B-092 Humoral Immune Response after Prime and Boost Vaccination against SARS-CoV-2: a Follow-up Study among Healthcare Workers

Abstract Background Vaccination is an efficient strategy to control the COVID-19. In Korea, vaccine distribution started with ChAdOx1 and followed by BNT162b2. An option to obtain a booster dose with BNT162b2 was later offered to people fully inoculated with prime vaccines. There were few simultaneous and comparative real-world data for these booster regimens in Korea. This study aimed to evaluate the humoral immune responses after these vaccination regimens. Methods In this prospective observational cohort study to measure SARS-CoV-2 anti-spike receptor binding domain IgG and neutralizing antibody titers, healthcare workers (HCWs) receiving routine COVID-19 vaccinations were recruited from two hospitals in Gwangju, Korea. Participants were recruited and voluntarily came for testing after vaccination on the scheduled visit day. Demographics, underlying diseases or medical histories, and side reactions after the first dose of prime vaccination were collected by questionnaires. Antibody levels were compared between ‘prime ChAdOx1-boost BNT162b2’ and ‘prime BNT162b2-boost BNT162b2’ groups over time. Results Recruitment, sampling, and data collection were performed from May 2021 to June 2022. At 180 days after prime vaccination, the antibody titers decreased more in ChAdOx1 vaccinated than in the BNT162b2 vaccinated. Following the booster vaccination with BNT162b2, both groups experienced a significant increase in antibody titers, reaching comparable levels. At 90 days after the boost, the titers in the ChAdOx1 group decreased more than those in the BNT162b2 group, but they were comparable at day 180. The antibody titers in the infected group after the boost vaccination for both regimens increased to a comparable level by day 180. However, the titers in the uninfected group on day 180 differed between the two regimens, showing lower levels in the heterologous regimen. Conclusion The BNT162b2 booster vaccine can enhance the lower immune response generated by the ChAdOx1 prime vaccination, bringing it to a level comparable to that with the BNT162b2 prime vaccination. However, it is important to note that the immune response associated with the heterologous regimen tends to decline more rapidly than that of the homologous regimen. This insight can guide future vaccination approaches to ensure optimal immune protection.