Impact evaluations of mass oral cholera vaccination campaigns
Study design
Project comprising several studies is currently underway at two sites in the Democratic Republic of the Congo: Goma (urban area) and Bukama (rural area).
Context
Cholera remains a global health threat, persisting in settings with inadequate water, sanitation and hygiene (WASH) access. Killed oral cholera vaccines (OCV), in combination with WASH improvements, have become a standard component in cholera control programs. Most available evidence on OCV impact has been related to individual-level protection from clinical disease. Mass vaccination campaigns have the potential to also reduce transmission and occurrence of pandemic Vibrio cholerae in the environment. In 2020, mass OCV campaigns were deployed in cholera endemic regions of the Democratic Republic of the Congo. The study, scheduled to run for 2 years and assess the impact of mass OCV deployment in Goma and rural Ex-Katanga, has been amended and extended to 2024.
The transformed project will aim to evaluate the strategies and impact of major preventive vaccination campaigns against OCV in different cholera outbreak contexts in DRC, and will include various activities:
1. Continued clinical surveillance of cholera in sites targeted by different vaccination strategies.
2. Case-cohort study in Goma with the aim of reinforcing estimates of vaccine efficacy
3. Regular surveys, including vaccination coverage, mortality, incidence of cholera symptoms and influencing factors, registration of population movements and access to care 4. Qualitative approach (focus groups and in-depth interviews).
Our role
Coordination
Partners
Ministry of Health, DRC
PNECHOL
INRB
- Funders: Wellcome, UKaid and FCDO (Foreign and Commonwealth and Development Office).
Objective
The study is expected to generate evidence on the long-term epidemiological and environmental impacts of mass OCV administration. The combination of clinical surveillance, serological surveys, and household contacts and environmental sampling will also contribute to improving our understanding of cholera transmission dynamics in endemic settings characteristic of transmission hotspots and inform articulated WASH-OCV strategies.
Objective of the amended project
Evaluate the impact of major preventive cholera vaccination campaigns by calculating vaccine effectiveness and monitoring appropriate indicators, in an urban cholera outbreak in the Democratic Republic of Congo (DRC).
Methods
Our protocols combine (1) clinical surveillance, (2) serological surveys, and (3) household follow-ups. We will collaborate with health authorities to strengthen clinical surveillance systems. Culture of V. cholerae, molecular assays and cholera serology will be conducted in North Kivu (including optimisation of a qPCR assay for the simultaneous detection and differentiation of toxigenic V. cholerae O1 and O139). A sequence of serological surveys will be conducted at each study sites to assess cholera infection rates in the community, providing a measure of transmission independent of infection severity and health-seeking behaviour. Finally, a series of follow-up visits to the households of confirmed cholera cases will be conducted estimate household secondary attack rates and cholera shedding time among vaccinated and non-vaccinated persons. Environmental samples will be collected from households and water sources to monitor changes in V. cholerae detection frequency. Clinical, household, and environmental isolates will be sequenced to better understand V. cholerae circulation within households and study sites.
For the amended part of the project, this will be a prospective observational study, following a case-cohort design, with a dynamic aspect to control for certain biases.
Cholera dynamics and impact of preventive vaccine in endemic zones of DRC, 2021-2026
Results
In Goma, OCV coverage level was estimated at 65% [61.7-68.2] in 2024 following a 1- dose campaign. In Bukama, a 2-doses campaign in 2022 reached 89.3% [88.3-90.3] coverage during the 1st round. Over 2021-2026, 28’630 and 2’684 suspected cholera cases were included in Goma and Bukama, respectively. At individual level, preliminary results show that vaccination status was significantly protective of laboratory-confirmed cholera in both contexts. In Goma, massive internal population displacements at the end of 2022 were followed by two epidemic peaks at the beginning of 2023 and 2024. Camps were identified as the main epidemic driver in the city, and OCV impact remains unclear at populational level. In Bukama, there was a strong seasonal pattern to cholera incidence; preventive vaccination was effective in preventing future outbreaks, but only under optimal conditions.
Conclusion
Early results in these two cholera endemic sites in the DRC provide elements to guide the implementation of future vaccination campaigns. For instance, targeting incoming displaced population might be relevant in contexts such as Goma. Additional analyses, such as the estimation of vaccine effectiveness, will help consolidate preliminary findings and further help design efficient control strategies.
Assessing cholera transmission in endemic areas following mass preventive OCV campaigns in DRC
Preliminary results for 2023
In 2023, in Goma, vaccine coverage two years after last vaccination is lower than expected at 49.5% in the targeted zones. Over 8000 suspect cases were included in clinical surveillance in Goma, with cases reported across most of the city. In Bukama, high levels of vaccination coverage were reported through a community approach survey, with most areas reporting 80 to 90% coverage. Close to 1000 cases were reported, and while an epidemic was ongoing at the time of vaccination, notification levels have remained low and stable ever since. In Goma, drinking surface or tank-delivered water appeared to be a risk factor for cholera infection, while in Bukama, the associated risk was the public distribution system.
Conclusion
The study sites present different pictures in terms of their OCV campaigns as well as their cholera surveillance profile and transmission. Preliminary results offer elements to guide the implementation of vaccination campaigns. In Goma, for instance, patchy vaccination targets and population movements may have diluted coverage. Early findings illustrate risk factors for cholera transmission, providing operational insights to enhance control strategies. Future results will incorporate other types of data and help design efficient vaccination strategies.
