This is a Phase II, prospective, randomized, observer-blinded, multi-center study, to evaluate the safety, tolerability, and immunogenicity of a booster vaccination with AZD1222, mRNA-1273, or MVC-COV1901 vaccine. Approximately 960 participants aged 18 ~ < 80 years, who received homologous two doses of vaccines 150 ~ 365 days ago, will be enrolled and divided into three groups. Each group will consist of 320 eligible subjects, and for each group the randomization will be stratified according to study site and age to four treatments (AZD1222, half dose of mRNA-1273, full dose or half dose of MVC-COV1901 in 1:1:1:1 ratio). Therefore, within a group, for either age stratum, there will be at least 30 participants for each treatment.
The purpose of this study is to assess the safety, tolerability, and immunogenicity of booster dose of vaccine in participants who are generally healthy or with stable pre-existing health conditions. Study details include:~The study duration per participant will be approximately 209 days (28 days screening, 1 day vaccination, and 180 days follow-up).~The treatment will include 1 booster dose only.~The visit frequency will be 6 on-site visits and 1 phone visit.