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 one-year boostability of a 3-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 3-dose intramuscular PrEP schedule (days 0,7,21-28) with 1 mL Rabipur®, followed by a 2-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 3rd PrEP dose), Day 60, Month 12 and Month 12 + 7 days. The primary outcome was one-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 and factors associated with the primary endpoint.

RESULTS: We included 56 individuals, of whom 52 completed the study. The one-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 3-dose rabies PrEP schedule followed by a 2-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 administration of RIG by 73%.