One dose of a live cholera vaccine was 87% effective in Mayotte's 2024 outbreak

Cholera is an acute diarrhoeal infection caused by consuming food or water contaminated with the bacterium Vibrio cholerae, and an oral cholera vaccine is one of the tools used to prevent and control it [s2]. A global resurgence of the disease alongside a shortage of the usual killed vaccines has pushed health agencies toward reactive, single-dose campaigns during outbreaks [s1]. A study published in Nature Communications on 19 October puts a number on how well one of the newer options worked when it was deployed that way for real [s1].

The vaccine being tested

CVD 103-HgR, sold as Vaxchora, is a live-attenuated oral cholera vaccine. It has shown high efficacy in human challenge studies — the controlled setting in which volunteers are deliberately exposed — but its effectiveness in an actual outbreak was, until now, unknown [s1]. The 2024 cholera outbreak in Mayotte, a French territory in the Indian Ocean, offered the first real-world test, and the authors focused on Koungou village, which combined high cholera incidence with high vaccine uptake [s1].

What the modelling found

Of Koungou's at-risk population of 4,733 people, 3,354 — 71% — received a single dose of CVD 103-HgR [s1]. Seventy-seven cases were confirmed, with symptom onset falling between 19 April and 7 July 2024 [s1]. The researchers built several stochastic transmission models and calibrated them to the daily case counts broken down by vaccination status, which is what lets a study like this separate the vaccine's effect from everything else moving at once [s1].

At baseline, they estimated a basic reproduction number — the average number of further cases one case generates in a fully susceptible population — of 1.5, with a 95% credible interval of 1.2 to 1.9 [s1]. The headline result is the effectiveness estimate: a single dose was 87% effective at preventing cholera, with a 95% credible interval of 74% to 99% [s1]. That central figure held up when the authors varied their assumptions, staying between 75% and 91% across the scenarios they tested [s1].

The population-level impact is larger but less certain. Counterfactual simulations — rerunning the outbreak as if no one had been vaccinated — estimated 536 cases averted at baseline, with a 95% credible interval of 240 to 824 [s1]. Across the full set of sensitivity analyses, the point estimates for cases averted ranged more widely, from 152 to 1,228 [s1]. In other words, the evidence that the vaccine protected the people who received it is strong; the estimate of how many infections the campaign prevented across the whole village is real but should be read as a range rather than a single figure.

Why this matters

The practical question hanging over cholera response is which vaccines can be used, and how. The world's stockpile of the standard killed oral cholera vaccines has repeatedly run short as outbreaks have multiplied, which is why WHO switched to a single-dose strategy to stretch supply. The Mayotte analysis supplies the first field evidence that the live-attenuated CVD 103-HgR can be dropped into that same reactive, one-dose role and deliver high protection [s1]. The authors frame their finding as supporting the inclusion of live-attenuated oral cholera vaccines in reactive strategies when necessary — a widening of the toolkit rather than a replacement for it [s1].

What the study can and cannot show

This is a modelling study of a single village, and its strength and its limits both follow from that. The strength is that the models were anchored to observed daily cases by vaccination status, which is a demanding standard; the 87% effectiveness estimate is not an extrapolation from the challenge studies but a measurement taken during an outbreak [s1]. The limits are that Koungou was chosen precisely because it had both high incidence and high uptake, so the setting was favourable, and that the population-impact figure carries wide uncertainty [s1].

None of this changes the foundations of cholera control, which remain access to safe water, basic sanitation and hygiene, with rapid oral rehydration keeping the case fatality rate in treatment centres below 1% [s2]. Vaccination is an addition to that, not a substitute for it. What the Mayotte data add is a usable answer to a supply-driven question: when the killed vaccines are not available, a single dose of the live-attenuated vaccine is a defensible reactive option, and in this outbreak it worked [s1].