Objectives | Outcome measures | Timepoint(s) |
---|---|---|
Primary objective | ||
 To determine whether heterologous prime-boost-boost COVID-19 vaccine regimens lead to non-inferior humoral immunity compared with homologous prime-boost-boost vaccine regimen against wildtype SARS-CoV-2 and/or 1≥ VOC | Level of SARS-CoV-2 anti-spike immunoglobulins | Day 28 |
Secondary objectives | ||
 To determine whether heterologous prime-boost-boost COVID-19 vaccine regimens lead to non-inferior humoral and cellular immunity compared with homologous prime-boost-boost vaccine regimen against wildtype SARS-CoV-2 and/or 1≥ VOC | Level of SARS-CoV-2 anti-spike immunoglobulins Level of SARS-CoV-2 neutralizing antibodies Quantitative T cell responses to spike proteins | Days 1, 7, 180, 360 Days 1, 7, 28, 180, 360 Days 1, 7, 28, 180, 360 |
 To assess the reactogenicity and safety of heterologous and homologous prime-boost-boost COVID-19 vaccine schedules | Solicited local and systemic reaction Changes from baseline in laboratory safety measures Unsolicited adverse events (AEs) Serious adverse events (SAEs), AEs of special interest (e.g., myocarditis, pericarditis), medically attended AEs | Day 7 Day 7 Day 28 Throughout the study |
Exploratory objectives | ||
 To determine whether vaccine efficacy differs between heterologous and homologous prime-boost-boost regimens | PCR-confirmed COVID-19 infections as recognized by the Ministry of Health, Singapore | Throughout the study |
To determine whether the administered mRNA vaccine (control group and intervention group 1 only) can be detected in the blood at ~1 week post-vaccination | qRT-PCR using primers targeting the sequence of the mRNA for SARS-CoV-2 spike protein | Day 7 |