Measles outbreak associated with an infectious travel

Dear Editor,

In 2025, the United States experienced its worst year for measles in over three decades, recording 2,255 confirmed cases—the highest since 1992. The situation remains critical in 2026, with 416 confirmed cases reported as of late January (https://www.cdc.gov/measles/data-research/index.html).

Measles outbreaks linked to infectious travelers are preventable public health failures, driven by immunity gaps and weak systems rather than by travel itself.

Measles is one of the most contagious human infections, with an R₀ estimated at 12–18, far exceeding influenza or SARS-CoV-2 [1]. Airborne particles can infect around 90% of susceptible contacts and remain viable in enclosed spaces for up to two hours [1, 2]. Infectiousness begins several days before the rash, when clinical suspicion and isolation are unlikely [2]. In crowded settings such as airports, aircraft, and health facilities, these properties turn a single undetected case into a potent spark for transmission where susceptibility exists [2, 3].

Global analyses show that measles resurgence since 2010 is tightly linked to declining measles-containing vaccine (MCV) coverage, especially in low- and middle-income countries, and to vaccine hesitancy in wealthier settings [1]. A large share of recent US and European outbreaks have been triggered by travel-related importations into undervaccinated communities, not by unusual travel patterns [4].

Modeling from Canada indicates that when two-dose coverage exceeds about 95%, 99% of introductions do not generate outbreaks; below ~ 85%, even strong interventions cannot prevent large outbreaks [5]. A global burden analysis from 1990 to 2019 found a consistent negative association between vaccine coverage and measles incidence, and highlighted that subpopulations missing vaccination—including travelers—can trigger localized outbreaks wherever immunity gaps persist [1].

Systematic reviews of parental hesitancy underscore social and informational drivers—fear of adverse effects, mistrust of institutions, and online misinformation—that cluster in specific communities and create pockets of susceptibility that imported cases can readily ignite [4]. In low-income settings, disrupted services, underfunded health systems, and COVID-19–related interruptions have further widened immunity gaps and driven outbreaks [6, 7].

These patterns show that unvaccinated or under-vaccinated travelers act as amplifiers, but only when they enter communities with immunity gaps and delayed detection [2, 5]. Where routine immunization is strong and evenly distributed, imported cases tend to cause few or no secondary infections [5]. Failures in surveillance—fragmented data, delayed reporting, and poor integration across borders—allow introductions to be recognized only after substantial spread [3, 7].

Strengthening pre-travel vaccination advice and requirements, especially for children and adults without documented MMR doses, is an immediately actionable prevention measure [2]. Once importation occurs, rapid diagnosis, airborne isolation, contact tracing, and timely cross-border information sharing are essential to prevent hospital-based and community amplification [3, 7].

Preventing traveler-linked measles outbreaks demands: (1) robust routine immunization achieving ≥ 95% two-dose coverage in every group; (2) enforced or strongly promoted measles vaccination for international travelers, supported by accessible pre-travel services; and (3) modern, interoperable surveillance and outbreak-preparedness systems, including near-real-time platforms, genomic tools, and coordinated cross-border response [1, 3, 7]. In this framework, infectious travelers are signals, not culprits—exposing preventable weaknesses in global measles control.

Author contributions

A.M.A & M.A.A conceptualized and designed the study. A.M.O conducted the literature review. A.M.A wrote the first draft of the manuscript. Y.B.A critically revised the manuscript for important intellectual content. All the authors have read and approved the final manuscript.

Funding

The authors have not been recipients of any funding for this specific study. Not applicable.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethical approval

This study did not involve any human or animal subjects, and thus, did not require review by an Institutional Review Board (IRB).

Competing interests

The authors declare no competing interests.