Epidemic measles in a highly vaccinated population

Area of Science:

• Immunology
• Epidemiology
• Public Health

Background:

• Measles vaccination is a key public health intervention.
• Assessing vaccine efficacy in real-world outbreaks is critical for understanding population immunity.

Purpose of the Study:

• To evaluate the effectiveness of measles vaccination, particularly regarding age at vaccination and the impact of a second dose.
• To determine antibody prevalence and protection levels in a population experiencing a measles outbreak.

Main Methods:

• Conducted a serologic survey and historical review of a population during a measles outbreak (November 1975-May 1976).
• Assessed measles antibody levels based on vaccination history, age at vaccination, and time since vaccination.
• Compared measles incidence rates between groups with different vaccination schedules.

Main Results:

• 9% lacked measles history/vaccination; 18% had no detectable antibody despite purported immunization.
• Antibody prevalence decreased with younger age at first vaccination (≥13 months: 92%, 12 months: 80%, <1 year: 67%).
• A second dose significantly increased antibody prevalence in those vaccinated <9 months (42% to 80%) and protected against illness (35% vs. 2% in unvaccinated vs. vaccinated).

Conclusions:

• Single measles vaccination before 12 months of age provides inadequate protection.
• A second measles vaccination dose is essential for conferring protection to infants vaccinated at less than 12 months old.