A comparative analysis of vaccine immunity induced by heterologous booster with Ad5-nCoV via different routes of administration

Heterologous boost COVID-19 vaccination can solved the problem of decreased efficacy caused by single dose of vaccine. Heterologous booster with adenoviral-vectored COVID-19 vaccine (Ad5-nCoV) following primary immunization with inactivated COVID-19 vaccines is a widely-used vaccination strategy in clinic, while different routes of Ad5-nCoV administration exist and pose a question which route could be more optimal. In this study, we comprehensively evaluated and compared the vaccine immunity induced in mice immunized according to three different vaccination regimens: “3×phosphate buffered solution(3× PBS)”, “2×inactivated vaccine + 1×inactivated vaccine (3×INA)”, “2×inactivated vaccine + 1×Ad5-nCoV (intramuscular)2×INA+Ad5(im)”and“2×inactivated vaccine + 1×Ad5-nCoV (intranasal)2×INA+Ad5(in)”. We found that heterologous booster with Ad5-nCoV, irrespective of the route of administration, induced significantly higher levels of anti-Spike IgG and subclasses (IgG1and IgG2c), Spike-specific T cells, class-switched Spike⁺ memory B cells (MBCs) than homologous booster with 3^rd dose of inactivated COVID-19 vaccine. Of note, compared with the intramuscular given, intranasal given of Ad5-nCoV as a booster dose clearly induced higher levels of serum and bronchoalveolar bavage fluid anti-spike immunoglobulin A, and moreover, induced stronger infiltration of major innate effector cells like neutrophils, natural killer cells and dendritic cells into the lung tissue, which suggested that mucosal vaccine responses are generated upon intranasal booster with Ad5-nCoV. Altogether, our study analyzed the vaccine immunity induced by different COVID-19 vaccines administered using different regimens, which may guide the clinical use of other types of prophylactic vaccines aiming to mount improved vaccine responses.