Phase 2 clinical trial in adults previously vaccinated against SARS-CoV-2 in Chile with an initial schedule of two doses of CoronaVac® plus two booster doses with different vaccines. Subjects will randomly receive a third booster dose with Omicron, trivalent, or CoronaVac® vaccine. The humoral immunogenicity against COVID-19 will be compared in subjects that received the Omicron or the Trivalent vaccines with subjects that received CoronaVac® to determine the superiority of the two candidate vaccines versus CoronaVac®. Subjects will be followed for 6 months after the booster dose administration.

This study will be a phase 2 randomized, double-blind, multicenter clinical trial in fully vaccinated adults, males, and females, who have previously received an initial schedule of two doses of CoronaVac® plus two booster doses with different vaccines against SARS-CoV-2 in Chile. Two groups of participants will be included: Heterologous group: subjects who had received two booster doses with mRNA or viral vector-based vaccine); and homologous group: subjects who have received two booster doses of CoronaVac® in a previous clinical trial (CoronaVac03CL).

Subjects of the heterologous group will randomly receive a booster dose with Omicron, trivalent, or CoronaVac® vaccine. On the other hand, subjects of the homologous group will randomly receive a booster dose of Omicron or trivalent vaccine.

The humoral immunogenicity against COVID-19 will be compared in subjects of the heterologous group that will receive Omicron or trivalent vaccine with subjects that will receive CoronaVac®, to determine the superiority of the two candidate vaccines versus CoronaVac®.

Subjects will be followed for 6 months after the booster dose administration.

Inclusion Criteria:

• Adults, male or female, over 18 years of age;
• Fully vaccinated against SARS-CoV-2 with two initial doses of CoronaVac® vaccine and that have received two booster doses of a different vaccine (heterologous group) or with CoronaVac® (homologous group) at least 5 months before enrollment;
• Capable of understanding and signing the informed consent form;
• Availability and commitment to comply with study procedures and in-person and remote appointments.

Exclusion Criteria:

• Symptomatic COVID-19 diagnosed 60 days before enrollment (confirmed COVID-19 by RT-PCR or antigen test, or by being in close contact with a confirmed case);

• Pregnant women (confirmed by urine pack test) or women who plan to get pregnant within the first 3 months of the study;

• Known allergies to the vaccine components;

• Evidence of uncontrolled metabolic, neurologic#, cardiac, pulmonary, hepatic, or renal disease. Significant changes in medical treatment or hospitalizations due to worsening conditions in the last three months are indicators of uncontrolled disease or unsuccessful adherence to the treatment;

• Alteration of the immune system (neoplasms, except basal cell cancer), congenital or acquired immunodeficiencies, and uncontrolled autoimmune diseases. Significant changes in medical treatment or hospitalizations due to worsening conditions in the last three months are indicators of uncontrolled disease;

• Behavioral, psychiatric, or cognitive conditions# that, according to the study physician, impair the ability to understand and cooperate with the requirements of this trial;

• Intake of immunosuppressants within 6 months before enrollment or prescribed to be taken within the next 2 years of the study. Antineoplastic therapy, radiation, and immunosuppressants that induce tolerance to transplants, among others, are included in this category;

• Intake of corticosteroids within 3 months before enrollment or prescribed to be taken within 3 months after enrollment. The equivalent of 20 mg/day of prednisone for more than one week will be considered an immunosuppressive dose. Topical or inhaled steroids are not considered immunosuppressive drugs;

• History of anatomic or functional asplenia;

• Coagulation disorders such as coagulation factor deficiency, coagulopathy or platelet disorders, or a history of significant bleeding or bruising from intramuscular injections or venipunctures;

• Alcohol or drug abuse reported 12 months before enrollment that caused medical, professional, or family consequences;

• Have been treated with blood or immunoglobulin transfusions within 3 months before enrollment;

• Have you received any live attenuated or inactivated vaccine within 28 and 14 days prior to the enrollment or planned to receive one within the first 28 days of the study;

• Participation in another clinical trial 6 months before the enrollment or plan to participate in one 6 months after the enrollment in this trial;

• Prior participation in a SARS-CoV-2 vaccine trial different than the CoronaVac03CL trial;

• Any fever episode (body temperature ≥37.8℃) within 3 days before enrollment;

• Any other condition that according to the primary care physician, could jeopardize the safety/rights of the subject or prevent him/her from completing the protocol

  • Alzheimer's and senile dementia are exclusion criteria.

Note: Obese and smoker participants are allowed to participate.

A specialized company designed the randomization algorithm of vaccines (IRT algorithm). The algorithm was designed considering the vaccination schedule of the subjects and the number of subjects assigned to each center. Once randomized, each participant will be assigned to a treatment branch/group. Neither the personnel administering it nor the rest of the team nor the participant will know which product was administered.