SARS-CoV-2 specific antibody response after an mRNA vaccine as the third dose: Homologous versus heterologous boost

•Waning immunity after inactivated vaccine can be recovered by an mRNA boost.•Both homologous and heterologous boost induced robust humoral immune response.•Heterologous boost induced higher antibody levels up to three months post-boost.•Both strategies provided protection against severe COVID-19 during Omicron wave. The aim of this study was to evaluate immunogenicity and longevity of the humoral immune response within six months after the homologous (BNT162b2/BNT162b2) or heterologous (BBIBP-CorV/BNT162b2) third dose, and to assess breakthrough infections among vaccinees during the Omicron wave in Serbia. Serum samples were analyzed at four timepoints: five months after the primary series; three weeks, three months, and six months after the boost. IgG antibodies against the receptor-binding domain of the spike protein were detected using enzyme-linked fluorescence assay. Both homologous (n = 55) and heterologous group (n = 36) showed a highly significant increase in antibody concentrations (p