What proportion of the measles outbreak is due to unvaccinated pediatric and young adult patients?

Proportion of Measles Outbreaks Due to Unvaccinated Patients

The overwhelming majority of measles outbreak cases—approximately 89-96%—occur in unvaccinated or unknown vaccination status patients, with the most recent 2025 U.S. outbreak data showing 96% of cases in this category. 1

Current Outbreak Data (2025)

The most recent and definitive evidence comes from the ongoing 2025 multistate measles outbreak:

• 771 of 800 cases (96%) occurred in patients who were unvaccinated or had unknown vaccination status 1
  • 77% were confirmed unvaccinated
  • 14% had unknown vaccination status (excluding Texas cases due to opt-in registry requirements)
  • Only 4% had documented vaccination 1

This outbreak represents the second highest annual case count in the United States in 25 years, predominantly affecting close-knit communities with low measles vaccination coverage. 1

Historical Outbreak Patterns

The 2019 outbreak data corroborates this pattern:

• 1,107 of 1,249 cases (89%) occurred in unvaccinated or unknown vaccination status patients 2
• The NYC and NYS outbreaks in Orthodox Jewish communities with low vaccination coverage accounted for 75% of all 2019 cases 2

Earlier outbreak data from 2001-2008:

• 285 of 438 U.S. resident cases (65%) were considered preventable (occurred in vaccine-eligible but unvaccinated persons) 3
• The remaining 35% were either vaccinated, had other evidence of immunity, or were not vaccine-eligible 3

Healthcare Setting Transmission

In medical settings during 1990-1991:

• 561 of 668 healthcare worker cases (84%) occurred in unvaccinated individuals 3
• 187 (28%) of these unvaccinated healthcare workers required hospitalization, and three died 3

Age-Specific Vulnerability

Unvaccinated preschool children represent a particularly high-risk group:

• In documented outbreaks, 38% of unvaccinated preschool children with measles had missed vaccination opportunities when they received other vaccines 4
• In rural African settings with 58% vaccination coverage, unvaccinated children under 5 had a case fatality rate of 16.2%, rising to 24% for those under 9 months 5

Clinical Implications

The data unequivocally demonstrates that unvaccinated populations drive measles transmission:

• Attack rates exceed 90% in susceptible household contacts 2
• Outbreaks consistently occur when travelers import measles into communities with low vaccination coverage 1, 2
• Even with high national vaccination coverage (>90%), pockets of unvaccinated individuals create vulnerability for sustained transmission 1

Common Pitfalls

Do not assume birth before 1957 confers immunity:

• 5-9% of healthcare workers born before 1957 lack measles immunity 3
• During 1985-1992,27% of measles cases among healthcare workers occurred in persons born before 1957 3

Do not underestimate the role of unknown vaccination status:

• Unknown vaccination status should be treated as unvaccinated for outbreak control purposes 1
• Registry opt-in requirements can artificially inflate "unknown" categories 1