Impact of oral cholera vaccine outbreak response immunization in sub-Saharan Africa, 2010-2020: a modeling analysis

Background Oral cholera vaccine (OCV) is a critical tool for controlling cholera outbreaks. However, the effectiveness and efficiency of reactive OCV campaigns depend strongly on implementation timing and targeting strategy. Methods Using data from 1,192 cholera outbreaks (2010-2020) in sub-Saharan Africa, we conducted stochastic simulations of single-dose OCV outbreak response immunization (ORI) strategies varying by timing, coverage, triggering criteria, and outbreak selection. Outcomes included impact (cases, deaths, and DALYs averted), efficiency (impact per 1,000 OCV doses), and cost-effectiveness (cost per unit of impact). Findings Week-1 ORIs with 75% vaccine coverage averted a median of 70% of cases (129 cases, IQR 41-375) whereas week-9 ORIs–comparable to historical response times–averted only ~8% (16 cases; IQR 0-112). Effectiveness declined exponentially by 23.6% per week (IQR 10.7-77.4), halving every 2.6 weeks. Efficiency fell from 0.69 to 0.09 cases averted per 1,000 doses, and cost-effectiveness worsened from US$4,662 to US$13,043 per case averted. When ORIs were triggered after case accumulation, effectiveness decreased by 6.4% (IQR 2.5-4.2) for every 10 additional cases, halving every ~104 cases. Prioritizing large or high-attack-rate outbreaks (top 10%) improved efficiency 14-27-fold and reduced cost per DALY averted by 95%-99%. Even with delays, week-9 ORIs deployed across multiple outbreaks, achieved a median 0.44 (IQR: 0.28-0.44) cases averted per 1,000 doses, which increasing further with targeted deployment. Interpretation Rapid deployment of OCV, with priority to large and high-attack-rate outbreaks, is essential to maximize the impact, efficiency, and cost-effectiveness of cholera response in Africa. Even delayed reactive vaccination can meaningfully reduce cholera burden.