Shortened Intradermal Rabies Vaccine Regimen Did Not Meet Equivalence Criterion

In a phase 4 randomized trial of 400 healthy adults in India, researchers compared a three-visit intradermal rabies vaccine regimen with a four-visit regimen. The shorter regimen did not meet the prespecified equivalence criterion for day-42 antibody concentrations, although all participants had antibody titres of at least 0.5 IU/mL from day 14 through one year.

400 healthy adults were randomized across four study groups.
The shorter regimen did not meet the equivalence criterion.
All groups had titres ≥0.5 IU/mL from day 14 to one year.
Findings may not generalize to post-bite prophylaxis.

A single-centre, open-label, randomized equivalence trial in India assigned 400 healthy adults to one of four groups: the three-session Institut Pasteur du Cambodge (IPC) or four-session Updated Thai Red Cross (UTRC) intradermal regimen, using either purified chick embryo cell vaccine (PCECV) or purified Vero cell rabies vaccine (PVRV). The primary outcome was the day-42 geometric mean concentration ratio (GMR) of rabies virus neutralizing antibodies; equivalence required the full 95% confidence interval to fall within 0.72–1.04. A total of 362 participants completed the regimen and day-42 blood draw.

The IPC-versus-UTRC GMR was 0.59 (95% CI, 0.47–0.75) with PCECV and 0.76 (95% CI, 0.61–0.95) with PVRV; neither met the prespecified equivalence criterion. All participants in the four groups had titres of at least 0.5 IU/mL at assessments from day 14 through one year. Adverse events were similar between regimens and mostly mild.

The findings do not establish equivalence of the shorter regimen in this population. Nearly one-quarter of participants had titres at or above 0.5 IU/mL at baseline, suggesting possible prior immune priming, and participants were healthy volunteers rather than people receiving post-exposure prophylaxis after animal bites. The authors say policy decisions should also consider compliance, cost, and vaccine availability.

In practice

For rabies post-exposure prophylaxis, this trial does not establish that the three-session IPC intradermal regimen is equivalent to the four-session UTRC regimen: day-42 antibody GMR confidence intervals fell below the prespecified equivalence range for both vaccines. Although every participant had titres of at least 0.5 IU/mL from day 14 through one year and adverse events were similar, the study involved healthy volunteers, not patients bitten by animals. Baseline antibody titres were also common, potentially affecting estimates; these results alone do not establish a reason to change routine PEP practice.

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Source

PLOS Medicine: Immunogenicity and safety of Institut Pasteur du Cambodge (IPC) and updated Thai Red Cross (UTRC) intradermal rabies vaccine regimens among healthy adults in India: A phase 4, open label, equivalence randomised controlled trial ↗

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