Illustration depicting disease clusters undermining vaccination efforts.

Is Your Community Vulnerable? Why Measles Outbreaks Defy Vaccination Rates

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Mira Elwood in Health & Wellness December 2025 • 4 min read.

"New research reveals how 'clustering' of unvaccinated individuals can undermine herd immunity and fuel outbreaks, even in highly vaccinated areas."


For years, public health officials have emphasized the importance of high vaccination coverage to achieve herd immunity and eliminate diseases like measles. The goal? To reach a critical vaccination threshold (Vc), ensuring enough people are immune to prevent outbreaks. However, the persistence of measles outbreaks, even in countries with seemingly high vaccination rates, suggests a more complex picture.

Traditional estimates of Vc assume that populations are evenly mixed, meaning everyone has an equal chance of interacting with everyone else. But what happens when this assumption is violated? What if susceptible individuals—those not vaccinated or immune—are more likely to come into contact with each other? This is where the concept of "clustering" comes in, and new research sheds light on its potentially dangerous impact.

A recent study published in Vaccine explores how spatial clustering of susceptibility can undermine measles elimination efforts. Researchers developed a method to estimate Vc and the effective reproductive number (R)—a measure of how many people an infected person is likely to infect—while accounting for this clustering effect. Their findings highlight the need to move beyond national vaccination goals and focus on subnational strategies to achieve true measles elimination.

The Clustering Effect: How Susceptibility Hotspots Fuel Measles

Illustration depicting disease clusters undermining vaccination efforts.

The study highlights a critical flaw in traditional approaches to vaccination planning: the assumption of evenly mixed populations. In reality, people tend to cluster geographically and socially. This means that unvaccinated individuals are more likely to interact with other unvaccinated individuals, creating pockets of susceptibility.

Think of it like this: imagine two communities with the same overall vaccination rate. In one community, the unvaccinated individuals are spread evenly throughout the population. In the other, they are concentrated in specific neighborhoods or social groups. Which community is more vulnerable to a measles outbreak? The answer is likely the community with clustering.

  • Higher Reproductive Number (R): Clustering increases the effective reproductive number, meaning a single infected person can infect more people than in a homogeneously mixed population.
  • Elevated Critical Vaccination Threshold (Vc): To achieve herd immunity, a higher percentage of the population needs to be vaccinated when susceptibility is clustered.
  • Increased Outbreak Risk: Even when national vaccination targets are met, communities with high clustering remain vulnerable to outbreaks following a single introduction of the virus.
To illustrate the impact of clustering, researchers used data from Tanzania Demographic and Health Surveys, they analyzed data of clustering and non-vaccination contributing to endemic transmission of measles during last two decades. That shift from national to subnational focus needed, as countries approach elimination.

Beyond National Goals: A Targeted Approach to Measles Elimination

The study's findings underscore the need to move beyond a one-size-fits-all approach to vaccination. High national vaccination coverage is essential, but it's not enough to guarantee protection against measles outbreaks.

Instead, public health efforts must focus on identifying and addressing areas with high susceptibility clustering. This may involve:

<ul><li>Micro-Targeting Vaccination Efforts: Tailoring vaccination campaigns to reach specific communities or social groups with low coverage.</li><li>Addressing Root Causes: Identifying and addressing the underlying factors contributing to clustering, such as lack of access to healthcare, misinformation, or social barriers.</li><li>Enhanced Surveillance: Implementing robust surveillance systems to detect and respond to outbreaks quickly, especially in high-risk areas.</li></ul>

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.vaccine.2018.12.012, Alternate LINK

Title: Characterizing The Impact Of Spatial Clustering Of Susceptibility For Measles Elimination

Subject: Infectious Diseases

Journal: Vaccine

Publisher: Elsevier BV

Authors: Shaun A. Truelove, Matthew Graham, William J. Moss, C. Jessica E. Metcalf, Matthew J. Ferrari, Justin Lessler

Published: 2019-01-01

Everything You Need To Know

1

What is herd immunity and the Critical Vaccination Threshold, and how do they relate?

The concept of herd immunity aims to protect those who cannot be vaccinated by having a sufficient portion of the population immune to a disease. The Critical Vaccination Threshold (Vc) is the percentage of the population that needs to be vaccinated to achieve herd immunity. However, this threshold can be higher in communities with significant clustering of unvaccinated individuals.

2

What is meant by 'clustering' in the context of measles outbreaks?

Clustering refers to the tendency of unvaccinated individuals to group together, whether geographically or socially. This creates pockets of susceptibility where measles can spread more easily. Even with high overall vaccination rates, if unvaccinated people are clustered, the community remains vulnerable to outbreaks because the virus can circulate rapidly within these groups.

3

How does 'clustering' impact the Reproductive number (R) of measles?

The Reproductive number (R) is a measure of how many secondary infections result from a single infected individual. In the context of measles, clustering increases the effective R, as an infected person is likely to infect more people within a cluster of unvaccinated individuals. This heightened R means that outbreaks can occur even when overall vaccination rates appear high.

4

Why are measles outbreaks still happening even with high vaccination rates?

Measles outbreaks persist despite high national vaccination rates because traditional vaccination strategies often assume an evenly mixed population. When susceptible individuals are clustered, the Critical Vaccination Threshold (Vc) increases. Therefore, to protect against measles, a focus on subnational vaccination strategies that address areas with clustering of unvaccinated individuals is necessary.

5

What does a subnational approach to measles vaccination involve?

A subnational approach to vaccination focuses on specific communities or regions with high clustering of unvaccinated individuals. Instead of solely relying on national vaccination goals, this strategy involves targeted vaccination efforts to reach those most at risk. This approach aims to increase vaccination coverage within susceptible clusters, reducing the effective Reproductive number (R) and better protecting the entire community from measles outbreaks.

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