GVHD was 70% +/- 16%; 95% CI following grade II to IV acute GVHD. Twenty-five of the 197 patients never developed chronic GVHD, resulting in a Kaplan-Meier projection of 30% +/- 8% (95% CI) cure of moderate/severe acute GVHD. Analysis of clinical features associated with complete response (CR) to acute GVHD therapy identified more favorable responses to therapy in patients without either liver or skin involvement, patients with acute lymphoblastic leukemia, and donor/recipient pairs other than male patients with female donors. Older recipient age was not associated with more resistance to GVHD treatment. CR to GVHD treatment was associated with significantly better 5-year survival: 51% +/- 14% versus 32% +/- 11% for patients with therapy resistant acute GVHD (P = .004). GVHD was a major contributing cause of death in 49 of the 90 patients who died and was often complicated by infection or interstitial pneumonitis. Control of acute GVHD through immunosuppressive therapy did not affect the risk of leukemic relapse after transplantation.