Study procedure
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Initially, 10 participants were randomly assigned into 4 groups each to receive two homologous or heterologous ID doses of CoronaVac (Sinovac), ChAdOx1 nCoV-19 (AstraZeneca), or BNT162b2 (Pfizer-BioNTech) 7 days apart. The vaccine regimens (first-second dose) for each arm were: BNT162b2-BNT162b2 (Group 1), ChAdOx1-BNT162b2 (Group 2), CoronaVac-ChAdOx1 (Group 3), and ChAdOx1-ChAdOx1 (Group 4). Participants received two injections of each vaccination, one in each arm within the deltoid region. The dosage for each injection was 10-20% of conventional IM doses based on previously published literature (Preliminary analyses of anti-RBD IgG were performed for all regimens and participants. Evidence of positive anti-NP or anti-RBG IgG at baseline were excluded. In the extended phase, additional participants were recruited to the two groups (10 per regimen) with the highest anti-RBD IgG measured 2 weeks after the second vaccination to meet statistical power. Additional blood sample were collected 12 weeks after the second dose for these two groups to evaluate the persistence of anti-RBG IgG.