Introduction. Nowadays the aspects of COVID-19 prophylaxis remain especially actual among medical personnel providing medical aid to patients with this disease, mainly because of high risk of contact with the pathogen. Aminodihydrophtalazindione sodium salt is one of promising drugs for providing such prophylaxis. Aim. Studying the effectiveness of medium and severe clinical forms of COVID-19 prophylaxis with the drug mentioned among medical personnel working in the «red zone» under circumstances of high epidemiological risk of COVID-19. Material and methods. The study is retrospective and observational. 250 participants are included in the study. Statistical analysis was conducted using Pearson’s χ test, Mann–Whitney U-test, multiple linear regression for creating the mathematical model of medium and severe COVID-19 forms risk determination. Results. Statistically significant lowering of the risk of medium and severe COVID-19 forms is shown for prophylaxis with sodium aminodihydrophtalazindione; no effect on PCR test result was shown, as well as on quantitative grade of immune response (IgG concentration). In mathematical model predicting medium and severe forms statistical significance was found for the fact of prophylaxis and the positive immunological test (with IgG concentration higher than the threshold), that confirms the fact of contact with SARS-CoV-2. Conclusion. A statistically significant reduction of the risk of moderate and severe forms of COVID-19 development was shown during prophylaxis with the investigated drug. 2