This is a multicenter, parallel groups, partially randomized, open-label,blank-controlled adaptive platform study to evaluate the effectiveness of a secondCOVID-19 vaccine booster in Chinese adults who are charactered as the majority of whomwith hybrid immunity of COVID-19 vaccination and COVID-19 breakthrough infection.Individuals aged 18 years and over, include the elderly over 60 years old or those withunderlying diseases (history of underlying medical conditions diagnosed by a clinician,including hypertension, diabetes, heart disease, etc). The eligible participants with aninterval ≥ 4 months after previous SARS-CoV-2 infection (or had never been infected) and≥ 6 months from the first COVID-19 vaccine booster will be recruited. Participants whoare not willing to receive the second booster but are consent to participate thesurveillance for COVID-19, will be included as a blank control. Informed consent will beacquired from eligible participants. Other participants who are willing to receive thesecond booster and participate the surveillance for COVID-19, will be randomly allocatedin a ratio of 1: k (k is the number of vaccine types) to the different investigationalvaccines, stratified according to age and history of COVID-19 infection. The symptomaticCOVID-19 cases will be reported and documented in both the investigational and controlgroups. The occurrence of serious adverse events within 6 months after vaccination willbe observed. Moreover, blood and nasal mucosa samples will be collected on the day 0before and day 14, month 3 and 6 after the booster vaccination in a subgroup for humoral,cellular and mucosal immunogenicity analysis. Moreover, oral specimens will be collectedonce for all participants on the day of enrollment.