Inclusion Criteria:  Men and women aged 18 to 65 years (inclusive); Signed Informed Consent Form;  Clinical symptoms of mild to moderate influenza and other viral URIs:  an increased axillary temperature of ≥38.0°С ≤39.5°С during the current illness without the use of antipyretics in the last 8 hours at the time of screening; at least one of the respiratory signs and symptoms of influenza or other viral URIs (cough, runny nose/nasal congestion, sore throat) scored at least 2 points on a 4-point scale; at least one of the systemic signs and symptoms of influenza or other viral URIs (headaches, muscle aches, chills/sweats, fatigue) scored at least 2 points on a 4-point scale. Absence of indications for hospitalization at the time of enrollment; Duration of illness from symptom manifestation to the first drug dose/placebo no more than 48 h; Negative pregnancy test for women of childbearing potential; A consent to use adequate birth control methods throughout the treatment and for 30 days thereafter; Patients who are able to understand and comply with treatment and procedures during the study.  Exclusion Criteria:  Pregnant and lactating women; Known or suspected hypersensitivity to the active substance or to excipients of the drug XC221 or placebo; An established or currently suspected diagnosis (based on the assessment of the patient's health and epidemiological medical history) of COVID-19; Oxygen saturation (SpO2) ≤ 95%, respiration rate (RR) ≥ 22/min. Complications of influenza or other viral URIs; Participation in another clinical trial within 90 days prior to screening; Known (based on the medical history) or suspected abuse of alcohol or psychotropic drugs; Patients with psychotic diseases, including in the medical history; Moderate viral URI (fever ≥ 38.5°С) accompanied by exacerbation of concomitant conditions; Clinically suspected pneumonia or other bacterial infections (including sinusitis, otitis media, urinary tract infection, meningitis, sepsis, etc.) requiring antibacterial therapy; Taking antibiotics, antivirals, or immunomodulatory drugs within < 48 h prior to treatment initiation and/or planning to use these types of drugs (except for the investigational products) during the study; Use of systemic, inhaled or nasal corticosteroids within 30 days prior to treatment initiation and/or planning to use corticosteroids (except for topical dermatological agents) during the study; The inability to cancel during the study period other drugs that can affect the result of this study, for example, antiviral drugs, or drugs that are incompatible with the study's therapy; Any diseases of cardiac, renal, hepatic, gastrointestinal, endocrine and nervous system, severe decompensated chronic (including chronic renal and liver diseases) or acute diseases, or any other conditions/diseases which would make participation in the study unsafe (in the investigator's opinion); Patients receiving any vaccines within 90 days prior to enrollment; Diabetes mellitus, grade 2-3 obesity; Patients who underwent surgery within 30 days prior to the screening and patients who are scheduled to undergo surgery during the study period, including diagnostic procedures or hospital stay; Patients with oncological diseases, HIV infection, or tuberculosis, including a medical history; Meningeal syndrome; Disorders of consciousness (in the form of stupor, sopor, delusion, delirium, etc.); Symptoms of pneumonia and possible acute respiratory distress syndrome (ARDS): cough with bloody frothy sputum, shortening of pulmonary sound with percussion, a large number of different-sized wet wheezing and profuse crepitus on auscultation, a sharp decrease in blood pressure (BP), deafness of heart sounds and arrhythmia; Positive rapid test result for SARS-CoV-2; Lactase deficiency, lactose intolerance, glucose-galactose malabsorption. Other reasons which would prevent the patient from participating in the study and make an unreasonable risk (in the investigator's opinion).