## Epidemiological Characteristics of MMR ### Measles **Key Point:** Measles has one of the highest R₀ values among infectious diseases, ranging from 12–18 in unvaccinated populations. This makes it highly contagious via respiratory droplets. ### Rubella **Key Point:** Congenital rubella syndrome (CRS) risk is gestational-age dependent: - First trimester (≤12 weeks): >85% risk of CRS - 13–16 weeks: ~50% risk - 17–20 weeks: ~15% risk - After 20 weeks: <10% risk ### Mumps **High-Yield:** Mumps outbreaks occur **predominantly in unvaccinated or under-vaccinated populations**, NOT in highly vaccinated populations. Mumps is a notifiable disease in India under the Integrated Disease Surveillance Programme (IDSP). Paradoxically, some outbreaks have been documented in vaccinated cohorts due to waning immunity or vaccine escape variants, but the primary epidemiological pattern remains clustering in low-vaccination-coverage areas. ### MMR Vaccine Efficacy **Clinical Pearl:** The MMR vaccine provides excellent long-term immunity: - Single dose: ~90% efficacy against measles, ~80% against mumps, ~97% against rubella - Two doses: >95% efficacy against all three diseases - Immunity is durable; booster doses are not routinely recommended in India's immunization schedule ### Why Option 2 is Wrong The statement claims mumps outbreaks are "most common in vaccinated populations with high herd immunity." This is **false**. Mumps outbreaks occur primarily in **unvaccinated or under-vaccinated populations**. While breakthrough infections can occur in vaccinated individuals (especially with waning immunity), the epidemiological pattern shows mumps clustering in low-coverage areas, not high-coverage areas. [cite:Park 26e Ch 10]
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