Safety and immunogenicity of inactive vaccines as booster doses for COVID-19 in Türkiye: A randomized trial.

What is the context? The timing of the primary and booster doses for each vaccine differs.We aimed to evaluate the safety and immunogenicity of CoronaVac and TURKOVAC vaccines used as homologous booster dose after CoronaVac primary vaccination.What is new? The neutralizing antibody titers against the Wuhan variant decreased below 1/6- the seropositivity threshold value- in more than 55% of the participants 4 months after administration of two doses of CoronaVac vaccine.Immunogenicity was re-stimulated and the neutralizing antibody titers increased rapidly and markedly with the administration of the CoronaVac or TURKOVAC as a booster dose 4 months after the second dose.While the increase in neutralizing antibodies against the Wuhan variant was similar with both CoronaVac and TURKOVAC, more antibodies developed against the Delta variant with TURKOVAC.What is the impact? With the Hybrid COV-RAPEL TR study, after the primary vaccination consisting of two doses of inactivated vaccine, antibody titers decreased in the long term; however, higher antibody titers are achieved than the primary vaccination after the booster dose administered after 4­6 month interval.Booster application with TURKOVAC provides antibodies at least as much as the CoronaVac booster dose, with an acceptable safety profile.