What Is Herd Immunity and How Does It Protect Communities?

Herd immunity — also called community immunity or population immunity — is one of the most important and frequently misunderstood concepts in public health. It describes the indirect protection that entire communities receive when a sufficiently large proportion of their members become immune to an infectious disease, making it difficult for the pathogen to spread and protecting even those who cannot be immunized. Understanding herd immunity is essential for understanding how vaccination campaigns work, why high vaccination coverage matters beyond individual protection, and why some infectious diseases have been eradicated while others persist.

The Basic Concept

When an infectious disease spreads through a population, each infected person typically infects some number of other susceptible people. The basic reproduction number (R₀ — pronounced "R-naught") describes how many secondary infections one case generates in a completely susceptible population under natural conditions.

Different pathogens have very different R₀ values:

• Measles: R₀ of 12–18 (extremely contagious — each case infects 12–18 others in a fully susceptible population)
• COVID-19 (original strain): R₀ of approximately 2.5–3
• Seasonal influenza: R₀ of 1.2–1.4
• Polio: R₀ of 5–7
• Smallpox: R₀ of 5–7