Herd Immunity

Herd immunity is indirect protection resulting when population immunity reduces transmission. Its extent depends on the pathogen, immunity against infection or transmission, contact patterns, and waning. Coverage is not identical to effective immunity, and no universal threshold applies.

Herd immunity describes indirect protection when immunity in a population reduces transmission of an infectious agent. The protection depends on how well immunity prevents infection or onward spread, how people mix, and how immunity changes over time. Vaccination coverage and effective immunity are related but not identical quantities.

In a simplified homogeneous model with complete durable immunity, the threshold fraction is one minus one divided by the basic reproduction number. Real settings can depart substantially from those assumptions through clustering, waning, variants, and heterogeneous contact. Benefits can occur below a threshold, and crossing a modeled threshold does not make protection permanent or eliminate every outbreak.

When to use it

When interpreting community protection or evaluating assumptions in an infectious-disease transmission model.

How it can help

Examine the transmission model and disease-specific evidence, distinguishing theoretical thresholds from practical coverage targets and individual recommendations.