Infectious Disease · Medical Literature
[Comment] New chapter in rabies vaccine development
Abstract
It has been more than 140 years since Pasteur, Vulpian, and Grancher conducted the first successful human immunisation against rabies with a live-attenuated vaccine derived from dried rabid rabbit spinal cord material. The current modern purified cell culture and embryonated egg-derived rabies vaccines were developed between the 1960s and 1980s. These inactivated rabies vaccines are highly immunogenic, induce a robust and long-lasting B-cell memory, and are highly effective in preventing rabies.
Clinical relevance
Does the simian adenovirus-vectored candidate rabies vaccine ChAdOx2 RabG, administered as a single intramuscular dose, achieve safety and immunogenicity outcomes that are non-inferior or superior to a licensed inactivated rabies virus (IRV) vaccine in healthy, rabies vaccine-naive adults and children in a rabies-endemic sub-Saharan African setting? __SNAPSHOT__ {"studyDesign":"Expert commentary (Comment/Editorial) accompanying a single-centre, age de-escalation, dose-escalation, partly randomised, open-label, phase 1b/2 controlled trial (RAB002; NCT04270838) — only the abstract of the primary article and the full Visser Comment are available for this summary; full primary article data accessed via secondary sources","setting":"Ifakara Health Institute Clinical Trial Facility, Bagamoyo, Tanzania (semi-urban, rabies-endemic); sponsored by the University of Oxford, UK","sampleSize":"n=174 total: adults n=63 (middle-dose ChAdOx2 RabG n=3; full-dose ChAdOx2 RabG n=45; single-visit IRV n=15); children aged 2–6 years n=111 (low-dose ChAdOx2 RabG n=3; middle-dose n=3; full-dose ChAdOx2 RabG n=45; single-visit IRV n=30; two-visit IRV n=30)","population":"Healthy, rabies vaccine-naive adults aged 18–45 years and children aged 2–6 years recruited from the semi-urban Bagamoyo area of Tanzania","intervention":"ChAdOx2 RabG — replication-incompetent simian adenovirus-vectored vaccine encoding rabies virus glycoprotein, administered as a single intramuscular dose at full dose (5×10¹⁰ virus particles) or middle dose (2.5×10¹⁰ virus particles) for adults; full, middle, or low dose (1×10¹⁰ virus particles) for children","comparator":"Licensed Vero-cell whole inactivated rabies virus (IRV) vaccine (Verorab, Sanofi Pasteur), given intradermally at two anatomical sites in a single visit (adults and children) or at two sites in two visits 7 days apart (children only)","primaryOutcome":"Safety assessed in the intention-to-treat population; secondary outcome was rabies virus neutralising antibody (VNA) titre at day 365 in single-visit full-dose ChAdOx2 RabG and single-visit IRV participants who attended the nominal 1-year visit","followUp":"Interim data reported at 1 year (365 days); full planned follow-up is 5.5 years per participant","fundingSource":"Not fully reported in available abstract/comment; primary phase 1 trial funded by UK Medical Research Council and Engineering and Physical Sciences Research Council; Visser discloses a grant from Bavarian Nordic to study longevity of B-cell memory after rabies PrEP"}
Key findings
Primary safety outcome: ChAdOx2 RabG was well tolerated across all dose groups in both adults and children. Reported reactogenicity was predominantly mild-to-moderate (most commonly injection-site pain and feverishness); no serious adverse reactions were observed during the 1-year interim period. Secondary immunogenicity outcome: In adults, day-365 geometric mean VNA titres were 2.0 IU/mL (95% CI 1.4–2.9) after full-dose single-visit ChAdOx2 RabG versus 0.4 IU/mL (95% CI 0.2–0.7) after single-visit IRV (geometric mean ratio [GMR] 5.1, 95% CI 2.5–10.4; p<0.0001). In children, day-365 geometric mean VNA titres were 6.1 IU/mL (95% CI 4.5–8.2) after full-dose ChAdOx2 RabG versus 0.7 IU/mL (95% CI 0.5–1.1) after single-visit IRV (GMR 8.6, 95% CI 5.4–13.9; p<0.0001). In a post-hoc analysis in children, day-365 VNA titres after ChAdOx2 RabG also exceeded those after the two-visit IRV regimen (geometric mean 3.0, 95% CI 2.2–4.1; GMR 2.0, 95% CI 1.3–3.1; p=0.0028). Nearly all children who received ChAdOx2 RabG maintained VNA ≥0.5 IU/mL (the established serological correlate of protection) at 365 days. Exploratory analyses showed unexpectedly rapid seroconversion after ChAdOx2 RabG vaccination, raising the hypothesis of possible post-exposure utility. NNT not calculable from available immunogenicity data. The rare but potentially severe adverse reaction of vaccine-induced immune thrombocytopenia and thrombocytopenia (VITT), a recognised class effect of adenovirus-vectored vaccines, could not be ruled out given the small sample size.
