In many countries, a booster dose against COVID-19 is recommended to reduce the risk of complications and death. A prospective epidemiological study of patients with COVID-19 who were previously vaccinated against the disease was conducted to compare those with a booster dose and those without. We analyzed 390 patients with COVID-19, hospitalized in various structures of the University Hospital "St. George”, in the city of Plovdiv, between October 2021 and April 2022, previously vaccinated against SARS-CoV-2. Only 12.1% (n=390) of all patients with COVID-19 (n=2835) were vaccinated. Onset of illness and hospitalization were, on average, 5.6 months after a full vaccination course. The average age of hospitalized vaccinated patients was 64.8 years (+-standard deviation 16.2) with a median of 69 years and a range of 15-95 years. The number of those with booster doses was 61 (15.6%). The data on fatal cases showed that 91.4% were unvaccinated and 8.6% were fully vaccinated. The average age of deceased vaccinated patients was 74 years, and they had 2.21 accompanying diseases. The booster dose reduces the risk of death by 2.5 times compared to the full vaccination course. The present epidemiological study confirms the importance of vaccination and revaccination against SARS-CoV-2. The ratio between vaccinated and unvaccinated hospitalized patients shows that the available vaccines against the disease in Bulgaria protect against severe complications and death.