Correct Answer: D. 96%
Measles eradication requires a vaccination coverage of 96% according to WHO guidelines. This threshold is based on the concept of herd immunity and the basic reproduction number (R₀) of measles, which is approximately 12–18 (one of the highest among vaccine-preventable diseases). The herd immunity threshold is calculated as: HIT = 1 − (1/R₀). With an R₀ of ~15, the HIT becomes approximately 93–95%, which WHO rounds to 96% coverage as the practical target for eradication. This exceptionally high coverage requirement reflects measles' extreme transmissibility—a single infected person can infect 12–18 susceptible contacts in an unvaccinated population. In the Indian context, achieving 96% coverage remains challenging due to vaccine hesitancy, cold-chain logistics in rural areas, and migrant populations. India's National Immunization Schedule targets 95% coverage with two doses of measles-containing vaccine (MCV1 at 9–12 months and MCV2 at 16–24 months) as per IAP guidelines. The 96% threshold is not arbitrary but epidemiologically derived and represents the minimum needed to interrupt measles transmission chains and prevent outbreaks, even when pockets of susceptibility exist.
Why the other options are wrong
A. 72% — This is wrong because 72% coverage is insufficient to achieve herd immunity for measles. This percentage might be confused with coverage targets for diseases with lower R₀ values (e.g., polio, diphtheria). NBE traps students who conflate measles eradication thresholds with other vaccine-preventable diseases that require lower coverage percentages. B. 84% — While 84% is a reasonable immunization coverage target for many diseases, it falls short of measles eradication requirements. This may represent intermediate coverage goals or targets for disease control (not eradication). The trap here is confusing disease control (reducing burden) with eradication (complete elimination), which demands the highest coverage threshold. C. 68% — This is wrong because 68% coverage is far below the herd immunity threshold for measles and would allow sustained transmission. This low percentage might be mistakenly associated with diseases with much lower transmissibility. NBE uses this as a distractor for students unfamiliar with measles' exceptionally high R₀ and corresponding high eradication threshold.
High-Yield Facts
- Measles R₀ is 12–18, the highest among vaccine-preventable diseases, necessitating 96% coverage for herd immunity and eradication.
- Herd immunity threshold (HIT) = 1 − (1/R₀); for measles with R₀ ~15, HIT ≈ 93–95%, rounded to 96% by WHO.
- India's National Immunization Schedule mandates two doses of MCV (at 9–12 months and 16–24 months) to achieve the 95–96% coverage target per IAP guidelines.
- Disease control (reducing cases) requires lower coverage (~90%); eradication (complete elimination) requires 96% for measles.
- Even 95% coverage is insufficient for measles eradication; the 96% threshold accounts for vaccine failures, cold-chain losses, and hard-to-reach populations in resource-limited settings like India.
Mnemonics
Measles Eradication = 96% (The 'Nearly Perfect' Rule) Measles is so contagious (R₀ 15) that you need **96%** coverage—nearly perfect vaccination. Remember: **9 out of 10 people vaccinated is NOT enough; you need 96 out of 100**. This is the highest threshold among vaccine-preventable diseases. **R₀ Drives Coverage: Measles R₀ = 15 → HIT = 93–96%** Higher R₀ = Higher coverage needed. Measles' R₀ of 15 is the highest, so its eradication threshold (96%) is also the highest. Polio (R₀ 5–7) needs 85%; measles needs 96%.
NBE Trap
NBE conflates measles eradication (96%) with disease control targets (84–90%) or pairs measles with lower R₀ diseases to trap students unfamiliar with measles' exceptional transmissibility. The trap is especially effective in India, where achieving 96% coverage remains a persistent challenge.
Clinical Pearl
In India, even states reporting 95% measles vaccination coverage have experienced outbreaks in migrant populations and vaccine-hesitant communities, illustrating why the 96% threshold is non-negotiable. A single unvaccinated cluster in a high-density urban area can seed transmission chains that overwhelm surveillance systems.
_Reference: Park's Textbook of Preventive and Social Medicine (Ch. Immunization & Vaccine-Preventable Diseases); WHO Measles Eradication Guidelines; IAP Immunization Schedule_