Supporting this idea, Ueland et al reported a significant negative correlation between plasma MMP-9 levels and P/F ratio in 39 hospitalised COVID-19 patients, with those who developed respiratory failure presenting at two study time-points (days 0-2 and 3-5 post-hospitalisation) with significantly higher circulating concentrations of MMP-9 than patients who did not experience respiratory failure (146). Here, MMP9 is linked to respiratory failure.