Which statement best describes herd immunity?

• A. The resistance of a group to attack by a disease because a majority are immune
• B. Requires complete vaccination
• C. The resistance of individuals to disease due to personal immunity
• D. It is achieved only when all individuals are immune

Answer: (C)

Herd immunity is about protection at the population level. When a large enough portion of the community is immune, the disease has fewer opportunities to spread, which breaks transmission chains and lowers the risk for people who aren’t immune. This collective protection doesn’t require everyone to be immune; the required proportion depends on how contagious the disease is. Vaccination helps raise that threshold, and both vaccination and prior infection can contribute to reaching it.

Think of it as group-level protection rather than individual immunity. The statement that describes herd immunity as resistance due to personal immunity refers to an individual's immune status, not the protection arising from many immune people around them. The idea that herd immunity is achieved only when everyone is immune is also incorrect, since the necessary proportion is typically less than 100% and varies by disease.