This is a multicenter, parallel groups, partially randomized, open-label, blank-controlled adaptive platform study to evaluate the effectiveness of a second COVID-19 vaccine booster in Chinese adults who are charactered as the majority of whom with 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 with underlying diseases (history of underlying medical conditions diagnosed by a clinician, including hypertension, diabetes, heart disease, etc). The eligible participants with an interval ≥ 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 who are not willing to receive the second booster but are consent to participate the surveillance for COVID-19, will be included as a blank control. Informed consent will be acquired from eligible participants. Other participants who are willing to receive the second booster and participate the surveillance for COVID-19, will be randomly allocated in a ratio of 1: k (k is the number of vaccine types) to the different investigational vaccines, stratified according to age and history of COVID-19 infection. The symptomatic COVID-19 cases will be reported and documented in both the investigational and control groups. The occurrence of serious adverse events within 6 months after vaccination will be observed. Moreover, blood and nasal mucosa samples will be collected on the day 0 before 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 collected once for all participants on the day of enrollment.

Inclusion Criteria:

1. Adults aged 18 years and over, including the elderly over 60 years and those with underlying diseases.
2. Volunteers are able and willing to comply with the requirements of the clinical trial protocol and sign the informed consent form.
3. ≥ 4 months from the last SARS-CoV-2 infection (or never been infected), and 6 months or more from the first booster immunization of the COVID-19 vaccine.

Exclusion Criteria:

1. Volunteers have suspected symptoms of COVID-19 when enrolled, such as dry throat, sore throat, cough, etc.
2. The COVID-19 Antigen Quick Test Kit is positive when volunteers are enrolled.
3. Fever, temperature > 37.0°C.
4. Have received a second COVID-19 vaccine booster immunization.
5. Have a history of serious adverse reactions related to the vaccine and/or have a history of severe allergic reactions to any component of the investigational vaccine (only applicable to the vaccine groups).
6. Pregnant or lactating women.
7. HIV infection, tuberculosis, low immunity caused by disease or long-term medication.
8. Acute disease or acute onset of chronic disease.
9. Epilepsy and other progressive neurological disorders.
10. Other situations that are not suitable for participating in this research, according to the judgment of the researcher.