The trial will be conducted in two stages (Stage IIb and Stage III).~• Stage IIb~Participants will be vaccinated one dose or two doses of the Convacell vaccine. Investigators will compare one dose and two doses groups and will choose the best vaccine dosage regimen in terms of immunogenicity and safety.~• Stage III~Participants will be vaccinated the Convacell vaccine or Placebo. Investigators will use the dosage regimen chosen in IIb stage.
The goal of this clinical trial is to assess the immunogenicity, efficacy and safety of the Convacell vaccine in healthy adult volunteers aged 18 years and older. The main questions it aims to answer are:~To assess the immunogenicity and safety of single and double dose intramuscular administration of the Convacell vaccine;~To assess the epidemiological effectiveness of the Convacell vaccine in the prevention of SARS-CoV-2 infection and development of severe COVID-19 compared with placebo when single or double intramuscular injection.