Introduction
More than 6.8 million people have died from COVID-19 worldwide since the start of the pandemic (More than 100 countries worldwide have already issued recommendations on booster or additional vaccination (In China, seven COVID-19 vaccines have been authorized for use, including five inactivated vaccines, an adenovirus-vectored vaccine (Ad5-nCoV, Convidecia) and a recombinant protein subunit vaccine (ZF2001, Zifivax). These vaccines have been shown to be efficacious in preventing mild to severe COVID-19 (Booster vaccination strategies based on inactivated vaccine priming have been well studied, and heterologous vaccination regimens induce immune responses that are superior to those induced by homologous regimens (To optimize the booster vaccination regimen in persons who have received two doses of inactivated vaccines in China, we performed a head-to-head immunological comparison of the COVID-19 vaccines available in China, including intramuscular Ad5-nCoV, aerosolized Ad5-nCoV, a recombinant protein subunit vaccine (ZF2001) and homologous CoronaVac booster administration in inactivated vaccine-primed recipients who were vaccinated 6 months prior.