EuCorVac-19 (ECV-19) is a recombinant receptor binding domain (RBD) COVID-19 vaccine that displays the RBD (derived from the SARS-CoV-2 Wuhan strain) on immunogenic liposomes. This study compares the safety and immunogenicity of ECV-19 to the COVISHIELD^TM (CS) adenoviral-vectored vaccine. Interim analysis is presented of a randomized, observer-blind, immunobridging Phase 3 trial in the Philippines in 2600 subjects, with treatment and biospecimen collection between October 2022 and January 2023. Healthy male and female adults who received investigational vaccines were 18 years and older, and randomly assigned to ECV-19 (n = 2004) or CS (n = 596) groups. Immunization followed a two-injection, intramuscular regimen with 4 weeks between prime and boost vaccination. Safety endpoints were assessed in all participants and immunogenicity analysis was carried out in a subset (n = 585 in ECV-19 and n = 290 in CS groups). The primary immunological endpoints were superiority of neutralizing antibody response, as well as noninferiority in seroresponse rate (defined as a 4-fold increase in RBD antibody titers from baseline). After prime vaccination, ECV-19 had a lower incidence of local solicited adverse events (AEs) (12.0% vs. 15.8%, p < 0.01), and solicited systemic AEs (13.1 vs. 17.4%, p < 0.01) relative to CS. After the second injection, both ECV-19 and CS had lower overall solicited AEs (7.8% vs. 7.6%). For immunological assessment, 98% of participants had prior COVID-19 exposure (based on the presence of anti-nucleocapsid antibodies) at the time of the initial immunization, without differing baseline antibody levels or microneutralization (MN) titers against the Wuhan strain in the two groups. After prime vaccination, ECV-19 induced higher anti-RBD IgG relative to CS (1,464 vs. 355 BAU/mL, p < 0.001) and higher neutralizing antibody response (1,303 vs. 494 MN titer, p < 0.001). After boost vaccination, ECV-19 and CS maintained those levels of anti-RBD IgG (1367 vs. 344 BAU/mL, p < 0.001) and neutralizing antibodies (1128 vs. 469 MN titer, p < 0.001). ECV-19 also elicited antibodies that better neutralized the Omicron variant, compared to CS (763 vs. 373 MN titer, p < 0.001). Women displayed higher responses to both vaccines than men. The ECV-19 group had a greater seroresponse rate compared to CS (83% vs. 30%, p < 0.001). In summary, both ECV-19 and CS had favorable safety profiles, with ECV-19 showing diminished local and systemic solicited AE after prime immunization. ECV-19 had significantly greater immunogenicity in terms of anti-RBD IgG, neutralizing antibodies, and seroresponse rate. These data establish a relatively favorable safety and immunogenicity profile for ECV-19. The trial is registered on ClinicalTrials.gov (NCT05572879).

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