The herd immunity threshold depends on R0; which statement is true?

• A. The herd immunity threshold is not influenced by R0.
• B. A higher R0 requires a lower immune proportion.
• C. The threshold is the same for all diseases.
• D. A higher R0 requires a higher immune proportion.

Answer: (D)

Herd immunity hinges on how easily the disease spreads, quantified by R0. R0 is the average number of new infections caused by one infected person in a fully susceptible population. To stop transmission, the effective reproduction number must be below one. If a fraction p of the population is immune, the remaining susceptible portion is (1 − p), so the effective R is R0 × (1 − p). Setting this less than 1 gives p > 1 − 1/R0. So the higher R0 is, the larger the fraction that must be immune to halt spread. For example, with R0 around 2, the threshold is about 50%; with R0 around 3, it’s about 67%. This explains why a higher R0 requires a higher immune proportion.

The other statements don’t fit because herd immunity isn’t independent of R0, and the threshold isn’t the same for all diseases. A higher R0 does not mean a lower required immune proportion; it means the opposite. Real-world factors like uneven contact patterns, vaccine effectiveness, and waning immunity can shift the exact threshold, but the fundamental relationship remains: higher R0 demands a higher fraction immune to stop transmission.