Efficacy, Safety, and Immunogenicity of Two Vaccination Schedules of an Inactivated Vaccine Against COVID-19 in Adults

Inclusion Criteria: * Adults over 18 years of age. * Demonstrate the capacity to understand and sign the Informed Consent document. * Agree to comply with the study procedures and visits. Exclusion Criteria: * History of confirmed symptomatic SARS CoV-2 infection. * Pregnant (confirmed by positive urine pregnancy test) or breastfeeding females, and/or expressing intention to have sexual practices with reproductive potential without using contraceptive methods in the three months following vaccination. * History of an allergic reaction to the vaccine or components of the study vaccine or placebo. * Evidence of uncontrolled neurological, cardiac, pulmonary, hepatic, or renal disease, according to anamnesis or physical examination; Significant changes in treatment or hospitalizations due to worsening of the condition in the last three months are indicators of uncontrolled disease. * Diseases that impair the immune system including neoplasms (except basal cell carcinoma), congenital or acquired immunodeficiencies, and uncontrolled autoimmune diseases not controlled according to anamnesis or physical examination. * Behavioral, cognitive, or psychiatric illness that, in the opinion of the principal investigator or his medical representative, affects the participant's ability to understand and collaborate with the requirements of the study protocol. * Use of immunosuppressive therapies six months before inclusion in the study or its scheduled use within two years of inclusion. Immunosuppressive therapies will be considered: antineoplastic chemotherapy, radiation therapy, immunosuppressants to induce tolerance to transplants, among others. * Have received an immunosuppressive dose of corticosteroids in the last three months before inclusion in the study or scheduled administration of an immunosuppressive dose of corticosteroids for the three months following inclusion in the study. The dose of corticosteroids considered immunosuppressive is equivalent to prednisone at a dose of 20 mg/day for adults for more than a week. The continuous use of topical or nasal corticosteroids is not considered immunosuppressive. * History of asplenia, either anatomic or functional. * History of bleeding disorders, as deficiency of clotting factors, coagulopathy, platelet dysfunction, or previous history of bleeding or significant bruising after IM injection or venipuncture. * Any alcohol or drug abuse in the last 12 months before inclusion in the study that has caused medical, professional, or family problems, as indicated by clinical history. * Have received blood products (transfusions or immunoglobulins) in the last three months before inclusion in the study. * Have received any vaccine with a live attenuated virus in the last 28 days or inactivated vaccine in the last 14 days before their inclusion in the study, or have immunization scheduled for the first 28 days after their inclusion in the study. * Participation in another clinical trial with product administration under investigation during the six months before its inclusion in the study or scheduled participation in another clinical trial in the two years following inclusion. * Previous participation in a COVID-19 vaccine evaluation study or previous exposure to a COVID-19 vaccine. * Fever (\>37.8°C) within 72 hours before vaccination. * Any other condition that, in the opinion of the principal investigator or his medical representative, could jeopardize the safety or rights of a potential participant or that would prevent him from complying with this protocol.