BACKGROUND Current vaccines against SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) and vaccine booster programs aim to reduce hospitalizations due to severe COVID-19 (coronavirus disease 2019). It is now accepted that vaccination does not completely prevent infection and that breakthrough COVID-19 does occur. This study included 53 vaccinated patients who were hospitalized at a single center in Poland with breakthrough COVID-19 and aimed to evaluate the factors associated with their clinical course. MATERIAL AND METHODS This study covered the period 26 November 2021 to 11 March 2022. All patients had been vaccinated against COVID-19 with one of the following 4 vaccines: the mRNA-1273 (Moderna) mRNA vaccine (Spikevax); the BNT162b2 (Pfizer-BioNTech) mRNA vaccine (nucleoside-modified) (Comirnaty); the Ad26.COV2.S (Janssen/J0ohnson & Johnson) recombinant vaccine (Jcovden); and the AZD1222 (ChAdOx1) (Oxford/AstraZeneca) recombinant vaccine (Vaxzevria). RESULTS The course of COVID-19 in vaccinated patients was relatively similar. The patients vaccinated more than 24 weeks earlier rarely needed a stay in the Intensive Care Unit (ICU) (P=0.021), and the occurrence of deaths was significantly lower in this group (P=0.046). Women remained in hospital considerably longer than men (P=0.011). Age and comorbidities did not affect the course of this infection. CONCLUSIONS Despite the many advantages of the COVID-19 vaccination, our observations indicate a potential risk of infection after vaccination. The assessment of the course of COVID-19 in vaccinated patients gives the possibility to compare different vaccines and indicate factors that can reduce immunity.