Impact of mass oral cholera vaccination in an endemic area of the Democratic Republic of the Congo: a surveillance-based counterfactual modeling analysis
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Background
Oral cholera vaccines (OCVs) are a key component of cholera control recommended in cholera-endemic areas. Yet evidence of their population-level impact is limited, especially in Africa where most cholera deaths occur. Here, we estimate the impact of mass administration of OCVs in the cholera-endemic city of Uvira, Democratic Republic of the Congo.
Methods
We conducted enhanced cholera surveillance at the two official cholera treatment facilities in Uvira between Jan 2017 and Dec 2023, centered around a mass vaccination campaign that achieved 66% coverage with at least one dose of Euvichol Plus vaccine in 2020. We combined systematic rapid diagnostic case testing with repeated, representative population surveys capturing healthcare-seeking behavior, vaccination, population mobility, and antibody profiles to estimate seroincidence. We developed a Bayesian framework that integrates these data into an ensemble of mechanistic cholera transmission models that account for time-varying transmissibility, realistic immunity dynamics and loss of vaccination coverage to population turnover. OCV impact was assessed through ensemble counterfactual modeling of alternative vaccination scenarios.
Findings
We estimate that the 2020 mass vaccination averted 56% (95% Credible Interval: 34-81) of infections and deaths over the subsequent three years. This corresponds to 2,350 (median, 95% CrI: 890-8,960) averted facility-attended cases, and 44 (median, 95% CrI: 16-150) averted facility and community deaths. Vaccination of all of Uvira’s eligible population would have averted 64% (median, 95% CrI: 43-87) of cases and deaths, with a negative but limited influence of vaccine coverage loss because of population turnover (71% median, 95% CrI: 52-90 at half the population turnover).
Interpretation
Although mass vaccination averted a significant fraction of cholera cases that would have otherwise occurred in this endemic setting, imperfect coverage, population turnover, and high transmission rates contributed in offsetting the larger potential benefits of OCV. Successful cholera control in Uvira hinges on multisectorial approaches including provision of safe water and sanitation. Setting and communicating realistic expectations for mass vaccination programs in highly endemic areas is critical for maintaining confidence in the current generation of OCVs.
Funding
Gavi (M&E 9166 09 20 A16) and the Wellcome Trust (221688/Z/20/Z).
Research in Context
Evidence before this study
We searched PubMed for studies published between January 1, 2000 and June 2026 using the terms (“oral cholera vaccine” OR “OCV”) AND (“vaccine effectiveness” OR “vaccine impact” OR “indirect protection” OR “herd immunity” OR “population-level protection”). We also reviewed World Health Organisation (WHO) position papers and reports from the Global Task Force on Cholera Control (GTFCC). Inactivated oral cholera vaccines provide strong direct protection against symptomatic cholera, with randomized controlled trials and observational studies reporting effectiveness estimates typically exceeding 50% in the first three years after vaccination. In a previous study we showed that one-dose vaccine effectiveness was 52% 12-17 months after vaccination in Uvira, DRC. However, evidence on the population-level impact of preventive mass vaccination campaigns, which encompasses both direct and indirect protection, is thin. The only published estimates from clinical trials, both conducted in South Asia, reported overall protection of 37% over a 2-year period in Dhaka, Bangladesh and 75% over a 3-year period in Kolkata, India. No rigorous real-world estimates of mass OCV impact have been published from sub-Saharan Africa, where the vast majority of global cholera deaths occur. A major obstacle behind this gap is the lack of high-quality pre- and post-vaccination surveillance to support robust estimation of impact. The WHO and GTFCC have identified this evidence gap as a major research priority.
Added value of this study
To our knowledge, this is the first study to provide a rigorous, data-driven estimate of the population-level impact of a preventive mass OCV campaign in a cholera-endemic African setting. Drawing on 7 years of enhanced clinical surveillance, three population-representative household surveys measuring vaccine coverage, population mobility and health-care seeking behaviors, a serological survey, and a novel ensemble of compartmental transmission models, we estimated that the 2020 campaign in Uvira, DRC — which achieved approximately 66% coverage of at least one dose of Euvichol Plus vaccine — averted 56% (95% CrI: 34-81) of cholera cases and deaths over the subsequent three years. This corresponds to 2,350 (median, 95% CrI: 890-8,960) averted cases that would have sought care at official cholera treatment facilities, and 44 (median, 95% CrI: 16-150) averted deaths. Our transmission modeling framework also yielded new insights into cholera natural history in this endemic setting: symptomatic infection conferred strong and durable immunity (mean duration 6 years), while immunity following asymptomatic infection was substantially weaker. We further quantify how vaccination coverage and population turnover jointly determine vaccination program impact, providing a practical framework for informing future campaign design.
Implications of all the available evidence
Taken together with prior evidence from South Asia, our findings indicate that preventive mass OCV campaigns can avert a substantial fraction of cholera cases and deaths in highly endemic settings across different epidemiological contexts. Nonetheless, the resurgence of cholera a year after the vaccination campaign underscores that OCV campaigns alone are insufficient for sustained cholera control in similar settings due to high transmission rates and high population mobility. Investments in water and sanitation infrastructure remains essential.
