Background: Since cancer patients (pts) are at increased risk of severe morbidity and mortality due to COVID-19, SARS-CoV-2 vaccination is crucial to prevent severe disease course. The efficacy and longevity of vaccine protection, however, is still in question. Methods: In this prospective observational study we assessed immunogenicity of cancers patients on active treatment for solid cancers in two Slovenian Oncology Centres receiving SARS-CoV-2 vaccine by detection of anti-SARS-CoV-2 S1 IgG antibodies in serum; before, after first and second dose, 3 and 6 months (mo) after complete primary course of vaccination and 1 mo after the 3rd booster dose. Samples with ≥ 175 ng/ml were considered positive. Results: There were 125 pts included in the study, 112 completed the primary course of vaccination with 2 vaccine doses and 34 received the booster 3^rd dose. Out of 34 pts receiving booster 3^rd dose, the majority received mRNA based vaccines (88%) for the primary course of vaccination and all patients received mRNA based vaccines for the booster 3^rd dose. Median time from 2^nd to booster 3^rd dose of vaccine was 6.75 mo, median age of pts was 66 yrs and 59% were male. Patients were receiving chemotherapy (ChT), immune checkpoint inhibitors (IO) or targeted therapy (TT) in 9%, 56% and 35%, respectively. They achieved seroconversion in 94% after the 2^nd and in 97% after the 3^rd booster dose. Overall, the median anti-SARS-CoV-2 S1 IgG titer was 11,191 ng/ml (mean 25,394; range 0 – 100,000) after complete primary course of vaccination and 1892 ng/ml (mean 6413; range 0 – 72,000) 6 mo after, P< 0.0001. However anti-SARS-CoV-2 S1 IgG antibodies increased significantly after the 3^rd booster dose with median antibody value 51,310 ng/ml (mean 51,787; range 0 – 100,000), P< 0.001. Only one pt in this group got infected with SARS-CoV-2 and had a mild disease course. Conclusions: Cancer pts treated for solid malignancies achieve a high rate of seroconversion through the primary course of vaccination. However, significant waning of immunity against SARS-CoV-2 is observed 6 months thereafter, which can be potentiated with additional 3^rd booster dose of vaccine.