Experimental, single-arm study in participants with comorbidities, previously immunized with 2 doses of CoronaVac, who will receive a booster vaccine with 1 dose of the Oxford/AstraZeneca vaccine.

Intervention: Oxford/AstraZeneca - 0.5 mL single dose, intramuscular in deltoid, with an interval of 180 days (+/- 30 days) from the 2nd dose of the initial vaccination schedule with CoronaVac

Inclusion criteria:

• Have voluntarily participated and be in regular follow-up monitoring by the COVACMANAUS study;
• Have completed the vaccination schedule with 2 doses of CoronaVac, as per the COVACMANAUS study protocol for at least 180 days (+/- 30 days);
• Demonstrate availability to be followed during the follow-up period defined in the study, through visits, telephone contacts or other digital means of communication;
• Accept to participate in this new study for 6 (six) months.

Exclusion criteria

• Confirmed diagnosis of COVID-19 within the last 28 days (antigen test or RT-PCR). In this situation, vaccination can be postponed until the participant completes 30 days;
• Report of fever within 72 hours prior to vaccination (inclusion can be postponed until the participant completes 72 hours without fever and COVID-19 is discarded);
• Having received live attenuated virus vaccine in the last 28 days or inactivated vaccine in the last 14 days prior to inclusion in the study;
• Any other condition that, in the opinion of the Principal Investigator or its medical representative, could jeopardize the safety or rights of a potential participant or prevent them from complying with this protocol;
• Pregnancy or lactation.

Type of study/study design: Experimental, single-arm study in participants with comorbidities, previously immunized with 2 doses of CoronaVac, who will receive a booster vaccine with 1 dose of the Oxford/AstraZeneca vaccine.

Sample size: all active participants from the COVACManaus study (up to 5071)

Primary outcome: Detection and titration of antibodies against SARS-CoV-2 before (D0), at D90 and D180 after booster vaccination

Secondary outcomes:

• Profile of the cell-mediated immune response in a subgroup of participants, before (D0), at D90 and D180 after booster vaccination;
• Ability to neutralize antibodies in a subgroup of participants before (D0), in D90 and D180 after the vaccine booster;
• Memory cell populations in a subgroup of participants before (D0), in D90 and D180 after booster vaccination.
• Detection and titration of antibodies against SARS-CoV-2 and its association with the history of physical activity or sedentary lifestyle.

Inclusion criteria:

• Have voluntarily participated and be in regular follow-up monitoring by the COVACMANAUS study;
• Have completed the vaccination schedule with 2 doses of CoronaVac, as per the COVACMANAUS study protocol for at least 180 days (+/- 30 days);
• Demonstrate availability to be followed during the follow-up period defined in the study, through visits, telephone contacts or other digital means of communication;
• Accept to participate in this new study for 6 (six) months.

Exclusion criteria

• Confirmed diagnosis of COVID-19 within the last 28 days (antigen test or RT-PCR). In this situation, vaccination can be postponed until the participant completes 30 days;
• Report of fever within 72 hours prior to vaccination (inclusion can be postponed until the participant completes 72 hours without fever and COVID-19 is discarded);
• Having received live attenuated virus vaccine in the last 28 days or inactivated vaccine in the last 14 days prior to inclusion in the study;
• Any other condition that, in the opinion of the Principal Investigator or its medical representative, could jeopardize the safety or rights of a potential participant or prevent them from complying with this protocol;
• Pregnancy or lactation.