This study evaluated the immune response to vaccination against COVID-19 in 534 healthcare frontline workers in Vilnius, Lithuania. The incidence of COVID-19 was reduced significantly after vaccination started in the healthcare sector. SARS-CoV-2 antibodies were detected in groups V-VII and this level of antibodies was found to be effective in preventing COVID-19. Sustained immune response was achieved after two vaccination doses, which remained stable for up to 6 months. After the booster dose, antibody levels remained high for an additional 12 months. Although SARS-CoV-2 antibody levels decreased after 6 months, even lower levels of antibodies provided protection against the Delta strain. The booster dose distributed the antibody titer in the high-level antibody groups, offering maximum protection at 12 months. However, even individuals with high antibody titers were observed to contract COVID-19 after vaccination with a booster dose and 6 months in the presence of the Omicron strain. Unfortunately, high levels of antibodies did not provide protection against the new strain of COVID-19 (the Omicron variant), posing a risk of infection. When comparing the antibody titer of vaccinated participants without COVID-19 and those with COVID-19, the change in antibodies after vaccination was significantly lower in infected participants. Individuals with comorbidities and specific conditions had lower antibody levels.