This study was aimed at evaluating the changes in the antibody titers and neutralization ability before and after inoculation with an mRNA COVID-19 vaccine (Pfizer, USA) among healthcare professionals (HCPs) in Japan. A total of 221 HCPs were enrolled and their blood samples were collected at three time points: 1) before vaccination, 2) 3 weeks after the first dose, and 3) 3 weeks after the second dose. The titers of anti-SARS-CoV-2 Spike protein IgG antibody were measured in all the samples using VITROS-Anti-SARS-CoV-2 S1 Quant IgG Antibody (CLEIA, Ortho-Clinical Diagnostics, Inc. ), and the surrogate Virus Neutralization Test (sVNT) (ELISA, GenScript, USA) was performed in the samples obtained after the vaccination. The anti-SARS-CoV-2 IgG titers were 1) median: 0.16; Interquartile (IQR): 0.10-0.27, 2) median: 250; IQR: 99.5-466 and 3) median: 2,400; IQR: 1,480-3,950, respectively, at the three time-points. Our study also revealed the relationship between the anti-SARS-CoV-2 IgG antibody titer and the antibody neutralization activity.