Inclusion Criteria:  Persons who have received a dose of Gam-COVID-Vac more than 30 days and less than 90 days ago. Age between 21 and 65 years. Both genders. Who have voluntarily agreed to participate in the clinical trial and have provided informed consent.  Exclusion Criteria:  Known history of COVID in the 6 months prior to study inclusion. Known or suspected immunocompromised status by the study investigator for any cause. Use of oral or parenteral corticosteroids in the last 30 days. Known history of allergy to any vaccine. History of anaphylaxis. Pregnant or lactating women. Known history of autoimmune diseases. Persons under treatment for any neoplastic disease within the last 6 months. Any serious illness or condition at the discretion of the study investigator (including but not limited to the presence of chronic obstructive pulmonary disease, heart failure, poorly controlled hypertension, poorly controlled diabetes, renal failure). Planned medical procedures within two months of randomisation. Previous vaccination within the last 30 days with any vaccine. Known participation in an ongoing clinical trial. Ongoing acute illness. Fever (≥37.8 C) at the time of randomisation.