Background: COVID-19 is a respiratory disease MESHD caused by a novel coronavirus (SARS-CoV-2) and causes substantial morbidity and mortality. At the time this clinical trial was planned, there were no available vaccine or therapeutic agents with proven efficacy, but the severity of the condition prompted the use of several pharmacological and non-pharmacological interventions. It has long been hypothesized that the use of convalescent plasma SERO (CP) from infected patients who have developed an effective immune response is likely to be an option for the treatment of patients with a variety of severe acute respiratory infections MESHD (SARI) of viral etiology. The aim of this study is to assess the efficacy and safety of convalescent plasma SERO in adult TRANS patients with severe COVID-19 pneumonia MESHD pneumonia HP. Methods/Design: The ConPlas-19 study is a multicenter, randomized, open-label controlled trial. The protocol has been prepared in accordance with the SPIRIT (Standard Protocol Items: Recommendations for Interventional Trials) guidelines. The study has been planned to include 278 adult TRANS patients hospitalized with severe COVID-19 infection MESHD not requiring mechanical ventilation (invasive or non-invasive). Subjects are randomly assigned in a 1:1 ratio (139 per treatment arm), stratified by center, to receive intravenously administered CP (single infusion) plus SOC or SOC alone, and are to be followed for 30 days. The primary endpoint of the study is the proportion of patients that progress to categories 5, 6 or 7 (on the 7-point ordinal scale proposed by the WHO) at day 15. Interim analyses for efficacy and/or futility will be conducted once 20%, 40%, and 60% of the planned sample size are enrolled and complete D15 assessment. Discussion: This clinical trial is designed to evaluate the efficacy and safety of passive immunotherapy with convalescent plasma SERO for the treatment of adult TRANS patients hospitalized with COVID-19. The results of this study are expected to contribute to establishing the potential place of CP in the therapeutics for a new viral disease MESHD. Trial registration: Trial registration at clinicaltrials.gov; Registration Number: NCT04345523; https://clinicaltrials.gov/ct2/show/NCT04345523; Registered on 30 March, 2020. First posted date: April 14, 2020. Keywords: COVID-19, randomized, controlled trial, protocol, convalescent plasma SERO (CP), antibodies SERO.