The humoral and cellular immune response of a third dose adenoviral-based vectored vaccine after a single or 2 shots of inactivated COVID-19 vaccine in healthy adults.

The coronavirus pandemic is a severe infectious respiratory disease which caused massive loss worldwide. Thailand was affected by the wild-type, and the Delta variant started in mid-2021. Due to the shortage of effective vaccines, the ministry of public health of Thailand suggested the heterologous vaccine scheme, which comprises inactivated COVID-19 vaccine (CoronaVac) and an adenoviral-based vectored vaccine (ChAdOx1). However, data on the humoral and cellular immune responses of the single or 2 shots of inactivated vaccine followed by the adenoviral-based vaccine are very limited. In this current study, the sera from participants who received either single or 2 shots of CoronaVac followed by the ChAdOx1 vaccine were evaluated for SARS-CoV-2 spike receptor-binding-domain (RBD) IgG. The cytokine level was also assessed using Luminex immunoassay. The PBMC were collected to evaluate spike-specific T-cell and B-cell responses. Participants who received 2 shots of CoronaVac followed by ChAdOx1 possessed significantly (P<0.0001) higher levels of spike RBD-specific IgG. They also exhibited a higher level of CD4+T-cell and IFN-gamma than those who received only 1 shot of CoronaVac followed by the ChAdOx1 vaccine. The volunteers who received two shots of CoronaVac followed by ChAdOx1 had a significantly (p<0.01) higher marginal B-cell response against wild-type SARS-CoV-2 S peptides than those who received only one shot of CoronaVac followed by ChAdOx1. Surprisingly, the class switch B-cell response to Delta variant SARS-CoV-2 S peptides of the volunteers who received 1 shot of CoronaVac followed by ChAdOx1 was significantly (p<0.01) higher than those who received 2 shots of CoronaVac. However, participants who received only a single shot of inactivated vaccine followed by an adenoviral-based vectored vaccine possessed a higher level of TNF-alpha and IL-6. This study indicated that boosting the ChAdOx1 as a third dose after completing 2 shots of CoronaVac induced strong humoral and cellular immune responses.