A one-week rabies regimen did not match the month-long standard on antibody levels

Rabies is almost always fatal once symptoms appear, and almost always preventable if a bitten person gets vaccinated in time. The obstacle is logistics: the standard post-exposure course means several clinic visits over weeks, and people drop out. For that reason the World Health Organization has endorsed shorter intradermal regimens — tiny doses injected into the skin rather than the muscle — that compress the schedule and stretch scarce vaccine across more patients [s2]. A phase 4 trial published in PLOS Medicine on October 29 tested whether the shortest of these holds its own against a longer one [s1].

The two schedules

The trial compared the one-week Institut Pasteur du Cambodge (IPC) regimen, with intradermal doses on days 0, 3 and 7, against the updated Thai Red Cross (UTRC) regimen, which adds a fourth visit on day 28 and so runs about a month [s1]. Both are intradermal; the question was whether dropping the fourth visit costs protection.

To make the test fair across products, the researchers ran it with two rabies vaccines widely used in India: a purified chick embryo cell vaccine (PCECV) and a purified Vero cell rabies vaccine (PVRV) [s1].

The design

This was a single-centre, open-label, randomised equivalence trial in healthy adult volunteers [s1]. Between October and December 2023, 400 participants were randomly assigned in equal numbers — 100 each — to four groups: PCECV with IPC, PCECV with UTRC, PVRV with IPC, and PVRV with UTRC [s1]. Rabies virus neutralising antibody (RVNA) titres were measured out to one year [s1]. Of the 400, 362 (90.5%) completed their regimen and the day 42 blood draw [s1].

The primary outcome was the geometric mean concentration ratio (GMR) of antibody on day 42, comparing the IPC regimen with UTRC, assessed separately for each vaccine [s1]. Equivalence was defined in advance as the entire 95% confidence interval for that ratio falling between 0.72 and 1.04 [s1]. A ratio near 1 would mean the schedules produced similar antibody levels; a ratio well below 1 would mean the one-week course produced less.

The result

The one-week regimen did not meet the bar. For the chick embryo cell vaccine, the day 42 ratio was 0.59 (95% confidence interval 0.47-0.75); for the Vero cell vaccine it was 0.76 (0.61-0.95) [s1]. Both intervals fall at least partly below the pre-specified lower limit of 0.72, so neither comparison demonstrated equivalence [s1]. In plain terms, the shorter schedule left people with lower antibody concentrations at six weeks.

But the clinically important number cuts the other way. Every participant in all four groups — regardless of their antibody status at the start — had an RVNA titre of at least 0.5 IU per millilitre, the threshold taken to indicate protection, at every assessment from day 14 through one year [s1]. The one-week course produced less antibody than the month-long one, yet still cleared the protective bar in everyone, and kept them above it for a year.

A wrinkle in the baseline

The authors flag an important limitation. Nearly a quarter of participants already had RVNA titres at or above 0.5 IU/mL at baseline, despite being screened to exclude anyone with a history of rabies vaccination or dog bite [s1]. That suggests unrecognised prior exposure or vaccination in the study population, which could have inflated the antibody estimates and muddied the comparison [s1]. And because the trial enrolled healthy volunteers rather than people actually bitten and seeking care, the findings may not fully reflect how the regimens perform in real post-exposure use [s1].

Adverse events were similar between regimens and mostly mild [s1].

How to read it

This is the tension at the heart of an equivalence trial. On the pre-registered antibody endpoint, the one-week regimen failed — it is not statistically equivalent to the month-long course [s1]. On the outcome that maps to protection, it succeeded: everyone was seroprotected, and stayed so for a year [s1]. The authors conclude that decisions about replacing the longer regimen should weigh these immunogenicity data alongside compliance, cost and vaccine supply [s1] — the very practical pressures that make a three-visit, one-week course attractive in the first place.

What to watch

Whether WHO and national programmes treat "lower antibody but universal seroprotection" as good enough to keep recommending the abridged regimen; and whether trials in people receiving genuine post-exposure prophylaxis, rather than healthy volunteers, reproduce the seroprotection result.

This article describes a single randomised trial and is informational only. It is not medical advice, and anyone bitten by an animal that might carry rabies should seek care immediately.