Study aim: To evaluate the immunogenicity and safety of Omicron-Based CovIran Barkat vaccine as a third injection dose in vaccinated population over 18 years of age Design: Non-randomized clinical trial with a parallel design on 210 volunteers over the age of 18 with a history of two-dose vaccination with an inactivated vaccine Settings and conduct: This Non-randomized clinical trial study will be conducted on 210 volunteers aged ≥18 years at Eram Hotel and Eshragh vaccination centre. They will be followed up for safety, immunogenicity, any adverse events, and COVID-19 incidence. Participants/Inclusion and exclusion criteria: Main inclusion criteria: Aged over 18 years old, In the volunteer’s vaccination history, the interval between the first and second dose is 4 to 8 weeks and at least 3 months and at most 5 months have passed since the injection of the 2nd dose of the volunteer vaccine, willing to participate, able to understand, sign the informed consent, medically stable condition for the past 3 months/ Main exclusion criteria: Confirmed, suspected, or asymptomatic COVID-19, History of SARS-CoV-2 (documented rtPCR) after the second dose, any abnormal paraclinical findings, history of allergy to the vaccine, any neurologic disease, immunodeficiency, coagulopathy, psychiatric and other chronic diseases, receiving the live vaccine in 14 days before inoculation, receiving immunoglobulins or blood products in 3 months before inoculation, women with a positive Beta HCG or breastfeeding or pregnancy intention. Intervention groups: 1. 3rd injection of CovIran Barkat of Wuhan strain 2. 3rd injection of CovIran Barkat of Omicron strain, both with a history of receiving two doses of CovIran Barkat 3. 3rd injection of COVIran Barekat of Omicron strain, with a history of receiving two doses of Sinopharm Main outcome variables: Humoral immunity (Seroconversion, Neutralizing Ab, Anti-RBD, Anti-SPIKE)