BackgroundFrom an epidemic outbreak, coronavirus disease-2019 (COVID-19) has quickly developed. Thymosin α1 (Tα1) has the ability to boost the T-cell numbers, support T-cell differentiation, maturation, and reduce cell apoptosis. In this study, we have investigated the efficacy and safety of Tα1 in moderate-to-severe COVID-19 patients.Patients and methodsIn this double-blind, multicenter, two-arm, randomized, placebo-controlled, phase III clinical study, patients were randomized to receive either Tα1 or placebo in combination with standard of care (SOC). The data on all-cause mortality, clinical progression/deterioration, duration of hospital/intensive care unit (ICU) stay, and safety data were collected. The patients were telephonically followed up on Day 28.ResultsA total of (n = 105) COVID-19 patients were included in the study, of which 40 and 65 were severe and moderate, respectively. Thymosin arm (11.1%) had a statistically lower death rate in comparison to the placebo arm (38.5%). A total of 67 adverse events were reported in 42 patients among 105 dosed patients during the study. Among them, 43 adverse events were of mild in nature, 16 adverse events were of moderate in nature, and 8 serious adverse events (death) occurred during the study.ConclusionThis study provides evidence that Tα1 can lower death rate in severe COVID-19 patients, reduce the load on hospitals by shortening the required number of days of hospitalization and help in abbreviating the requirement of oxygen support by positively impacting the recovery rate and time taken for recovery.How to cite this articleShetty A, Chandrakant NS, Darnule RA, Manjunath BG, Sathe P. A Double-blind Multicenter Two-arm Randomized Placebo-controlled Phase-III Clinical Study to Evaluate the Effectiveness and Safety of Thymosin α1 as an Add-on Treatment to Existing Standard of Care Treatment in Moderate-to-severe COVID-19 Patients. Indian J Crit Care Med 2022;26(8):913-919.