## Measles Transmissibility and Epidemiology **Key Point:** Measles is one of the most highly contagious diseases, with a basic reproduction number (R₀) of 12–18. This means one infected person will infect 12–18 susceptible individuals in an unvaccinated population. **High-Yield:** The R₀ of measles is exceeded only by varicella (R₀ 10–12) among common viral infections. This extremely high transmissibility explains why measles vaccination coverage must exceed 95% to achieve herd immunity. ### Transmission Characteristics - **Route:** Respiratory droplets (airborne transmission via coughing/sneezing) - **Infectivity period:** 4 days before rash onset to 4 days after rash onset - **Incubation period:** 10–14 days (range 7–21 days); NOT 21–28 days - **Contagiousness:** Highly contagious during the prodromal phase (fever, cough, coryza, conjunctivitis — the "3 Cs") **Clinical Pearl:** Measles is most infectious 2–3 days before the rash appears, when diagnosis is often missed and transmission is highest. ### Why Herd Immunity Threshold is So High With R₀ = 12–18, the herd immunity threshold = 1 − (1/R₀) = 83–94%, requiring >95% vaccination coverage for sustained disease elimination. [cite:Park 26e Ch Communicable Diseases]
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