Phase II/III Randomized Clinical Trial of Booster Dose of COVID-19 (Recombinant, Inactivated) Vaccine

Inclusion Criteria: 1. Age ≥ 18 years, of which 50% and 20% were aged ≥60 years in Phase II and Phase III studies, respectively, regardless of previous SARS-CoV-2 infection status, with proof of four doses of any monovalent vaccine against COVID-19, of which the last dose administered at least 120 days to 540 days ago. 2. If pre-existing medical conditions: be in a stable condition that does not require hospitalization or significant changes in therapy during the three months prior to enrollment. 3. Agree to regular contact by phone, electronic means, and/or home visits. 4. Intention to participate in the study, documented by the Informed Consent Form. Exclusion Criteria: 1. Administration of a vaccine of active or inactivated virus not provided for in the study regimen up to 30 days before the dose of the study vaccine. 2. Use of other COVID-19 vaccination regimen other than the one contemplated in item a. of the inclusion criteria. 3. Angioedema or anaphylactic reaction to previous immunizations. 4. Allergy to egg or chicken. 5. Severe allergic reaction or anaphylaxis to the vaccine or components of the study vaccine. 6. Suspected or confirmed fever within 24 hours prior to vaccination or an axillary temperature greater than 37.8°C\* on the day of vaccination (inclusion may be delayed until the subject is fever-free for 24 hours), as well as confirmation of SARS-CoV-2 infection (enrollment should be deferred until the participant has completed 24 hours without fever or until the participant resolves the SARS-CoV-2 infection documented by two negative RT-PCR tests). 7. Evidence of uncontrolled active neurological, cardiac, pulmonary, liver or kidney disease. Significant treatment changes or hospitalizations for worsening the condition in the last three months are indicators of uncontrolled disease. 8. Bleeding disorders (e.g., clotting factor deficiency, coagulopathy, platelet dysfunction), or previous history of significant bleeding or bruising after intramuscular injection or venipuncture. 9. Neoplastic diseases (except basal cell carcinoma and cervical carcinoma in situ) diagnosed or under investigation. 10. Suspected or confirmed immune compromising diseases including congenital or acquired immunodeficiencies and autoimmune diseases not under control according to the medical history or physical examination, including asplenia. Significant treatment changes or hospitalizations for worsening the condition in the last three months are indicators of uncontrolled disease. 11. Use of immunosuppressive therapies six months prior to study inclusion or scheduled to be of service within two years of inclusion. The dose of corticosteroid considered immunosuppressive is the equivalent of prednisone at a dose of 20 mg/day for adults for more than 14 days. Continued use of topical or nasal corticosteroids and other topical immunomodulators or immunosuppressants will not be considered immunosuppressive. The following are considered immunosuppressive therapies: antineoplastic chemotherapy, radiotherapy, immunosuppressants to induce transplant tolerance, immunosuppressive and immunobiological treatments in patients with autoimmune rheumatic diseases, among others. 12. Use of blood products (transfusions or immunoglobulins) within the last three months prior to study inclusion or scheduled blood product or immunoglobulin administration within six months of study inclusion. 13. Alcohol or drug abuse in the past 12 months prior to the subject's inclusion. 14. Behavioral, cognitive, or psychiatric illness that affects the subject's ability to understand and cooperate with the study protocol requirements. 15. Being team member conducting the study or having a dependent relationship with one of the study team members. 16. Any other condition that may jeopardize the safety or rights of a potential participant or prevent him/her from complying with this protocol. 17. Abnormalities in screening laboratory tests are considered to be excludable in the opinion of the principal investigator or his/her medical representative. If any changes in the tests are considered temporary, the tests may be repeated up to three times during the screening period (Phase II only) 18. Positive serology tests for human immunodeficiency virus (anti-HIV1/2 ELISA); Hepatitis B (HbsAg or Anti-HBc) or Hepatitis C (total Anti-HCV ELISA). 19. Any other findings that the investigator expect to would increase the risk of adverse outcomes from study participation. For women of childbearing potential: 20. Pregnancy (confirmed by positive β-hCG test), being a breastfeeding, and/or manifest intention to have sexual practices with reproductive potential without the use of a contraceptive method (abstinence, sterilization, intrauterine or implantable contraceptive device; oral contraceptives; diaphragm or condom in combination with contraceptive gel, cream, or foam) within 30 days before and 28 days after vaccination. * Note: \* The temperature measured with a temporal scanner skin thermometer is considered equivalent to the axillary temperature.