Burden and Impact of Infectious Complications After Allogeneic Hematopoietic Cell Transplantation in Older Patients: A GETH-TC Study.

BACKGROUND: Infectious complications remain a major cause of morbidity and mortality after allogeneic hematopoietic cell transplantation (allo-HCT). With the increasing use of allo-HCT in older adults, the burden and clinical impact of infections in this population remain poorly characterized. OBJECTIVES: To characterize the epidemiology of microbiologically documented infections during the first year after allo-HCT in patients aged ≥65 years and to identify risk factors for infectious complications. METHODS: This retrospective multicenter study included consecutive patients aged ≥ 65 years undergoing a first allo-HCT in 13 Spanish centers between 2011 and 2023. Microbiologically documented infections occurring within the first year post-transplant, or until relapse or death, were analyzed. Cumulative incidence (CI), infection density, and multivariable models were used to identify risk factors and assess clinical outcomes. RESULTS: Overall, 568 of 726 allo-HCT recipients (78.2%) experienced ≥ 1 infectious episode, with an overall infection density of 8.77 per 1,000 patient-days. Viral and bacterial infections were most common, affecting 60.6% and 54.8% of patients, respectively. At 1 year, the CI of bacterial infection was 53.6% (density 4.31 per 1,000 patient-days), invasive fungal disease (IFD) 11.7% (density 0.47 per 1,000 patient-days), and CMV infection 44.0%. Infection-related mortality at 1 year was 12.4%, mainly driven by Gram-negative bacterial and Aspergillus infections. The occurrence of any infectious episode was independently associated with inferior overall survival (OS; HR 1.6, 95% CI 1.3-2.1) and higher non-relapse mortality (NRM; HR 3.4, 95% CI 2.3-4.9). Bacterial infections, IFD, and CMV infection were independently associated with inferior OS and increased NRM. Across infection-specific multivariable analyses, poor functional status, haploidentical transplantation, and GvHD emerged as the principal determinants of infectious burden, whereas antibacterial prophylaxis and letermovir were protective. CONCLUSIONS: In older allo-HCT recipients, infectious complications affected a large proportion of patients and have a substantial impact on survival. Functional status and donor type independently modulate susceptibility to infection, highlighting the need for refined risk stratification and tailored preventive strategies. Infection density may provide a useful complementary measure to better capture the burden of recurrent infectious events in this population.