Differential Plasma MMP-3 Levels in H1N1 and COVID-19 Pneumonia

Background: Matrix metalloproteinase-3 (MMP-3) participates in extracellular matrix remodeling and lung injury, yet its behavior across different viral pneumonias remains unclear. We compared circulating MMP-3 levels in patients with coronavirus disease 2019 (COVID-19) and H1N1 pneumonia and evaluated their relationships with clinical severity. Methods: This retrospective study included hospitalized patients with laboratory-confirmed COVID-19 or H1N1 infection. Plasma MMP-3 concentrations were measured using enzyme-linked immunosorbent assay (ELISA). Clinical severity was assessed using the arterial oxygen partial pressure to fractional inspired oxygen ratio (PaO2/FiO2), Sequential Organ Failure Assessment (SOFA) score, Acute Physiology and Chronic Health Evaluation II (APACHE II) score, and hospital length of stay. Between-group comparisons and correlation analyses were performed among 109 patients, including 57 with H1N1 and 52 with COVID-19. Results: Plasma MMP-3 concentrations were higher in patients with H1N1 than COVID-19. Receiver operating characteristic analysis demonstrated excellent discriminatory ability of MMP-3 for differentiating H1N1 from COVID-19 pneumonia with an optimal cutoff value of 4.601 ng/mL. Higher MMP-3 levels were associated with lower bicarbonate levels in both cohorts, although the association reached statistical significance only in patients with H1N1. Conclusions: The circulating MMP-3 differs substantially between H1N1 and COVID-19 pneumonia and is substantially higher in H1N1 patients.