Practice implications
This Comment contextualises interim phase 1b/2 data showing that a single intramuscular dose of ChAdOx2 RabG produced VNA titres substantially exceeding both the established serological correlate of protection and those generated by licensed single-visit IRV at 1 year in Tanzanian adults and children. The current editorial and the primary trial both establish promising immunogenicity and an acceptable short-term safety profile in a rabies-endemic sub-Saharan African population, including in young children aged 2–6 years. What was NOT established: clinical (symptomatic) efficacy against rabies disease has not been demonstrated; the 1-year interim data do not yet confirm durability of protection over the full planned 5.5-year follow-up; the small sample size precludes reliable detection of rare but severe class-effect adverse events such as VITT; suitability for post-exposure prophylaxis use in vaccine-naive individuals remains exploratory and unconfirmed. The findings apply most directly to healthy rabies vaccine-naive adults and children in sub-Saharan African endemic settings; populations not represented include immunocompromised individuals, infants under 2 years, persons previously vaccinated against rabies or adenovirus, and high-income country settings. Visser's commentary also highlights that cost-effectiveness of programmatic rabies pre-exposure prophylaxis (PrEP) remains a barrier: current inactivated vaccines only become cost-neutral in settings with dog-bite incidences exceeding 5,000 per 100,000 people, far above the typical endemic incidence of 10–130 per 100,000. A thermostable, low-cost, single-dose vaccine such as ChAdOx2 RabG could meaningfully change this equation. Practice should not change at this time; confirmation of efficacy, long-term durability, and further safety assessment at scale are required. Note: This summary is based on the Visser editorial comment together with interim primary trial data available in the public domain; the full primary article text was not directly accessed. __LIMITATIONS__ 1. Phase 1b/2 interim data: small sample sizes in most subgroups preclude detection of rare adverse events (notably VITT, a recognised adenovirus-vector class risk), precise efficacy estimation, or subgroup analyses. 2. Open-label design: absence of participant and assessor blinding may introduce bias in reactogenicity reporting. 3. Immunogenicity surrogate endpoint: VNA titre ≥0.5 IU/mL is an accepted but imperfect correlate of protection; clinical efficacy against rabies disease was not directly assessed. 4. Only 1-year interim data reported: durability of antibody and memory B-cell responses over the full 5.5-year follow-up is unknown. 5. Single-centre study conducted at one Tanzanian site: generalizability to other African or Asian rabies-endemic regions is uncertain. 6. The two-visit IRV comparator analysis in children was post-hoc and should be interpreted cautiously. 7. The adult IRV comparator arm used a single-visit intradermal regimen not formally WHO-recommended for PrEP, potentially understating the performance of currently licensed vaccines. 8. The VITT risk associated with adenovirus-vectored vaccines was specifically flagged by Visser as unquantifiable given the trial's small size. 9. Funding relationships: Visser discloses industry grant from Bavarian Nordic (a competing rabies vaccine manufacturer), which should be considered when evaluating editorial framing. 10. Only abstract-level and editorial-level data were available for this summary; full primary article statistical results may contain additional important details. __BOTTOM_LINE_CATEGORY__ promising_but_preliminary __BOTTOM_LINE__ A single intramuscular dose of ChAdOx2 RabG demonstrated favourable safety and substantially higher rabies virus neutralising antibody titres at 1 year compared with licensed single-visit inactivated rabies vaccine in Tanzanian adults and children, robustly exceeding the established serological correlate of protection. These interim phase 1b/2 findings are promising for low-cost, single-visit pre-exposure prophylaxis in rabies-endemic low- and middle-income countries, but clinical efficacy, long-term durability, and VITT risk at scale remain unestablished. Practice change is not yet warranted; completion of the 5.5-year follow-up and larger-scale efficacy trials are needed before this vaccine can be considered for programmatic deployment.