Atici, SerkanSoysal, AhmetGonullu, ErdemAydemir, GokhanOner, NaciAlan, ServetKarabocuoglu, Metin2024-05-252024-05-25202300037-868210.1590/0037-8682-0046-20232-s2.0-85165719865https://doi.org/10.1590/0037-8682-0046-2023https://hdl.handle.net/20.500.14517/1105Gonullu, Erdem/0000-0002-6833-5646; ATICI, Serkan/0000-0002-3329-1866; Alan, Servet/0000-0002-8228-6350; karabocuoglu, metin/0000-0003-2854-0562Background: Heterologous COVID-19 booster vaccination is an alternative strategy to homologous vaccination, especially in developing countries, due to shortages, delays, or unequal distribution of COVID-19 vaccines. We compared cohorts vaccinated with different vaccine combinations to investigate whether a heterologous booster dose of mRNA-based BNT162b2 vaccine boosts the immune response in individuals primed with the CoronaVac vaccine. Methods: Anti-RBD IgG is generally measured 4 weeks after primary immunization and 4 weeks after booster vaccination. Data on anti-receptor-binding domain (anti-RBD) IgG antibody titers and clinical characteristics were provided by infection control units. Results: The highest median anti-RBD IgG antibody titers (14589 AU/mL) after primary immunization was observed in the group vaccinated with two doses of BNT162b2 vaccine. Antibody titers were lower 4 months or more after the second CoronaVac vaccine dose in CoronaVac recipients with or without previous COVID-19. In the homologous COVID-19 booster vaccine group (primed with two doses of CoronaVac 4 weeks apart and a single booster dose of CoronaVac) the median anti-RBD titers decreased from 1025 to 242 AU/mL before the booster dose. In the heterologous group (primed with two doses of CoronaVac 4 weeks apart and a single booster dose of BNT162b2), the median anti-RBD titer increased to 31624 AU/mL, a 132-fold increase, 16 days after the booster dose. Conclusions: After the second dose of CoronaVac, protective neutralizing antibody levels decrease over time, and a booster dose is required. Heterologous COVID-19 booster vaccination with BNT162b2 is effective at boosting neutralizing antibody levels.eninfo:eu-repo/semantics/openAccessCOVID-19 vaccinesCoronaVacBNT162b2Booster doseHeterologous vaccination scheduleHomologous vaccination scheduleArticleQ3Q356WOS:00108939440000137493731