# Investigating persistent measles dynamics associated with population immunity in Chad: a national secondary data model analysis from 2011 to 2025

## TL;DR

This study models measles immunity gaps in children in Chad from 2011 to 2025, showing persistent susceptibility due to low vaccination coverage.

## Contribution

The study provides a national analysis of measles immunity gaps in Chad using secondary data and modeling.

## Key findings

- Immunity gaps ranged from 5% to 6% in 2011-2013 and increased to 21% in 2016-2017.
- From 2019 to 2022, immunity gaps decreased to 6-8%, but increased again in 2023-2025.
- Low MCV1 coverage, late MCV2 introduction, and poor MCV2 coverage contribute to immunity gaps.

## Abstract

•Measles outbreaks are recurrent in Chad.•Nearly 33% children in Chad are underimmunized.•Significative cumulative cases of measles susceptibility in Chad.•High population measles immunity profile gaps in Chad.

Measles outbreaks are recurrent in Chad.

Nearly 33% children in Chad are underimmunized.

Significative cumulative cases of measles susceptibility in Chad.

High population measles immunity profile gaps in Chad.

To model the population measles immunity gaps in a birth cohort of children aged 0-14 years in Chad from 2011 to 2025.

This study used the Centers for Disease Control and Prevention–established population modeling analysis measles immunity profile approach, including data published by World Health Organization, United Nations Children’s Fund, and United Nations Population. Excel and R Software were used to build a stacked bar chart of population immunity using immunity by measles-containing vaccine (MCV) dose 1 (MCV1) (%), by MCV dose 2 (MCV2) (%), by supplemental immunization activities (%), by maternal antibodies (%), and the rate of unprotected individuals against measles.

The population immunity gaps ranged from 5% to 6% in the cohort of children born in 2011-2013. We estimated the immunity gaps to be 13% in 2014-2015. Our results revealed an increase in population immunity gaps of 21% in 2016 and 2017 and 20% computed in 2018. From 2019 to 2022, the immunity gaps are ranged from 6% to 8%, correlating with a decrease of three to four times less than 2016-2018. A significant increase in immunity was observed in 2023, 2024, and 2025, with 46%, 53%, and 36%, respectively. The overall cohort showed a significative cumulative number of cases of measles susceptibility correlated with population immunity gaps. The findings also revealed insufficient MCV1 coverage, a late introduction of MCV2, poor MCV2 coverage, and a high MCV1/MCV2 abandonment rate over time.

The risk of recurrent measles outbreaks remains high because of high population immunity profile gaps, increasing the population susceptible. To address immunity gaps, it is necessary to increase coverage by routine vaccination and high-quality supplemental immunization activities in Chad.