Herd immunity is

• A. the immune response of an individual after receiving a vaccine
• B. the immunity of a population achieved when disease is eliminated
• C. the resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, especially through vaccination.
• D. the immunity that develops after natural infection only

Answer: (C)

Herd immunity describes the resistance to the spread of a contagious disease within a population that arises when a large enough proportion of individuals are immune, especially through vaccination. When many people are immune, the chance that an infected person encounters a susceptible person is reduced, so transmission slows or stops. This protective effect extends to those who cannot be vaccinated or have weaker immune responses, because the disease has fewer opportunities to spread.

The exact level of immunity needed isn’t fixed; it depends on how contagious the disease is, quantified by the basic reproduction number, R0. The herd immunity threshold is roughly 1 minus 1 over R0, so more transmissible diseases require a higher proportion of immune people. Vaccination is a safe way to reach this threshold without people getting sick, though natural infection can also contribute to immunity.