Introduction and objective: The SARS-CoV-2 virus has triggered a global pandemic, particularly affecting individuals with comorbidities and those on renal replacement therapy. Vaccination has become a critical strategy, although evidence indicates suboptimal immunity in these groups compared to the general population. Materials and methods: The study assessed the post-vaccination response in 121 patients undergoing renal replacement therapy and 104 control individuals. IgG antibody levels against glycoprotein S were measured twice: the first sample was collected 4–8 months after two vaccine doses, and the second sample 6–8 months after third dose. Results: In both groups, antibody titres significantly increased after the third vaccine dose, with no notable difference between the control and study groups after two vaccine doses (p < 0.001). The findings revealed no statistically significant difference in antibody levels between the two groups. Furthermore, there was no significant difference in infection frequency after three vaccine doses (p = 0.072). Patients who contracted COVID-19 after the third vaccination had lower antibody levels during the first blood draw, suggesting a potential impact on immunity. Among dialysis patients, a correlation was found between IgG antibody titres (in the first blood draw) and a longer time interval between the first and second vaccine doses (p = 0.021). The regression analysis indicated that a 1-unit increase in antibody level resulted in a 0.1% reduction in the risk of infection. Conclusions: The results imply that achieving immunity comparable to the general population is possible among patients undergoing renal replacement therapy more than half a year after receiving three vaccine doses. The study also introduces the hypothesis of a delayed antibody response.