Introduction: Widespread immunization is critical to bringing the COVID-19 pandemic to an end. The objective of this cross-sectional data analysis was to investigate the impact of vaccination on the frequency of COVID-19 occurrence and the presence of its symptoms and clinical presentations.

Material and methods: This was a retrospective, cross-sectional study, analysing medical records of patients hospitalized at the Dr Antoni Jurasz University Hospital No. 1 in Bydgoszcz in the years 2019–2021. The analysis considered age, sex, vaccination status, the result of the reverse transcription polymerase chain reaction (RT-PCR) test for the detection of SARS-CoV-2 RNA, and disease symptoms and clinical presentations in accordance with ICD-10 codes.

Results: The study group consisted of 27,209 patients. There were 1,393 persons (5.12%) who tested positive for COVID-19. The vaccinated patients accounted for 17.3% (n = 4,704). Those vaccinated were significantly less likely to test positive for COVID-19 (3.93% vs. 5.38%; p < 0.0001). Among unvaccinated patients with a positive test result for COVID-19, symptoms and/or clinical presentations occurred in 359 study participants (29.72%), while in vaccinated COVID-19-positive patients only in 49 persons (26.49%). The symptoms that occurred in both vaccinated and unvaccinated patients were mainly respiratory and circulatory. The most common clinical presentation, both in the group of vaccinated and unvaccinated patients, was viral pneumonia, not elsewhere classified (J12), which occurred in 17.30% and 9.19% of patients, respectively (p = 0.005). Other symptoms and clinical presentations showing a statistically significant difference in the frequency of occurrence between the groups were pneumonia in diseases classified elsewhere (J17; p = 0.019) and abnormalities of breathing (R06; p = 0.001).

Conclusions: Vaccination against COVID-19 protects against symptomatic disease.