Safety and immunogenicity of a recombinant adenovirus type-5-vectored COVID-19 vaccine with a homologous prime-boost regimen in healthy participants aged 6 years and above: a randomised, double-blind, placebo-controlled, phase 2b trial.

Background We assessed the safety and immunogenicity of a recombinant adenovirus type-5 (Ad5)-vectored COVID-19 vaccine with homologous prime-boost regimens in healthy participants aged 6 years and above. Methods In this randomised, double-blind, placebo-controlled trial, participants received low-dose vaccine, middle-dose vaccine or placebo. Prime-booster regimens were given intramuscularly 56 days apart. ELISA antibodies to the receptor binding domain (RBD) and pseudovirus neutralising antibodies were detected. Adverse events were monitored for 28 days following each vaccination. Results A total of 430 participants were enrolled in the study, with 30 participants aged 18-55 years (MID cohort), 250 participants aged 56 years and older (OLD cohort), and 150 participants aged 6-17 years (MIN cohort). Ad5-vectored COVID-19 vaccine induced significant RBD-specific ELISA antibodies which decreased with increasing age, with geometric mean titres (GMTs) of 1037.5 in MIN cohort, 647.2 in MID cohort, and 338.0 in OLD cohort receiving 5×10 10 viral particles on day 28 following boost vaccination. Pseudovirus neutralising antibodies showed a similar pattern, with GMTs of 168.0 in MIN cohort, 76.8 in MID cohort, and 79.7 in OLD cohort. A single dose in children and adolescents induced higher antibody responses than that elicited by two doses in adults, with GMTs of 1091.6 and 96.6 in ELISA antibody and neutralising antibody, respectively. Homologous prime-boost vaccination was safety and tolerable. Conclusions Ad5-vectored COVID-19 vaccine with a single dose was safe and induced robust immune responses in children and adolescents aged 6-17 years. A prime-boost regimen needs further exploration for Ad5-vectored COVID-19 vaccine.