Safety concerns of COVID-19 ChAdOx1 nCoV-19 vaccine have led to a recommendation of heterologous boost with mRNA vaccine in several countries. Initial safety data of heterologous prime-boost with ChAdOx1 nCoV-19 and BNT162b2 vaccines showed well-tolerated. No major differences in reactogenicity were observed between homologous and heterologous vaccination. Immunogenicity of heterologous ChAdOx1 nCoV-19 and BNT162b2 vaccination is better compared with homologous ChAdOx1 nCoV-19 vaccination. Despite the scarcity of safety and immunogenicity of heterologous ChAdOx1 nCoV-19 and mRNA 1273 vaccination, the preliminary reports revealed similar findings. Although both T and B memory cells contribute to the protection, there is evidence that neutralizing serum antibody levels have high correlates of protection for symptomatic SARS-COV-2 infection. The SARS-COV-2 specific antibody and neutralizing antibody level induced by adenovirus vector vaccines were lower than mRNA vaccines. Vaccine efficacy of ChAdOx1 nCoV-19 was lower than BNT162b2 and mRNA 1273 in clinical trials. In the real world, the ChAdOx1 nCoV-19 vaccine appeared less effective than the BNT162b2 and mRNA1273 vaccine in preventing SARS-COV-2 infection. The emergence of highly transmissible and mutant variants of SARS-COV-2 has raised the concern of COVID-19 vaccine effectiveness. The S protein of the widespread SARS-COV-2 delta variant reduced the binding of neutralizing antibodies. Breakthrough infections are increasing worldwide. Vaccine effectiveness decreased significantly after the emergence of the SARS-COV-2 delta variant. Waning immunity is another potential cause of breakthrough infection. Centers for Disease Control of the United States has advocated 3rd booster dose of mRNA vaccine for people at risk of COVID-19 and those who work in high-risk settings including healthcare workers. Israel expanded its 3rd COVID-19 vaccine boost to those aged over 12 years old. The complete vaccination rate is still low in Taiwan. Strict border control measures are imposed by Central Epidemic Command Center. However, the measure of quarantine for flight crew is considered one of the breaches of COVID-19 infection control. Despite most of the flight crew has fully vaccinated, several episodes of breakthrough infection occurred among flight crew resulting in domestic infection recently. Low neutralizing antibody was found in a proportion of fully vaccinated flight crew and healthcare workers. A 3rd booster COVID-19 dose is considered for flight crew and healthcare workers. This study is to determine the safety, reactogenicity, and immunogenicity of heterologous 3rd booster of mRNA and protein COVID-19 vaccines.

With waning vaccine induced immunity and emergence of potent immune evasion SARS-CoV-2 omicron variant, previous virus and vaccine-induced immunity have failed to prevent transmission. Build-up of herd immunity through vaccination is no longer achievable so far. On the other hand, SARS- CoV-2 omicron induced immunity has been shown low cross-protective against other variants. Omicron breakthrough infection can boost existing immunity induced by vaccination. The neutralizing titer elicited by omicron breakthrough infection are much higher than that induced by 3 doses of mRNA vaccines. Hybrid immunity by vaccination and omicron breakthrough infection can elicit effective cross-neutralizing antibodies against most variants.

More than 10% of population in Taiwan have infected with omicron variant. Second booster vaccination is being advocated for health care workers. The study of dynamics of immunogenicity induced by third dose heterologous booster vaccination needs to be revised. Immunogenicity including SARS-CoV-2 anti- spike IgG, the 50% neutralizing antibody titer and IFN-secreting T cells specific to whole spike protein will be performed at 180 days after booster vaccination as schedule. Additional memory T cell epitopes and memory B cell epitopes will be studied. Some participants have been infected with the omicron variant. We will compare differences of memory T cell epitopes and memory B cell epitopes between uninfected participants and omicron breakthrough infected participants. Besides, second booster vaccination is planning to be administered in health care workers. We will compare differences of memory T cell epitopes and memory B cell epitopes between 2 boosters vaccinated participants and SARS-CoV-2 breakthrough infected participants. Immunogenicity study will be conducted 28 days after the second booster vaccination and 28 days after SARS-CoV-2 breakthrough infection. Immunogenicity study, including SARS-CoV-2 anti- spike IgG, the 50% neutralizing antibody titer (NT50) and IFN-secreting T cells specific to whole spike protein, will be compared between participants with 2 booster vaccination and SARS-CoV-2 breakthrough infection.