ABSTRACT
Background. SARS-CoV-2 ongoing pandemic and heterologous immunization approaches implemented worldwide for booster doses call for diversified vaccines portfolio. We report safety and immunogenicity of GRAd-COV2, a novel gorilla adenovirus-based COVID-19 vaccine, in a phase 2 trial aimed at identifying the appropriate dose and schedule. Method. 917 eligible adults aged 18 years or older, including participants with co-morbidities, were randomised to receive, 21 days apart, a single vaccine administration at 2x1011 viral particles (vp) followed by placebo, or repeated vaccine administration at 1x1011 vp, or two doses of placebo. Primary endpoints were the incidence of local and systemic solicited AEs for 7 days post each dose and the post-treatment (35 days after the first dose), geometric mean titers (GMTs) and geometric mean fold rise (GMFRs) of ELISA antibody responses to Spike protein. Additional humoral and cellular immune response parameters were monitored for up to six months. Results. The safety profile of GRAd-COV2 was characterized by short-term, mild-to-moderate pain and tenderness at injection site, fatigue, headache, malaise, and myalgia. Neither related SAEs nor deaths were reported. Humoral (binding and neutralizing) Ab responses peaked at day 35 after a single administration, were boosted by a second vaccination, were sustained until day 57 to then decline at day 180. Potent, VOC cross-reactive T cell responses peaked already after first dose with high frequencies of long-lived CD8 T cells. Conclusion. GRAd-COV2 was safe, and induced robust immune responses after a single immunization; the second administration increased humoral but not cellular immune responses.