Background Chronic lymphocytic leukaemia is the most common leukaemia subtype and associated with profound immunosuppression. Bruton Tyrosine Kinase inhibitors have revolutionised chronic lymphocytic leukaemia management; however, Bruton Tyrosine Kinase inhibitors therapy impairs vaccine-induced immunity. We evaluated if a 3-week pause of Bruton Tyrosine Kinase inhibitors treatment improved spike protein receptor binding domain immunity to severe acute respiratory syndrome coronavirus 2 vaccination while maintaining disease control. Method We performed an open-label, two-arm parallel group, randomised trial in secondary-care haematology clinics in 11 United Kingdom hospitals. Patients aged 18 and over, diagnosed with chronic lymphocytic leukaemia and currently taking Bruton Tyrosine Kinase inhibitors therapy for at least 12 months were eligible. Participants were randomised (1 : 1 stratified by Bruton Tyrosine Kinase inhibitors therapy line) to pause Bruton Tyrosine Kinase inhibitors for 3 weeks or continue their Bruton Tyrosine Kinase inhibitors therapy as usual, starting 6 days before their COVID-19 vaccine booster date. The primary outcome measure was anti-spike-receptor binding domain-specific antibody titre 3 weeks post-vaccination and analysed using linear regression on the log scale. Findings Between 10 October 2022 and 8 June 2023, 99 individuals were randomised to pause ( n = 50) or continue ( n = 49) their Bruton Tyrosine Kinase inhibitors therapy. At 3 weeks post vaccine, the geometric mean (standard deviation) anti-spike-receptor binding domain antibody titre was 218.8 (122.9) and 153.4 (103.2) U/ml in the continue and pause arms respectively with geometric mean ratio (95% confidence interval) 1.104 (0.565 to 2.158), p = 0.772, mixed-effects model and no difference in seroconversion between the two arms observed. Therapy interruption did not improve antibody neutralisation against Ancestral B.1 virus at 3 or 12 weeks [geometric mean ratio (95% confidence interval) in those that paused compared to those that continued: 1.073 (0.626 to 1.837) and 0.647 (0.378 to 1.109)] respectively. Cellular interferon gamma response to ancestral B.1 or variants of concern at 3 weeks post vaccine did not differ either. No intervention-related serious adverse events were reported, although four participants in the pause arm self-reported lymphadenopathy at 3 weeks, whereas none reported it in the continue arm. No significant differences in other features of disease flare were observed between the two arms. Interpretation This study suggests that pausing Bruton Tyrosine Kinase inhibitors around the time of vaccination is not beneficial for COVID-19 immunity and, therefore, this should not be recommended in clinical practice. It highlights the heterogeneity in response to vaccination among this profoundly immunocompromised group and the need for further research to understand the mechanisms underpinning this. Future work Understanding why the heterogeneity in vaccine responses is now required, along with assessing generalisability of the finding for other patients with chronic lymphocytic leukaemia. Limitations This study was performed when most patients had already received multiple vaccinations. A difference in the primary immune response to the first three doses may have differed. Patients were not blindied and self-reporting measure may have been affected. This study does not completely rule out the possibility of some effect. Funding This synopsis presents independent research funded by the National Institute for Health and Care Research (NIHR) Efficacy and Mechanism Evaluation (EME) programme as award number NIHR151892.