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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, double-blinded, 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., Competing Interests: Declaration of competing interest CL, Yang HY, Zhao MS and Qiu YZ are the full-time employees of CSPC Megalith Biopharmaceutical Co., Ltd. Li CL and Yang HY are the stock owners. All other authors declare no competing interests., (Copyright © 2024. Published by Elsevier Ltd.)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mRNA vaccine enables quick upgrade of antigen sequence to combat emerging new variants. In an observer-blinded, randomized, placebo-controlled phase 2 trial, immunologically naïve 300 adults and 150 older participants were enrolled and randomized (1:1:1) to receive two doses of 20 µg or 30 µg of a SARS-CoV-2 mRNA vaccine (SYS6006) or placebo. Adverse events (AEs) were recorded through 30 days after the second dose. Live virus neutralizing antibody (Nab), S1 protein-specific binding antibody (S1-IgG) and cellular immunity were tested. Results showed that robust wild-type Nab response was elicited with geometric mean titers of 91.3 and 84.9 in the adults, and 74.0 and 115.9 in the elders, 14 days following the second dose (Day 35) in the 20-µg and 30-µg groups, respectively. All seroconverted for wild-type Nab except two participants. Nab against Omicron BA.5 was mild. Robust wild-type S1-IgG response was induced with geometric mean concentrations of 2751.0 and 3142.2 BAU/mL in adults, and 2474.1 and 2993.5 BAU/mL in elders at Day 35 in the 20-µg and 30-µg groups, respectively. S1-IgG against Omicron BA.2 was induced. Cellular immunity was elicited, particularly in enzyme-linked immunospot assay. The most frequent AEs were injection-site pain and fever. Most reported AEs were grade 1 or grade 2. The AE incidences were similar following the first dose and second dose. No vaccination-associated serious AE was reported. In conclusion, two-dose vaccination with SYS6006 demonstrated good safety, tolerability and immunogenicity in immunologically naïve healthy participants aged 18 years or more., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yuliang Zhao, Qi Li reports financial support was provided by Hebei Provincial Key Research Project. Yuanzheng Qiu, Chengye Cai, Shaonan Ni, Maosheng Zhao, Lixian Guo, Li Ji, Hanyu Yang, Chunlei Li reports a relationship with CSPC Megalith that includes: employment and equity or stocks., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Vaccination plays a key role in preventing morbidity and mortality caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We aimed to evaluate the safety and immunogenicity of a SARS-CoV-2 messenger ribonucleic acid (mRNA) vaccine SYS6006. In the two randomized, observer-blinded, placebo-controlled phase 1 trials, 40 adult participants aged 18-59 years and 40 elderly participants aged 60 years or more were randomized to receive two doses of SYS6006 or placebo (saline). Adverse events (AEs) were collected through 30 days post the second vaccination. Immunogenicity was assessed by live-virus neutralizing antibody (Nab), spike protein (S1) binding antibody (S1-IgG), and cellular immunity. The result showed that 7/15, 9/15 and 4/10 adult participants, and 9/15, 8/15 and 4/10 elderly participants reported at least one AE in the 20-µg, 30-µg and placebo groups, respectively. Most AEs were grade 1. Injection-site pain was the most common AE. Two adults and one elder reported fever. No vaccination-related serious AE was reported. SYS6006 elicited wild-type Nab response with a peak geometric mean titer of 232.1 and 130.6 (adults), and 48.7 and 66.7 (elders), in the 20-µg and 30-µg groups, respectively. SYS6006 induced moderate-to-robust Nab response against Delta, and slight Nab response against Omicron BA.2 and BA.5. Robust IgG response against wild type and BA.2 was observed. Cellular immune response was induced. In conclusion, two-dose primary vaccination with SYS6006 demonstrated good safety and immunogenicity during a follow-up period of 51 days in immunologically naive population aged 18 years or more. (Trial registry: Chictr.org.cn ChiCTR2200059103 and ChiCTR2200059104).