It is important to control the levels of specific IgG against SARS-CoV-2 to ensure the timely monitoring of immunity in patients with COVID-19. Yet it is unclear what antibody levels protect against new infection and how long the protection is maintained. The study was aimed to assess the dynamic changes in the levels of IgG against SARS-CoV-2 by the two-year controlled observation. Healthy individuals (n = 70), COVID-19 survivors (n = 42), and people vaccinated with Sputnik V (n = 43) were enrolled. They were followed-up from April 2020 to April 2022. Serum IgG levels were defined (n = 312) using immunochip and the commercially available test system. Significance of differences was estimated using the Mann–Whitney U test for р ≤ 0.05. IgG levels in the disease survivors (median 97.1; 95% CI: 80–162 BAU/mL) and vaccinated individuals (103.1; 78–139 BAU/mL) were significantly higher than in healthy people (4.3; 4.1–4.5 BAU/mL). Intensity of immune response significantly increased after vaccination of the disease survivors (up to 1023; 657–1191 BAU/mL) or administration of booster dose to vaccinated individuals (413; 213–545 BAU/mL). In elderly convalescents (60+), IgG levels were significantly higher, and in vaccinated people these were significantly lower, than in people under the age of 60. IgG levels decreased faster in vaccinated individuals (after 3–4 months), than in the disease survivors, and stabilized at <100 BAU/mL in 60% of subjects within 5–9 months. Thus, intensity and duration of immune response in COVID-19 survivors and vaccinated people vary significantly depending on age, observation period, and additional vaccinations/revaccinations. Three cases of infection after full vaccination were reported over the entire follow-up period, including infection in a patient having a history of the disease and subsequent vaccination.