Introduction and objective: The role of immunoglobulin G antibodies in combating SARS-CoV-2 infection, modulation of COVID-19 disease severity, and persistence of humoral response after primary infection and vaccination is unclear. This study aimed to evaluate the role of antibodies in limiting the infection, modulating disease severity, and determining the durability of the immune response depending on the clinical status of patients, their age, sex, and vaccination status. Material and methods: The study involved 156 patients, 99 men and 57 women, aged 58.3 ± 12.5 years old, hospitalised for pneumonia in the course of COVID-19, with infection confirmed by real-time polymerase chain reaction test. The concentration of anti-SARS-CoV-2 IgG antibodies was tested at 3, 6, and 9 months from the day of hospitalisation. Results: The concentration of anti-SARS-CoV-2 IgG antibodies in patients with severe COVID-19 was higher compared to the group of patients with mild to moderate disease. The level of anti-SARS-CoV-2 IgG antibodies was comparable in men and women. In patients over 60 years of age, a lower concentration of antibodies was observed than in patients under 60 years of age. In the vaccinated group, the level of antibodies was higher than in the unvaccinated group. Conclusions: The study findings showed that the concentration of IgG anti-SARS-CoV-2 antibodies was not a parameter dependent on the sex of the patients, but rather on the severity of the disease and the age of the patients and their vaccination status.