Discussion
T-cell immunity, infection, dimer vaccine
INFECTION
In China, where more than 95% of individuals vaccinated against COVID-19 received inactivated vaccines, we evaluated the immunogenicity of homologous and heterologous boosters in adults who received prime vaccination with two doses of the inactivated COVID-19 vaccine approximately 6 months prior. Both homologous and heterologous booster vaccination led to an increase in levels of spike RBD-specific binding antibodies, neutralizing antibodies, the B-cell response and T-cell responses from day 14 after booster vaccination, but these increases were highest in participants who received heterologous regimens with an adenovirus-based COVID-19 vaccine.Booster vaccination with the Ad5-nCoV vaccine induced a superior T-cell response and neutralizing antibody responses compared to those induced by the homologous inactivated vaccine booster or heterologous recombinant protein vaccine booster, regardless of whether intramuscular injection or aerosol inhalation was used (The durability of the humoral response is crucial for COVID-19 vaccines. In our study, participants who received two-dose of CoronaVac 6 months ago showed a low seropositivity rate for neutralizing antibodies against SARS-CoV-2, which is similar to other findings (The highly transmissible Omicron variant severely impairs the neutralizing activity of priming two-dose COVID vaccines, with a more than 10-fold reduction compared to that with the wild-type strain (T-cell immunity is required for viral clearance and supports the generation and maintenance of high-affinity antibodies (In the present study, intramuscular or aerosolized Ad5-nCoV-induced neutralizing antibodies and T-cell responses after booster vaccination were significantly higher than those from the recombinant RBD dimer vaccine ZF2001. However, ZF2001 booster vaccination induced 2-fold more neutralizing antibodies than the homologous inactivated vaccine booster. Low cellular responses were detected with the aluminum-adjuvanted recombinant protein and inactivated COVID-19 vaccine. In the case of ChAdOx1 nCoV-19 and BNT162b1-primed vaccination, the immune responses upon booster vaccination with the recombinant protein vaccine (NVX-CoV2373) were also inferior to those upon administration of the adenovirus-vectored boosters, including neutralizing antibodies and cellular immune responses (So far, few studies have reported the longevity of antibody response to the Omicron variant boosted by SARS-CoV-2 vaccines. Immune recall by a homologous 3rd dose of mRNA vaccine in COVID-naive vaccinees greatly enhanced great and stable Omicron-specific antibody response (There are some limitations to this study. First, this study only evaluated the immune response of different boosters in young male subjects, although similar results have been observed in subjects of different ages and sexes (In summary, in the face of waning immunity and the circulation of SARS-CoV-2 variants, the mucosal IgA, neutralizing antibody and T-cell responses were boosted most efficiently with aerosolized Ad5-nCoV in those who received inactivated vaccines as initial doses. Aerosolized Ad5-nCoV booster probably provides a new tool against infection and transmission of the SARS-CoV-2 Omicron variant.