Study of Immunogenicity Equivalence of a Homologous Third Dose of Covid-19 (Recombinante) Vaccine

Inclusion Criteria: * Employee of Fundação Oswaldo Cruz, stationed in Manguinhos campus, in Rio de Janeiro. * Previous two-dose vaccine schedule with Covid-19 (recombinante) in both doses, with an interval of8 or 12 weeks between doses. * A 6-month interval (5 to 7 month window) between the second and the third dose of Covid-19 (recombinante) vaccine. * Availability to participate during the entire study, and ability to follow study protocol strictly. * Consent to supply personal contact information, such as telephone numbers, address and other information so the research team may contact the participant (For example, in case the participant fails to attend a scheduled visit without previous notice) * Ability to understand and sign the volunteered and informed consent form, and ability to fill in the adverse events journal at home, according to the evaluation of the principal investigator ou delegated research team members. * Understanding the impossibility of participating in another clinical trial while participating in this clinical trial. Exclusion Criteria: * Receiving another Covid-19 vaccine after inclusion in the study * Receiving any vaccine 28 days after administration of a third dose of Covid-19 (recombinante) vaccine. * Refusal to sign the voluntary and informed consent form or refusal to permit collection of blood samples before the third dose of the Covid-19 (recombinante) vaccine. * Receiving any other vaccine 28 days before the inclusion in the study * Previous vaccine schedule with other Covid-19 vaccines, in the first and/or second dose. * Covid-19 disease confirmed by RT-PCR (Real time polymerase chain reaction) up to 28 days before inclusion in the study. * Covid-19 symptoms up to 10 days before the inclusion in the study. * Fever (Axillary temperature above 37,8 º C / 100,04 °F) 72 hours before vaccination in the study. * Contraindications to the Covid-19 (recombinante) vaccine - Fiocruz/AstraZeneca * Use of immunosuppressive medication, such as systemic corticosteroids or chemotherapy, or immunosuppressive diseases. We will consider as immunosuppressive doses of systemic corticosteroids daily doses of prednisone of 10mg or more, for more than 14 days. * Incapacitating mental illness * Any discoveries made by the principal investigator that would enhance the risk of an adverse result following the participation in the study, or that in some other way justifies exclusion from the study.