Zambia's cholera resurgence in late 2023 and early 2024 emptied shelves faster than doses could be replaced, and it exposed a disagreement that has quietly divided cholera specialists for years: whether reactive mass vaccination during an outbreak is the best use of a scarce global stockpile, or whether it draws money and attention from the water and sewerage work that ends outbreaks for good.

The outbreak that outran the stockpile

Cholera is endemic in parts of Zambia, with cases reported most years in the peri-urban settlements around Lusaka and in parts of the Copperbelt. The 2023-24 season was different in scale. Transmission accelerated through densely packed neighbourhoods where piped water is intermittent and pit latrines sit close to shallow wells.

The oral cholera vaccine is given in two doses, and the global stockpile that supplies it is managed collectively by WHO and partners. During the 2023-24 period that stockpile was under simultaneous pressure from outbreaks in several regions. Zambia's request competed with others, and deliveries arrived in tranches rather than at once.

Health authorities shifted to a single-dose strategy to stretch what was available, a move WHO has endorsed during shortages. That decision buys coverage at the cost of durability, because a single dose protects for a shorter window than the two-dose course. The trade-off was made under time pressure, not in a seminar room.

The supply constraint is structural, not incidental. For years the global stockpile has been supplied by a small number of manufacturers, and expanding production takes time that an outbreak does not allow. When several large emergencies land in the same season, the allocation mechanism becomes a queue, and the countries at the back of it are usually the ones with the least capacity to argue their case. Zambia's experience illustrates how a country can do the epidemiological work of identifying its hotspots and still find that the doses it counted on are committed elsewhere.

What the vaccine actually does

Oral cholera vaccines work locally in the gut, prompting intestinal antibody production against Vibrio cholerae. The older injectable vaccines did this poorly; the oral formulations developed from the 1980s onward do it well. For roughly the first six months after a full course, protection sits near 85 percent, then declines over the following few years.

That waning matters for planning. A campaign that protects a neighbourhood through one season may leave it vulnerable two or three years later, which is precisely when a stockpile is again under strain and donors have moved on. Repeat campaigns are possible but expensive, and each one competes for the same limited doses.

There is also a herd effect. When enough people in a crowded settlement are vaccinated, transmission slows even among the unvaccinated, because the bacterium runs out of susceptible hosts. Documented in several settings, this is the strongest argument for vaccinating whole communities rather than selected individuals.

Protection is not the same as immunity to the underlying conditions. A vaccinated person in a settlement without safe water remains exposed to the same faecal contamination that generates cases in the unvaccinated, and the vaccine reduces the chance that exposure becomes severe disease rather than eliminating exposure. This is why campaigns are described as one component of a response package rather than a substitute for the rest of it.

The case for reactive mass campaigns

Reactive campaigns target an outbreak that is already underway. WHO recommends them as part of a response package, and the logic is straightforward: in a dense settlement with shared water sources, slowing transmission for even a few weeks buys time to chlorinate wells, distribute household treatment, and stand up treatment centres.

Field experience from Haiti after 2010 and from parts of Bangladesh supports the idea that campaigns can be delivered quickly in difficult conditions. Teams have vaccinated large numbers of people in days using mobile approaches, cold-chain improvisation, and community health workers going door to door.

Timing is the weak point. A reactive campaign that starts weeks after the epidemic curve has peaked delivers far less than one launched early, and delays are common. Doses have to be requested, allocated, shipped, and stored. Each step costs days that the outbreak does not surrender.

There is a practical argument on the other side of the timing problem. Even a late campaign can protect a population that remains at risk after the peak, because cholera in endemic settings often produces a second wave or smoulders through the dry season before flaring again. Doses delivered after the first peak are not wasted if the settlement's water supply is still compromised, which in most outbreak settings it is.

Skeptics question reactive dosing

The objection is not that the vaccine fails. It is that vaccination absorbs the political attention that water and sanitation would otherwise receive. Cholera is a faecal-oral disease, and the durable fix is piped, chlorinated water and sewers that separate waste from drinking supply. A vaccine campaign can be photographed; a sewer trunk main cannot.

Some specialists favour reserving stockpile doses for preventive campaigns in places known to be at high risk before the season starts, rather than chasing outbreaks after they begin. Preventive dosing avoids the timing problem entirely and can be planned around supply. The counterargument is that preventive campaigns protect people who may never be exposed, while reactive ones target actual transmission.

Waning immunity complicates the preventive case too. A campaign mounted in a good year may have faded by the bad one. The same related piece on antibiotic formularies diverging between hospitals and clinics makes a parallel point about how local capacity shapes what guidance can achieve.

The cost comparison is often invoked and rarely settled. A course of oral cholera vaccine is inexpensive per dose, but a campaign carries delivery costs that dwarf the product: cold chain, staff time, community mobilisation, and the logistics of reaching settlements without formal addresses. A chlorination programme at a single well serves everyone who draws from it for as long as it is maintained. Comparing the two on price alone flatters whichever intervention has the cheaper delivery channel, which is rarely the vaccine.

The evidence gap both sides acknowledge

Randomised trials of reactive campaigns are scarce, and for understandable reasons. Randomising a neighbourhood to no vaccine during an outbreak raises obvious ethical problems, and outbreaks are unpredictable, so trials are hard to plan. Most of what guides policy is observational: cohort studies, outbreak reconstructions, and modelling.

Observational data can show that vaccinated areas fared better, but they struggle to separate the vaccine's effect from everything else happening at the same time, including water interventions, treatment access, and population movement. The uncertainty is not about whether the vaccine reduces individual risk. It is about how much a reactive campaign changes an epidemic's course, and under what conditions.

Questions about optimal timing, target group, and dose number remain genuinely open. A careful reader should treat confident claims in either direction with suspicion. This site has argued in a related piece on why Ghana's stroke wards see hypertension diagnosed too late that late presentation is often a systems problem rather than a patient failing.

One methodological point deserves more attention than it gets. Outbreak reconstructions often compare vaccinated and unvaccinated individuals within the same settlement, which controls for water supply and geography but not for the fact that people who accept vaccination may differ from those who do not in ways that affect their exposure. The bias can run in either direction, and studies rarely have the data to correct it. Modelling fills the gap by making assumptions explicit, but the assumptions are doing much of the work, and different models of the same outbreak can reach different conclusions about how many cases a campaign averted.

What a reader can do with this

Support organisations that fund water and sanitation infrastructure in endemic regions, not only emergency vaccine appeals. The long-run reduction in cholera burden comes from pipes, chlorination, and sewers.

Advocate for preventive vaccination planning in places where cholera is endemic, so that campaigns are scheduled before the season rather than improvised during it. Ask implementing organisations how they handle the waning-immunity problem.

Follow WHO's published stockpile reports and allocation decisions. The stockpile is a shared global resource, and shortages are visible in the public record before they reach the news.

Press donors to fund both vaccines and infrastructure rather than treating them as alternatives. The dispute between reactive campaigns and sanitation investment is real, but it is a dispute about sequencing and proportion, not about choosing one and abandoning the other.

This article is informational and does not constitute personalised medical advice. Readers with questions about vaccination or outbreak response should consult qualified public health or clinical professionals.