Outcomes of ‘neglected’ respiratory viruses (PIV, hMPV) vs. Influenza and RSV lower respiratory tract infections in hematologic malignancies: a systematic review

Background Respiratory viral lower respiratory tract infections (LRTI) are a major cause of mortality and morbidity in adults with hematological malignancies. While influenza and respiratory syncytial virus (RSV) are well-characterized, the comparative clinical burden of parainfluenza virus (PIV) and human metapneumovirus (hMPV) remains poorly defined. Methods We conducted a systematic review (PROSPERO: CRD420251152933) following PRISMA 2020 guidelines. We searched four databases from inception to October 2025 for observational studies comparing clinical outcomes of PIV/hMPV LRTI versus RSV/influenza pneumonia in adults undergoing chemotherapy for hematological malignancies. Primary outcomes were all-cause mortality, ICU admission, mechanical ventilation, and length of stay. Dual independent review was performed for study selection, data extraction, and quality assessment (Newcastle-Ottawa Scale). A narrative synthesis was conducted. Results From 1,071 screened records, 12 studies (11 ‘Good’ quality) comprising a total of 4,841 patients were included. Data for mechanical ventilation were particularly scarce, and notably absent for PIV and Influenza. All-cause mortality for PIV (0–16%) and hMPV (1.4%) was not higher than for influenza (2.2–33.3%) or RSV (0–25%). ICU admission rates were highly variable and overlapping across all viruses. A salient finding, derived from a single study, was a substantially prolonged median length of hospital stay for PIV (28 days) compared to RSV (18 days), hMPV (9 days), and influenza (10–13 days). Conclusion PIV and hMPV LRTI present a distinct clinical profile in this vulnerable population. While mortality is comparable to more common viruses, PIV infection was reported in a single, small-cohort study to be associated with a prolonged hospital stay, suggesting a potential morbidity burden that requires further validation. These findings challenge the perception of these pathogens as uniformly less severe and highlight an urgent need for prospective studies to guide optimized management and resource allocation.