Efficacy and safety of tixagevimab/cilgavimab as passive immunisation against COVID-19 infections in patients with hematological malignancies

Monoclonal antibodies, as tixagevimab/cilgavimab, have been introduced as prophylaxis against COVID-19 infections in high-risk populations. However, data on efficacy are limited. This study investigates efficacy and tolerability of tixagevimab/cilgavimab in hematological patients under real-life conditions. Tixagevimab/cilgavimab was administered to 155 hematological patients (March-August 2022) at two Austrian centres. S/RBD-antibody assessments were performed before (T0), four weeks (T1), and six months (T2) after application. Side effects, the occurrence of COVID-19 infections, and the course of S/RBD-antibody titres were analysed retrospectively in relation to clinical variables. 155 hematological patients, who refused tixagevimab/cilgavimab, were included as a control group to compare the frequency of COVID-19 infections. Of all immunised patients (52.3% males; 91% triple vaccinated), 25.8% had a COVID-19 breakthrough infection (76% mild) compared to 43.9% in the control group. Patients with chronic lymphocytic leukaemia (CLL)/lymphoma were at highest risk of a COVID-19 infection (OR = 2.21; 95% CI 1.05-4.65; p = 0.037). After immunisation, a steep increase in median antibody levels (1193.4BAU/ml, IQR 0-2318.94) was observed in 67.8%, followed by a rapid decrease between T1 and T2 (465.95BAU/ml, IQR 0-1900.65.3) with the greatest declines in CLL/lymphoma (848.7BAU/ml, IQR 0-1949.6, p = 0.026). Side-effects occurred in 21.2% (CTCAE I/II). These real-world data indicate that S/RBD antibodies respond rapidly after passive immunisation in all hematological patients without safety concerns. Given the rapid decline in S/RBD antibodies, early booster immunisations should be considered for future scenarios in this vulnerable group.