Immunogenicity and Safety of Heterologous and Homologous Boosting With ChAdOx1-S and CoronaVac or a Formulation of SCB-2019 (COVID-19)

SARS-CoV-2 is a highly transmissible and pathogenic coronavirus that emerged in late 2019 and has caused a pandemic of acute respiratory disease, collectively called coronavirus disease-19 (COVID-19). SARS-CoV-2 has a high transmission rate, and severe cases of COVID-19 require admission to hospital intensive care units with the need for mechanical ventilation and associated high mortality. Currently cases continue to rise in many countries as the 'second and third waves' of SARS-CoV-2 infection evolve. The authorized vaccines and most vaccines in development are focused on the major antigenic target of the virus, the SARS-CoV-2 spike (S) protein. Authorization was granted in Brazil by ANVISA for the Fiocruz/Oxford-AstraZeneca ChAdOx1-S COVID-19 vaccine as a 2-dose homologous vaccination regimen, 28- to 84-days apart. Emergency Use Authorization (EUA) was also granted for Sinovac Biotech's CoronaVac vaccine as a 2-dose homologous vaccination regimen, 28 days apart. Further vaccines, using different platforms are approved or expected to be approved for use against SARS-CoV-2. Most of the vaccines are expected to be authorized as 2-dose, homologous vaccination series. SCB-2019 is Clover's adjuvanted recombinant SARS-CoV-2 trimeric S-protein subunit vaccine. The SCB-2019 antigen includes SARS-CoV-2 S protein as a trimer fused to Trimer-Tag and is produced in Chinese hamster ovary cells (CHO). SCB-2019 preserves the native trimeric structure of S-protein in the prefusion form and induces neutralizing antibodies to SARS-CoV-2. Trimer-Tag is derived from the fully-human C-propeptide domain of pro-collagen and is capable of self-trimerization, thus fusing any biologically-active proteins in-frame with Trimer-Tag. The resulting fusion proteins expressed in mammalian cells are secreted as disulfide bond-linked homotrimers. The immunogenicity and safety of different dose levels (3, 9, and 30 μg) SCB-2019 vaccine, administered as 2-dose regimen 21-days apart was assessed in