ABSTRACT
Background: Patients with haemato-oncological malignancies are one of the high-risk groups for a severe course in case of Covid-19 infections. Furthermore, vaccination results in signifcantly lower response rates in haematological malignancies and lower antibody levels in patients with solid cancer. We investigated efcacy and safety of a heterologous booster vaccination with Ad26.COV2.S DNA vector vaccine in haemato-oncological patients without antibody response after doubledose BNT162b2 mRNA Covid-19 vaccine. Methods: A total of 32 haemato-oncological non-respond-ers to double-dose BNT162b2 received a heterologous booster vaccination with Ad26.COV2.S. Blood samples were assessed directly before the vaccination (T0) and 4 weeks after (T1). Safety assessment was performed using a standardised questionnaire. Results: The overall response rate was 31 %, with a mean (SD) antibody titre of 693.79 (1096.99) BAU/ml. Patients with chronic lymphocytic leukaemia or lymphoma showed a sig-nifcantly lower response rate (P = 0.048). Adverse events were reported in 29.6 % of patients, whereby 7.1 % were graded as severe, which includes grade III and IV events following CTCAE. Conclusions: The heterologous booster vaccination with Ad26.COV2.S led to a serological response in 9 out of 29 patients without response after double-dose BNT162b2. Furthermore, the vaccination was safe in our cohort, leading to mainly mild local and systemic reactions. Overall, this vaccination regimen should be further evaluated to increase the response rate in the highly vulnerable population of haemato-oncological patients.