background: This study aimed to determine factors affecting serum levels of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies 2 months after coronavirus disease 2019 (COVID-19) vaccination in young and middle aged healthy adults. methods: Healthcare workers who have no history of SARS-CoV-2 infection, were enrolled at 2 months after second shot of BNT162b2 mRNA COVID-19 vaccine. Antibody immunoglobulin G against the spike protein subunit of SARS-CoV-2 was semi-quantitatively measured using 4 commercial enzyme-linked immunosorbent assay kits. Factors affecting anti-SARS-CoV-2 antibodies levels were investigated. results: Fifty-one persons (22 - 54 years, male sex; 19.6%) were enrolled and all participants acquired anti-SARS-CoV-2 antibodies in four diagnostic kits. Anti-SARS-CoV-2 antibodies were strongly correlated between diagnostic kits; SG Medical and Genscript (r = 0.942), SG Medical and HB Healthcare (r = 0.903), and HB Healthcare and Genscript (r = 0.868). We investigated factors affecting antibody level using SG medical kit. The median inhibition was 93.1%, and 84.0% of participants showed >90.0% inhibition. Systemic adverse event severity had no association with the anti-SARS-CoV-2 antibodies level. Antibody level was inversely correlated with weight (-0.312, = 0.027), body mass index (BMI) (r = -0.303, = 0.032), and body surface area (r = -0.285, = 0.044). In multivariate analysis, the upper 50% of anti-SARS-CoV-2 antibodies (≥93.1%) was inversely associated with weight (odds ratio [OR]: 0.19; 95% confidence interval [CI]: 0.04 - 0.83 in weight ≥55kg) and BMI (OR: 0.12; 95% CI: 0.03 - 0.61 in BMI ≥22 kg/m²). conclusions: Anti-SARS-CoV-2 antibody was inversely correlated with weight and BMI, which may be used as a marker to predict immune response of BNT162b2 mRNA vaccination in young and middle aged adults. background: ClinicalTrials.gov Identifier: NCT05083026.