The clinical features, risk factors and outcomes of polyomavirus encephalitis after allogeneic haematological stem cell transplantation

Summary Polyomavirus (PyV) encephalitis is a rare but life‐threatening opportunistic infection linked to progressive multifocal leucoencephalopathy (PML). We analysed 54 patients diagnosed with PyV encephalitis after allogeneic haematopoietic stem cell transplantation (allo‐HSCT) in a 1:4 case–control retrospective cohort study. Median time to diagnosis was 77 days post‐HSCT, with seizures, headache and motor dysfunction as the most common presenting symptoms. PyV‐associated mortality at 6 months and 1 year was 18.5% and 24.2% respectively. BK virus (BKV) and JC virus (JCV) encephalitis showed comparable 3‐year non‐relapse mortality (NRM: 31.5% vs. 25.0%) or overall survival (OS: 61.5% vs. 62.5%). Compared to controls, PyV encephalitis patients had a significantly higher cumulative incidence of 3‐year NRM (32.0% vs. 13.5%, p = 0.034) and lower 3‐year OS (60.6% vs. 76.6%, p = 0.012). Additionally, PML occurred in 51.8% (28/54) of cases, but no significant differences in NRM or OS compared to non‐PML. In conclusion, PyV encephalitis is associated with poorer clinical outcomes, and BKV/JCV encephalitis exhibit comparable, unfavourable outcomes.