Objective To clarify both the histologic changes in primary viral pneumonia other than COVID-19 and whether patients with severe lung injury (SLI) on biopsy specimens progress to severe respiratory insufficiency.

Methods Patients with primary viral pneumonia other than COVID-19, who underwent lung tissue biopsy, were retrospectively studied.

Patients Forty-three patients (41 living patients and 2 autopsied cases) were included in the study.

Results Nine patients had SLI, whereas most of patients who recovered from primary viral pneumonia showed a nonspecific epithelial injury pattern. One patient underwent a biopsy under mechanical ventilation. Two of 8 (25.0%) patients on ambient air or low-flow oxygen therapy progressed to a severe respiratory condition and then to death, while only 1 (3.1%) of 32 patients without SLI progressed to a severe respiratory condition and death (p=0.096). The proportion of patients who required O₂ treatment for ≥2 weeks was higher in patients with SLI than in those without SLI (p=0.033). The 2 autopsy cases showed a typical pattern of diffuse alveolar damage, with both showing hyaline membranes. Non-specific histologic findings were present in 32 patients without SLI.

Conclusion Some patients with SLI progressed to severe respiratory insufficiency, whereas those without SLI rarely progressed to severe respiratory insufficiency or death. The frequency of patients progressing to a severe respiratory condition or death did not differ significantly between those with and without SLI. The proportion of patients who required longer O₂ treatment was higher in SLI group than in those without SLI.