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Continuous emergence of new variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), enhanced transmissibility, significant immune escape, and waning immunity call for booster vaccination. We evaluated the safety, immunogenicity, and efficacy of heterologous booster with a SARS-CoV-2 mRNA vaccine SYS6006 versus an active control vaccine in a randomized, open-label, active-controlled phase 3 trial in healthy adults aged 18 years or more who had received two or three doses of SARS-CoV-2 inactivated vaccine in China. The trial started in December 2022 and lasted for 6 months. The participants were randomized (overall ratio: 3:1) to receive one dose of SYS6006 ( N  = 2999) or an ancestral receptor binding region-based, alum-adjuvanted recombinant protein SARS-CoV-2 vaccine ( N  = 1000), including 520 participants in an immunogenicity subgroup. SYS6006 boosting showed good safety profiles with most AEs being grade 1 or 2, and induced robust wild-type and Omicron BA.5 neutralizing antibody response on Days 14 and 28, demonstrating immunogenicity superiority versus the control vaccine and meeting the primary objective. The relative vaccine efficacy against COVID-19 of any severity was 51.6% (95% CI, 35.5-63.7) for any variant, 66.8% (48.6-78.5) for BA.5, and 37.7% (2.4-60.3) for XBB, from Day 7 through Month 6. In the vaccinated and infected hybrid immune participants, the relative vaccine efficacy was 68.4% (31.1-85.5) against COVID-19 of any severity caused by a second infection. All COVID-19 cases were mild. SYS6006 heterologous boosting demonstrated good safety, superior immunogenicity and high efficacy against BA.5-associated COVID-19, and protected against XBB-associated COVID-19, particularly in the hybrid immune population. Trial registration: Chinese Clinical Trial Registry: ChiCTR2200066941.

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.)

The emerging new variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) needs booster vaccination. We evaluated the long-term safety and immunogenicity of heterologous boosting with a SARS-CoV-2 messenger RNA vaccine SYS6006. A total of 1000 participants aged 18 years or more who had received two (Group A) or three (Group B) doses of SARS-CoV-2 inactivated vaccine were enrolled and vaccinated with one dose of SYS6006 which was designed based on the prototype spike protein and introduced mutation sites. Adverse events (AEs) through 30 days and serious AEs during the study were collected. Live-virus and pseudovirus neutralizing antibody (Nab), binding antibody (immunoglobulin G [IgG]) and cellular immunity were tested through 180 days. Solicited all, injection-site and systemic AEs were reported by 618 (61.8%), 498 (49.8%), and 386 (38.6%) participants, respectively. Most AEs were grade 1. The two groups had similar safety profile. No vaccination-related SAEs were reported. Robust wild-type (WT) live-virus Nab response was elicited with peak geometric mean titers (GMTs) of 3769.5 (Group A) and 5994.7 (Group B) on day 14, corresponding to 1602.5- and 290.8-fold increase versus baseline, respectively. The BA.5 live-virus Nab GMTs were 87.7 (Group A) and 93.2 (Group B) on day 14. All participants seroconverted for WT live-virus Nab. Robust pseudovirus Nab and IgG responses to wild type and BA.5 were also elicited. ELISpot assay showed robust cellular immune response, which was not obviously affected by virus variation. In conclusion, SYS6006 heterologous boosting demonstrated long-term good safety and immunogenicity in participants who had received two or three doses of SARS-CoV-2 inactivated vaccine., (© 2024 Wiley Periodicals LLC.)

The development of vaccines that can efficiently prevent the infection of SARS-CoV-2 is necessary to fight the COVID-19 epidemic. mRNA vaccine has been proven to induce strong humoral and cellular immunity against SARS-CoV-2. Here, we studied the immunogenicity and protection efficacy of a novel mRNA vaccine SYS6006. High expression of mRNA molecules in 293T cells was detected. The initial and boost immunization with a 21-day interval was determined as an optimal strategy for SYS6006. Two rounds of immunization with SYS6006 were able to induce the neutralizing antibodies against the SARS-CoV-2 wild-type (WT) strain, and Delta and Omicron BA.2 variants in mice or non-human primates (NHPs). A3 rd round of vaccination could further enhance the titers of neutralization against Delta and Omicron variants. In vitro ELISpot assay showed that SYS6006 could induce memory B cell and T cell immunities specifically against SARS-CoV-2 in mice. FACS analysis indicated that SYS6006 successfully induced SARS-CoV-2-specific activation of T follicular helper cell (Tfh) and Th1 cell, and did not induce CD4 + Th2 response in NHPs. SYS6006 vaccine could significantly reduce the viral RNA loads and prevent lung lesions in Delta variant infected hACE2 transgenic mice. Therefore, SYS6006 could provide significant immune protection against SARS-CoV-2., Competing Interests: Authors BK, HY, YW, YL, LL, YS, XW and MD were employed by company CSPC Pharmaceutical Group Co., Ltd. The authors from CSPC Pharmaceutical Group have ownership interests. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Xu, Lei, Kang, Yang, Wang, Lu, Lv, Sun, Zhang, Wang, Yang, Dan and Wu.)