The Safety and Immunogenicity of SARS-CoV-2 mRNA Vaccine (SYS6006) as Heterologous Booster in Participants Aged 18 Years and Older Vaccinated With Inactivated SARS-CoV-2 Vaccine

This study plans to enrol 1000 participants 18 years and above, with ≥10% participants ≥60 years old. According to SARS-CoV-2 vaccine vaccination history, they will be evenly divided into 2 groups, Group A and Group B. Group A: will enrol 500 participants who have received 2 doses of inactivated SARS-CoV-2 vaccine according to national immunization planning, whose last dose was given at least 6 months ago. They will be given one dose of the study vaccine (SYS6006) after enrolment. Group B: will enrol 500 participants who have received 3 doses of inactivated SARS-CoV-2 vaccine according to national immunization planning, whose last dose was given at least 6 months ago. They will be given one dose of the study vaccine (SYS6006) after enrolment. The study is devided into two stages, the first stage will enrol 200 participants with 100 in Group A and 100 in Group B. They will undergo laboratory examination, immunogenicity observation and safety observation. The first 30 participants in each group will take extra cellular immune testing; the second stage will enrol the remaining 800 participants for safety observation.