Immunosuppression is associated with an increased risk of delayed SARS-CoV-2 viral clearance, severe COVID-19 and related death. This heterogeneous group of affected patients includes but is not limited to those with a haematological malignancy, people on immunosuppressive therapy for the treatment of autoimmune/inflammatory diseases and those following bone marrow transplantation (BMT). Immunosuppression is associated with decreased rates of anti-spike IgG seroconversion following COVID-19 vaccination. While clinical guidelines have been established to guide vaccination pre-splenectomy and post-BMT, there are limited data to guide timing of COVID-19 or other booster vaccines in adults commencing new or intensified moderate to severe immunosuppression. The comparison of immunity-boosting regimens for COVID-19 upon initiation of immunosuppressive therapy (CIRCUIT) study was designed to address this knowledge gap. CIRCUIT investigates whether administration of a third (or subsequent) COVID-19 booster vaccine ≤2 weeks prior to immunosuppression provides greater anti-spike IgG-mediated immunity than a booster given 24 weeks after new or intensified immunosuppression, that is, week 24 timepoint (Group 1; n=280). Additionally, the research will investigate whether giving a fourth post-BMT COVID-19 booster vaccine at 9 months post-transplant provides greater anti-spike IgG-mediated immunity than a booster given 15 months post-transplant (Group 2; n=40).

The CIRCUIT study is an open-label, multicentre randomised clinical trial. Participants will be randomised 1:1 to receive either an additional COVID-19 booster ≤2 weeks pre-immunosuppression and a diphtheria/tetanus toxoids (DT) booster at 24 weeks following new or intensified immunosuppression (week 24 timepoint) or receive a DT booster ≤2 weeks pre-immunosuppression and an additional COVID-19 booster at week 24 (Group 1). Group 2 participants who underwent autologous or allogenic BMT in the last 9 months will be randomised 1:1 to either receive a fourth post-BMT COVID-19 booster at 9 or 15 months post-transplant. The primary outcome will be the integrated time-weighted area under the curve anti-SARS-CoV-2 neutralising antibody (NAb) response over 12 months from a SARS-CoV-2 booster as assessed by a high-throughput SARS-CoV-2 NAb platform assay. Key secondary outcomes of the CIRCUIT randomised control trial will include safety and generation of SARS-CoV-2 antigen specific T and B cell responses.

The research protocol was approved by the Western Sydney Local Health District Human Research Ethics Committee on 25 August 2022 (Ref no. 2022/PID00782 – 20022/ETH0069). Study results will be published in peer-reviewed medical journals and presented at local and international conferences. All findings regardless of the outcome will be reported.

NCT05415267.