Open Multicentre Study of the Safety and Efficacy Against COVID-19 of Nirmatrelvir/Ritonavir in the Adult Population

Inclusion Criteria: * Availability of PIS Informed Consent Form signed and dated by a patient. * Male and female subjects aged 18 to 80 inclusive as of the time of signing the PIS Informed Consent Form. * Confirmed COVID-19 at screening stage: positive for SARS-CoV-2 RNA laboratory test with nucleic acid amplification techniques (NAAT) or SARS-CoV-2 antigen with immunochromatographic assay. * SARS-CoV-2 test results obtained up to 5 days prior to randomization may be taken into account upon availability of supporting documentation. * Mild or moderate SARS-CoV-2 induced infection. * At least one of the following COVID-19 symptoms as of the time of screening and randomization: nasal congestion or rhinitis; sore throat; dyspnoea or respiratory distress on exertion; cough; tiredness; pain in muscles or throughout the body; headache; chills; fever (body temperature \> 38 ⁰С); nausea; vomiting; diarrhoea; anosmia; ageusia. * Disease onset (first symptom) within not more than 5 days prior to randomization * The patient agrees and is able to take oral drug products. * Patient's consent to use reliable contraception methods during the entire study and for 3 weeks after the end of the study. Reliable contraception methods are: sexual abstinence, the use of a condom combined with a spermicidal agent. * Female subjects incapable of bearing children (history: hysterectomy, tubal ligation, infertility, menopause for more than 2 years), as well as male subjects with infertility or a history of vasectomy may participate in the study. Exclusion Criteria: * Hypersensitivity to the study drug components. * Lactase deficiency, lactose intolerance, glucose-galactose malabsorption. * Use of direct-acting antiviral drugs within 10 days prior to screening. * The use of drugs at the time of screening whose clearance is highly dependent on the CYP3A isoenzyme, or which are strong inducers of CYP3A (for more information, see the section "Unauthorized pharmaceuticals"). * Necessity to use unauthorized pharmaceuticals. * The need for oxygen therapy at the time of screening. * Hospitalization required at time of screening, or hospitalization expected to be required for COVID-19 within 48 hours of randomization, excluding hospitalization to the observation facility for social reasons (e. g., living in a hostel, cohabiting with people, including those with risk factors for aggravated COVID-19, who, after contact with the patient, have a negative result for the SARS-CoV-2 RNA/antigen, etc.). * Severe and extremely severe disease signs as of the time of screening. * Vaccination within less than 4 weeks prior to screening. * Possible or confirmed moderate COVID-19 within 6 months prior to screening. * Possible or confirmed history of severe or very severe COVID-19. * Patients with moderate to severe renal insufficiency (estimated glomerular filtration rate (GFR) \< 60 ml/min/1.73 m2 per CKD-EPI formula) or receiving renal replacement therapy at the time of screening. * Severe liver failure (class C per Child-Pugh) at the time of screening or elevated ALT and/or AST levels of ≥ 2.5 UNL and/or elevated total bilirubin levels of ≥ 2 UNL (≥ 3 UNL with Gilbert's syndrome) in past medical history (within 6 months prior to screening). * Positive test result for HIV, syphilis, hepatitis B and/or C at the time of screening. * Alcohol, pharmacological and/or drug dependence (history and/or at the time of screening). * Schizophrenia, schizoaffective disorder, bipolar disorder, or other psychiatric disorder (history or suspected) at the time of screening. * Any history data that may complicate, in the opinion of the investigator, interpretation of the study results or create additional risk for the patient due to his/her participation in the study. * Unwillingness or inability of a patient to comply with the Protocol procedures (in the opinion of the investigator). * Pregnant or lactating women, or women planning a pregnancy. * Participation in another clinical study within 3 months prior to enrolment in the present study. * Other conditions that prevent the patient from inclusion in the study.