Introduction
RBD
CORONAVIRUS, SEVERE ACUTE RESPIRATORY SYNDROME
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continuously evolves to acquire mutations that may change its infectivity and antigenicity. Since the beginning of SARS-CoV-2 pandemic, several variants with increased transmissibility or immune escape capability have emerged, such as Beta, Gamma, Delta, and Omicron.The receptor-binding domain (RBD) of spike (S) protein directly interacts with the receptor of the host cells, and several studies have indicated that more than 90% of the neutralization in the serum of both convalescent and vaccinated individuals targets the RBD.Guided by structural and computational analyses, we have designed a trimeric RBD vaccine, named NVSI-06-07, in which three homologous RBDs derived from the prototype SARS-CoV-2 strain were connected tandemly into a trimeric structure (Supplementary Fig. Here we reported the interim analysis results of a randomized, double-blind, controlled phase 2 trial conducted in the UAE to evaluate the safety and immunogenicity of NVSI-06-09 as a booster dose in the inactivated vaccine BBIBP-CorV recipients, using the homologous boost with one additional dose of BBIBP-CorV as the control arm. Considering the dominant prevalence of Omicron worldwide currently, immune responses against multiple Omicron sub-lineages were evaluated. Furthermore, as an exploratory study, the cross-neutralizing antibody responses against other VOCs were also detected to assess the broad-spectrum immune responses elicited by NVSI-06-09.