OUTBREAK response immunisation (ORI) campaigns have averted millions of infections and hundreds of thousands of deaths from vaccine-preventable diseases in low and middle-income countries (LMICs) since 2000, according to the most comprehensive global modelling study of its kind.
Researchers analysed 210 outbreaks, spanning measles, cholera, yellow fever, meningococcal meningitis, and Ebola, across 49 LMICs between 2000–2023. Using publicly available outbreak reports and published literature, agent-based models were developed for each disease. These models simulated outbreak dynamics under two scenarios: the real-world response with ORI, and a counterfactual scenario assuming no vaccination. Calibrations accounted for disease-specific features, vaccine coverage, environmental context, endemicity, and response time, ensuring that simulated outbreaks reflected observed outcomes.
ORI programmes are estimated to have prevented 5.81 million cases (95% uncertainty interval [UI]: 5.75M–5.87M), 327,000 deaths (95% UI: 317K–338K), and 14.6 million disability-adjusted life years (95% UI: 14.1M–15.1M). Economic modelling suggested these interventions saved USD 31.7 billion in societal costs. The majority of this impact came from yellow fever (1.5M cases, 300K deaths, USD 30.7 billion saved), followed by measles (4.01M cases, 20K deaths, USD 710M), cholera (283K cases, 5.2K deaths, USD 156M), meningitis (21.3K cases, 1.6K deaths, USD 97.6M), and Ebola (820 cases, 381 deaths, USD 6.7M). Yellow fever responses were particularly cost-effective due to the high fatality rate and large-scale urban outbreaks.
A critical factor in campaign success was the speed of deployment. The modelling consistently showed that the shorter the time to ORI implementation, the greater the health and economic benefits. Delays reduced the effectiveness of vaccination in curbing transmission and mortality.
This study fills a major gap in outbreak response literature. Unlike prior modelling studies, which were often disease-specific and methodologically inconsistent, this research used a unified agent-based approach across multiple pathogens and geographies. It offers decision-makers consistent, quantitative evidence to support future investment in vaccine stockpiles and rapid deployment infrastructure.
The findings underline the need for continued global commitment to vaccine preparedness and funding for ORI programmes. As outbreaks remain a persistent threat in under-immunised populations, rapid vaccination remains one of the most powerful tools to save lives and limit economic disruption.
Reference
Delport D et al. Estimating the historical impact of outbreak response immunisation programmes across 210 outbreaks in low and middle-income countries. BMJ Global Health. 2025;10:e016887.
