Safety and immunogenicity of heterologous boosting with a bivalent SARS-CoV-2 mRNA vaccine (XBB.1.5/BQ.1) in Chinese participants aged 18 years or more: A randomised, double-blinded, active-controlled phase 1 trial

Continuous emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants urges the development of new vaccines. We assessed the safety and immunogenicity of SYS6006.32, a bivalent vaccine (XBB.1.5/BQ.1), in healthy adults who had received SARS-CoV-2 primary vaccination. In a randomised, doubleblinded, active-controlled trial, 200 participants were randomised to receive one dose of SYS6006.32 (N = 100) or a prototype-based, monovalent control vaccine SYS6006 (N = 100). Adverse events (AEs) were collected through the study. Immunogenicity was assessed by live -virus neutralising antibody (Nab) and pseudovirus Nab. 61 (61.0 %) and 60 (60.0 %) participants reported AE in the SYS6006.32 and SYS6006 groups, respectively. Most AEs were grade 1 or 2. Pain and fever were the most common injection -site and systemic AEs, respectively. No serious AEs were observed. SYS6006.32 heterologous boosting induced robust Nab responses against BA.5, XBB.1.5 and EG.5 with live -virus Nab geometric mean titres (GMTs) increased by 17.1-, 34.0-, and 48.0-fold, and pseudovirus Nab GMTs increased by 12.2-, 32.0-, and 35.1-fold, respectively, 14 days after vaccination. SYS6006.32 demonstrated a superior immunogenicity to SYS6006. SYS6006.32 also induced robust pseudovirus Nab responses against XBB.1.16, XBB.2.3, and BA.2.86, with GMTs 3- to 6-fold higher than those induced by SYS6006. In conclusion, SYS6006.32 showed good safety profile and superior immunogenicity to the monovalent vaccine SYS6006.