This study has been designed to assess immune response and safety of third dose vaccination with AstraZeneca ChAdOx1AZD1222 vaccine or Pfizer/BioNTech BNT162b2 vaccine among Thai subjects who have received two doses of Sinovac.~The types of vaccines provided by the government included 7.7 million doses of inactivated vaccine manufactured by Sinovac and 6.5 million doses of AstraZeneca ChAdOx1 AZD1222 vaccine.~With the limited supplies of COVID vaccines in many regions of the world especially in LMIC including Thailand and the evidences of waning immunity of especially inactivated vaccine have raised the concerns whether third dose is needed.~The third dose that available now in Thailand are AstraZeneca ChAdOx1AZD1222 vaccine (AZ)/ Pfizer/BioNTech BNT162b2 vaccine (PF) and whether this can be provided with half dose so that the vaccination coverage is going to be higher in spite of limited vaccine supplies.~A number of studies have proved that COVID-19 vaccines are effective at preventing people from getting severe COVID-19 disease. However, the vaccines do not only reduce the chance of infection, but they also help to mitigate disease severity.~Study population: Male and female adults aged equal or more than 20 years who received two doses of Inactivated COVID-19 vaccine developed by Sinovac (given at 21-28 days apart) at different intervals of 60 to less than 90 days, 90 to less than120 days and 120 to 180 days~This prospective, multi-center, randomized, observer-blind Phase 2 study, A total of 1320 participants will be divided into 2 groups (660 each) receiving either full dose or half dose of either AZ or PF.~Each group is further stratified into 3 subgroups according to three interval duration in term of days after second dose of SV for 60-less than 90 days, 90-less than120 days and 120-180 days respectively. Subjects who fulfilled eligibility criteria will be randomly assigned to receive either full dose or half dose of AZ or PF in 1:1 ratio as an IM injection in the deltoid muscle at Visit 1 (V1).~All participants will be randomized based on dose given either full dose or half dose and further stratify accordingly by Interactive web-based response system (IWRS). There will be unblinded team which consists of pharmacist and nurse who will give injection. All the safety assessment will be performed independently by clinical team.~Subjects will be follow-up for assessing immunity at day 28 (V3), day 60 (V4) and day 90 (V5) and for safety at day 7 (V2), day 28 (V3), day 60 (V4) and day 90 (V5).
This prospective, multi-center, randomized, observer-blind Phase 2 study. A total of 1320 participants will be divided into 2 groups (660 each) receiving either full dose or half dose of either AZ or PF.~Each group is further stratified into 3 subgroups according to three interval duration in term of days after second dose of SV for 60 to less than 90 days, 90 to less than120 days and 120 to 180 days. Each group will be randomized to receive either AZ or PF in 1:1 ratio.~Subjects who fulfilled eligibility criteria will be randomly assigned to receive either full dose or half dose of AZ or PF in 1:1 ratio as an IM injection in the deltoid muscle at Visit 1 (V1). Subjects will be follow-up for assessing immunity at day 28 (V3), day 60 (V4) and day 90 (V5) and for safety at day 7 (V2), day 28 (V3), day 60 (V4) and day 90 (V5). At least 50% from each subgroup will be randomly selected to provide additional blood at baseline (V1, day 0) and day 28 (V3) to be used for assessment of T-cell-mediated immunity (CMI)