In this study, researchers retrospectively evaluated the role of serum angiotensin-converting enzyme (ACE) activity in the progression of COVID-19 within 136 patients with confirmed COVID-19 and 60 age- and sex- matched normal controls. The COVID-19 patients were split into 16 severe cases and 120 non-severe cases, and the authors demonstrated a correlation between disease severity and older age (57.50 ± 11.70 years vs 49.19 ± 15.98 years), higher body mass index (BMI) (26.04 ± 5.63 kg/ m2 vs 23.60 ± 3.33 kg/m2), and higher proportion of hypertension (8 [50%] vs 25 [20.83%]). Hypertensive patients, older patients, and patients with a higher BMI exhibited lower baseline serum ACE activity. Overall, baseline serum ACE activity was much lower in patients with COVID-19 compared to controls, with severe COVID-19 patients expressing the lowest activity levels. Patients with COVID-19 also showed a large change in immune-inflammatory parameters--including fibrinogen, percentage of neutrophils, and lymphocyte count--compared to controls. After treatment and during recovery, serum ACE activity levels increased in the COVID-19 group. The authors suggest that since serum ACE activity can be used as a marker to monitor other respiratory illnesses, such as influenza and acute respiratory distress syndrome, it can similarly be used to monitor the progression of COVID-19.