Estimating the impact of decreasing vaccination response times for outbreaks in low- and middle-income countries
Authors: Delport Dominic , Muellenmeister Alina M. , Greig Jane , Abeysuriya Romesh Gerald , Scott Nick 
Published: February 2025

Background

Immunisation is an effective tool for responding to disease outbreaks, but in low- and middle-income countries (LMICs) accessing vaccine stockpiles can be slowed by logistical challenges. The 7-1-7 outbreak timeliness targets propose that outbreaks be detected within seven days; notification occur within one day; and responses be started within another seven days. For outbreaks of measles, cholera, yellow fever, and meningococcal meningitis, we aim to estimate the impact of initiating outbreak response immunisation (ORI) programs within 15 days of outbreak emergence, relative to the mean ORI response time for each disease since 2000.

Methods

Using calibrated agent-based models for the four diseases, a status-quo and series of ‘Faster response’ scenarios were compared for each disease, with a 15-day response time (7-1-7) as the minimum.

Findings

A 15-day ORI response could avert: 80% of cases from cholera outbreaks relative to a response time of 105 days; 35% of cases from meningococcal meningitis outbreaks relative to a response time of 75 days; 0–35% of cases from yellow fever outbreaks relative to a response time of 105 days (depending on routine vaccine coverage and environmental suitability); and 0–55% of cases from measles outbreaks relative to a response time of 120 days (depending on routine vaccine coverage).

Interpretation

Improvements made to ORI response time while aiming for 7-1-7 could reduce disease burden and decrease the risk of large outbreaks of vaccine-preventable diseases in LMICs. Efforts to improve response time should be prioritized to higher risk settings.

Funding source

Nothing to disclose.

Research in context

What is already known on this topic

We searched PubMed and Google Scholar on November 18, 2024, without date limits, using the search terms “((((outbreak response) AND (impact) AND (response time OR delay)) AND (model)) AND (countries)) AND (mortality OR morbidity)”. Studies were included if they used modelling approaches to estimate the health impact of varying the response time for reactive vaccination for at least one pathogen. We found there to be a lack of work using mathematical models to investigate the relationship between outbreak response time and vaccine impact. Of 302 results from PubMed, three studies met the inclusion criteria and in the first 50 pages (500 results) of Google Scholar, two studies met the criteria. A further eight studies were found incidentally over the course of the literature review. Of these 13 studies, three focused on measles, three on meningococcal meningitis, and one on each of cholera, Ebola, poliovirus, MERS-CoV, SARS-CoV-2, yellow fever, and a generalised SEIR model. No study considered multiple diseases, and none were considering the impact of achieving the 7-1-7 goal.

What this study adds

This study contributes to the existing literature by estimating the impacts of achieving the 7-1-7 targets for outbreaks of four high burden vaccine preventable diseases (measles, cholera, meningococcal meningitis, and yellow fever) in low- and middle-income countries, using agent-based models calibrated to historical outbreak data. We use agent-based models to capture the stochastic effects present in the early stages of an outbreak, and report the reductions in cases, deaths, and disability adjusted life years achieved by achieving 7-1-7 relative to the mean historical response time for each disease. We also investigate the incremental impacts of improvements to response time in steps down to a minimum of 15 days (7-1-7).

How this study might affect research, practice or policy

Outbreaks of vaccine preventable diseases still occur frequently in low- and middle-income countries because routine immunization programs are not available or there are gaps in coverage. When outbreaks do occur, vaccination responses can take weeks to months to begin. This study shows that if outbreak response immunization can be initiated within 15 days, then much more of the health burden can be averted relative to what is typically prevented. Furthermore, we found that incremental improvements in outbreak response immunization times can still have impacts reducing cases, even if a 15-day target is not met, and that the impacts of improvement are heterogeneous with greater benefits in higher risk settings. This serves as motivation for the implementation of outbreak response timeliness targets like 7-1-7, and suggests that nuanced and context-specific approaches to measuring and improving outbreak response timeliness may be an effective way to prioritize resources.