Measles has an R0 of approximately 15. Calculate the herd immunity threshold (HIT) and interpret whether a community with 90% MMR vaccination coverage is above or below the threshold and thus at risk for sustained transmission. Show calculations.

• A. HIT ≈ 85%; 90% coverage is above the threshold so the community is protected and outbreak risk is negligible.
• B. HIT ≈ 99.9%; because measles is extremely infectious, any coverage below 99.9% will allow outbreaks, so 90% is dangerously low.
• C. HIT = 50%; herd immunity is achieved at half the population immune for measles, so 90% is well above this and outbreak risk is impossible.
• D. HIT ≈ 93.3%; the community at 90% vaccination is below herd immunity and therefore at risk for sustained measles transmission if introduced.

Answer: (D)