## Measles Epidemiology in Pre-Vaccination Era **Key Point:** In the pre-vaccination era, measles was a highly endemic disease affecting primarily young children aged 1–5 years, with the highest incidence in this age group. ### Age-Specific Attack Rates - **Peak incidence:** 1–5 years of age (highest morbidity and mortality) - **Secondary peak:** 5–10 years in school-age children - **Rare in infants <6 months:** Due to maternal antibodies - **Rare in adults:** Due to prior infection and lifelong immunity ### Transmission Characteristics - **Highly contagious:** R₀ = 12–18 (one infected person infects 12–18 susceptible individuals) - **Airborne route:** Respiratory droplets; 90% of susceptible contacts develop disease - **Endemic pattern:** Continuous circulation in unvaccinated populations with periodic epidemic surges - **Seasonal variation:** Peaks in winter and spring in temperate climates; year-round in tropical regions **High-Yield:** The 1–5 year age group bore the highest burden of measles mortality in unvaccinated populations; this is why MMR vaccination targets this age group for maximum impact. **Clinical Pearl:** In countries with high vaccination coverage, measles has shifted to older age groups (adolescents and young adults) due to waning immunity or missed vaccination doses. [cite:Park 26e Ch 7]
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