Patients undergoing allogenic hematopoietic stem cell transplantation (allo-HSCT) are highly susceptible to infections. The use of masks on wards for allo-HSCT has been controversial in the past decades and was not common before the covid-19 pandemic. We retrospectively compared incidence and outcomes of viral respiratory infections during allo-HSCT on our specialized ward between 2018 – 2020 to the era of FFP2 masking between 2021 – 2023 covering similar seasons of the year. Each group consisted of 150 matched patients. The usage of FFP2 masks reduced the incidence of viral respiratory infections from 22,1% to 2,1% (p < 0.005). This reduced the time on ward from a median of 26 days to 23,5 days (p = 0,002). It also resulted in less use of CT-scans (p = 0.003) and bronchoalveolar lavage procedures (p = 0,057). Median time to proof of infection was 21 days after admission in both groups. No difference was detected in progression free survival, hospital survival or non-relapse mortality (p = 0,78). Our retrospective results indicate that FFP2 masks worn by patients and hospital staff significantly reduce the incidence of viral respiratory infections, including covid-19, shorten the time spent in hospital and reduce costs without affecting survival.