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Herd immunity — USMLE Step 3 MCQ

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EasyPreventive Medicine, Epidemiology, BiostatisticsHerd immunityUSMLE Step 3

In a community, vaccination coverage against measles declines. Several infants too young for vaccination develop measles after exposure at a clinic. Public health officials explain that protection of susceptible individuals was reduced when population immunity fell. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: B — Reduced herd immunity

Herd immunity protects susceptible persons when a high proportion of the population is immune; declining coverage increases outbreak risk.

Reference: CDC Principles of Epidemiology