The novel coronavirus pneumonia (COVID-19) is an infectious disease caused by the infection of a novel coronavirus known as Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), which has resulted in millions of deaths. We aimed to evaluate the safety and immunogenicity of the COVID-19 mRNA vaccine (CS-2034, CanSino, Shanghai, China) in adults without COVID-19 infection from China.

This is a multicenter Phase I clinical trial with a randomized, double-blinded, dose-exploration, placebo-controlled design. The trial recruited 40 seronegative participants aged 18–59 years who had neither received any COVID-19 vaccine nor been infected before. They were divided into a low-dose group (administered with either the CS-2034 vaccine containing 30 μg of mRNA or a placebo of 0.3 ml type 5 adenovirus vector) and a high-dose group (administered with either the CS-2034 vaccine containing 50 μg of mRNA or a placebo of 0.5 ml type 5 adenovirus vector). Participants were randomly assigned in a 3:1 ratio to receive either the mRNA vaccine or a placebo on days 0 and 21 according to a two-dose immunization schedule. The first six participants in each dosage group were assigned as sentinel subjects. Participants were sequentially enrolled in a dose-escalation manner from low to high dose and from sentinel to non-sentinel subjects. Blood samples were collected from all participants on the day before the first dose (Day 0), the day before the second dose (day 21), 14 days after the second dose (day 35), and 28 days after the second dose (day 49) to evaluate the immunogenicity of the CS-2034 vaccine. Participants were monitored for safety throughout the 28-day follow-up period, including solicited adverse events, unsolicited adverse events, adverse events of special interest (AESI), and medically attended adverse events (MAE). This report focuses solely on the safety and immunogenicity analysis of adult participants aged 18–59 years, while the long-term phase of the study is still ongoing. This study is registered at ClinicalTrials.gov, NCT05373485.

During the period from May 17, 2022, to August 8, 2022, a total of 155 participants aged 18–59 years were screened for this study. Among them, 115 participants failed the screening process, and 40 participants were randomly enrolled (15 in the low-dose group, 15 in the high-dose group, and 10 in the placebo group). Throughout the 28-day follow-up period, the overall incidence of adverse reactions (related to vaccine administration) in the low-dose group, high-dose group, and placebo group was 93.33% (14/15), 100.00% (15/15), and 80.00% (8/10), respectively. There was a statistically significant difference in the incidence of local adverse reactions (soreness, pruritus, swelling at the injection site) among the low-dose group, high-dose group, and placebo group (P = 0.002). All adverse reactions were mainly of severity grade 1 (mild) or 2 (moderate), and no adverse events of severity grade 4 or higher occurred. Based on the analysis of Spike protein Receptor Binding Domain (S-RBD) IgG antibodies against the BA.1 strain, the seroconversion rates of antibodies at day 21 after the first dose were 86.67%, 93.33%, and 0.00% in the low-dose group, high-dose group, and placebo group, respectively. The geometric mean titer (GMT) of antibodies was 61.2(95%CI 35.3–106.2), 55.4(95%CI 36.3–84.4), and 15.0(95%CI 15.0–15.0), and the geometric mean fold increase (GMI) was 4.08(95%CI 2.35–7.08), 3.69(95%CI 2.42–5.63), and 1.00(95%CI 1.00–1.00) for each group. At day 28 after the full vaccination, the seroconversion rates of antibodies were 100.00%, 93.33%, and 0.00%, and the GMT of antibodies was 810.0(95%CI 511.4–1283.0), 832.2(95%CI 368.1–1881.6), and 15.0(95%CI 15.0–15.0), and the GMI was 54.00(95%CI 34.09–85.53), 55.48(95%CI 24.54–125.44), and 1.00(95%CI 1.00–1.00) for each group, respectively. Based on the analysis of CD3+/CD4+ cell cytokine response, the percentages of IL-2+, IL-4+, IFN-γ+, and TNF-α+ cells increased after 14 days and 28 days of full vaccination in both the low-dose group and high-dose group. The increase was most pronounced in the high-dose group.

At day 28 after the full vaccination, both the low-dose and the high-dose CS-2034 vaccine were able to induce the production of high titers of S-RBD IgG antibodies against the BA.1 strain. Adverse reactions in the low-dose and high-dose groups were mainly of severity grade 1 or 2, and no trial-limiting safety concerns were identified. These findings support further development of this vaccine.