Inclusion Criteria:  Signed Informed Consent Form. Patients of both sexes aged 18 to 75 years inclusive. Diagnosed COVID-19 based on positive laboratory tests confirming the SARS-CoV-2 presence, performed no earlier than 3 days before screening. It is allowed to determine the SARS-CoV-2 by the PCR or by another method according to the MoH Temporary Guidelines. Patients having a total score ≥ 5 according to the COVID-19 Major Symptom Rating Scale. Patients with mild COVID-19, as defined in The MoH Temporary Guidelines. Presence of at least two criteria: SpO2 ≥ 95% (required criterion), 37.5°С ≤ t ˂ 38°С, or respiratory rate (RR) ≤ 22 / min. Absence of moderate/severe progression criteria.  Disease duration is no more than 3 full days since the onset of one or more of symptoms below before the first dose of the drug administration:  body temperature increase; dry cough or cough with little phlegm; dyspnoea; myalgia; fatigue; feeling of congestion in the chest; reduced sense of smell and / or taste. For women only: negative pregnancy test result. Pregnancy testing is not required for women of not childbearing potential (WONCBP): women who are in menopause (defined as an absence of menstruation for at least 2 years or more), or women who undergone surgical sterilization (hysterectomy, bilateral oophorectomy, tubal ligation), or women with a clinical diagnosis of "infertility". The presence of surgical sterilization and infertility should be confirmed by patient's claim or by relevant document confirming this condition. Consent to use reliable method of contraception throughout the study period. Patients who are able to understand and comply with treatment and procedures during the study.  Exclusion Criteria:  Known or suspected hypersensitivity to the active substance or to excipients of the drug XC221 or placebo Known or suspected hypersensitivity to standard therapy drugs specified in The MoH Temporary Guidelines. Body temperature ˂37.5°С. Lactase deficiency, lactose intolerance, glucose-galactose malabsorption. Presence or suspicion of oncological diseases by the day of criteria assessment or in medical history (within the last 2 years). Presence of autoimmune diseases by the day of criteria assessment or in medical history. Pregnancy. Lactation period. Presence of serious lung diseases, including, but not limited to the following diseases: moderate and severe bronchial asthma, severe and extremely severe COPD, interstitial lung disease, pulmonary hypertension, pulmonary fibrosis, surgical interventions on the lungs, tuberculosis (including suspicion of tuberculosis based on the results of CT examination at screening). Patients with type 1 diabetes mellitus and / or decompensated type 2 diabetes mellitus. Heart failure, NYHA functional class III - IV. Chronic liver failure stage II (decompensated) and higher. The need of replacement renal therapy at enrollment. Organ transplantation in medical history. Medical history of epilepsy or the need for anticonvulsant therapy. Major depressive disorder, anxiety, other mental disorders requiring medical correction. Acute cerebrovascular accident, stroke or transient ischemic attack within 90 days before screening. Use of any antiviral and / or immunomodulatory drugs after the manifestation of COVID-19. Any immunosuppressive therapy (including tocilizumab / sarilumab) within 90 days prior to randomization, or the need of immunosuppressive therapy at the time of randomization. Use of anti-COVID-19 plasma within 14 days prior to the screening visit. Use of systemic glucocorticosteroids within 90 days prior to randomization, or the need of systemic glucocorticosteroids at the time of randomization. Use of vaccines against viral infections within 90 days prior to randomization. Patients receiving other experimental drugs, drugs not approved in the Russian Federation, or participating in other clinical trials within 30 days before screening. Patients abusing alcohol or psychotropic drugs and other drugs by the day of criteria assessment or during the last year. Patients with other serious, unstable, or clinically significant medical or psychological conditions that, in the opinion of the investigator, may preclude the patient's participation in the study.