Immunogenicity and 1-year boostability of a three-dose intramuscular rabies pre-exposure prophylaxis schedule in adults receiving immunosuppressive monotherapy: a prospective single-centre clinical trial

BACKGROUND: For immunocompromised patients (ICPs), administration of rabies immunoglobulins (RIG) after exposure is still recommended regardless of prior vaccination, due to a lack of data. We aimed to assess the 1-year boostability of a three-dose rabies pre-exposure prophylaxis (PrEP) schedule in individuals using immunosuppressive monotherapy. METHODS: In this prospective study, individuals on immunosuppressive monotherapy with a conventional immunomodulator (cIM) or a TNF-alpha inhibitor (TNFi) for a chronic inflammatory disease received a three-dose intramuscular PrEP schedule (days 0,7,21–28) with 1 mL Rabipur®, followed by a two-dose simulated post-exposure prophylaxis (PEP) schedule (days 0,3) after 12 months. Rabies neutralizing antibodies were assessed at baseline, on day 21–28 (before the third PrEP dose), day 60, month 12 and month 12 + 7 days. The primary outcome was 1-year boostability, defined as the proportion of patients with a neutralizing antibody titre of ≥ 0.5 IU/mL at month 12 + 7 days. Secondary outcomes were geometric mean titres (GMTs) and factors associated with the primary endpoint. RESULTS: We included 56 individuals, of whom 52 completed the study. The 1-year boostability was 90% (47/52) with a GMT of 6.16 (95% CI 3.83–9.91). All participants seroconverted at some point in the study. Early response to PrEP (at day 21–28) was significantly associated with 100% boostability (Odds Ratio 51; 95% confidence interval [5.0–6956], P < 0.01). The vaccination schedule was safe and well tolerated. No vaccine-related serious adverse events occurred. CONCLUSION: In patients using immunosuppressive monotherapy, a three-dose rabies PrEP schedule followed by a two-dose PEP schedule is immunogenic, with all patients seroconverting at some point in the study. Although boostability 7 days after PEP was not 100%, nobody would wrongly be denied RIG when only administered to those who responded early to PrEP while reducing the administration of RIG by 73%.