Questions: Herd Immunity and Vaccination Programs

Question 1

Measles has an R₀ of approximately 15. What is the approximate herd immunity threshold (HIT) for measles?

• A. 50%
• B. 75%
• C. 93%
• D. 99%

Answer: (C)

HIT = 1 − 1/R₀ = 1 − 1/15 ≈ 0.933, or about 93%. This is why measles vaccination programs require very high coverage—a small drop below this threshold can enable sustained outbreaks, as observed when vaccine hesitancy reduces community coverage into the 80s.

Question 2

Once a population's vaccination coverage exceeds the herd immunity threshold, localized outbreaks of that disease cannot occur.

• T. True
• F. False

Answer: False

The HIT assumes homogeneous (random) mixing across the population. In reality, unvaccinated individuals often cluster geographically, socially, or within religious communities. Even if overall coverage exceeds the HIT, a dense pocket of susceptibles can sustain local transmission. This is why monitoring coverage heterogeneity matters as much as the aggregate rate.

Question 3

Why does pursuing 'natural herd immunity' by allowing an infectious disease to spread widely carry a fundamentally different cost-benefit tradeoff than achieving the same immunity threshold through vaccination?

Model answer: Vaccination produces immunity without causing disease, so the herd threshold can be reached without the morbidity, mortality, and complications that natural infection imposes. Allowing disease to spread harms or kills many people—especially vulnerable individuals who cannot be vaccinated—to confer immunity on survivors. Vaccines achieve the epidemiological benefit at a fraction of the human cost.

This misconception arises from treating immunity as the only variable that matters. The path to that immunity matters enormously: natural infection kills, hospitalizes, and disables a predictable fraction of those infected (especially infants, the immunocompromised, and the elderly), whereas vaccination confers equivalent immunity with rare, typically mild adverse events.