OBJECTIVE: An observational study was conducted from 2021 to 2022 to track the dynamics of the production of IgG antibody against the SARS-CoV-2 S protein in individuals vaccinated with two doses of the Pfizer-BioNTech mRNA vaccine BNT162b2 (Comirnaty). The study included a group of volunteers without any previous signs of SARS-CoV-2 infection as well as a group with a history of natural COVID-19 infection. The primary objective was to monitor antibody levels up to 12 months after the second dose and determine the proportion of vaccinated individuals who underwent seroconversion. The study also evaluated cases of vaccine failure within 13 months post-vaccination. METHODS: Before the vaccination began, participants had laboratory tests for IgG antibodies against the SARS-CoV-2 S protein, and their medical history related to COVID-19 was taken. Based on negative test results, a cohort of immunologically naïve individuals was formed and subsequently vaccinated with two doses of BNT162b2 (Comirnaty). Venous blood samples were collected at six time points: before the first dose, 2-3 weeks after the first dose, one month after the second dose, 3-4 months after the second dose, 6 months after the second dose, and 12 months after the second dose. A supplementary cohort included volunteers with positive antibody findings or confirmed COVID-19 infection. In some of these individuals, the dynamics of post-vaccination antibody response was also monitored. Participants filled out monthly questionnaires about respiratory infection symptoms to detect vaccine failure. RESULTS: The study included 166 participants who did not have SARS-CoV-2 S protein antibodies prior to vaccination. The median age was 52 years, with a higher proportion of women (71.1%). After the first and second doses of the vaccine, antibody levels showed a significant increase, followed by a gradual decline over 12 months. Seroconversion occurred in all participants except for one immunosuppressed individual. The vaccine failure rate against COVID-19 within 12 months was 13.3%. The second cohort included 60 participants with prior SARS-CoV-2 infection. In this group, post-vaccination antibody levels increased significantly, more than in individuals without prior infection. The second dose did not result in further statistically significant increase in antibody levels for this cohort. CONCLUSIONS: The study confirmed that the BNT162b2 mRNA vaccine induces a strong antibody response, with the majority of participants experiencing seroconversion after the first dose. Older individuals exhibited a lower antibody response, highlighting the importance of booster doses. In individuals with prior SARS-CoV-2 infection, antibody levels were significantly higher after the first vaccine dose than in naïve individuals. These findings add to understanding antibody response dynamics and suggest the need for further research focused on optimizing vaccination schedules.