This is a multicenter, randomized, observer-blind, and parallel-controlled clinical study to evaluate the immune responses and safety profiles in adults (≥ 18 years) receiving investigational products (intramuscular injection or nebulized inhalation) ≥ 180 days after the immunization of 2 doses of BNT162b2 vaccines plus one dose of booster AZD1222 vaccine. This study will enroll about 30% participants aged 60 years and above.~Subjects who have completed the primary mRNA vaccine immunizations and the 1st booster AZD1222 immunization for more than 6 months will be randomized to receive a 2nd booster dose of Ad5-nCoV/O, Ad5-nCoV/O-IH or mRNA-based COVID-19 vaccine in a ratio of 2:2:1.
Subjects who have completed the primary mRNA vaccine immunizations and the 1st booster AZD1222 immunization for more than 6 months will be randomized to receive a 2nd booster dose of Ad5-nCoV/O, Ad5-nCoV/O-IH or mRNA-based COVID-19 vaccine in a ratio of 2:2:1.