Herd immunity

Herd immunity is an important yet often misunderstood concept in epidemiology. As immunity accumulates in a population — naturally during the course of an epidemic or through vaccination — the spread of an infectious disease is limited by the depletion of susceptible hosts. If a sufficient proportion of the population is immune — above the ‘herd immunity threshold’ — then transmission generally cannot be sustained. Maintaining herd immunity is therefore critical to long-term disease control. In this primer, we discuss the concept of herd immunity from first principles, clarify common misconceptions, and consider the implications for disease control. Much is being said about herd immunity these days. But the concept, though seemingly simple, is often misunderstood. Ben Ashby and Alex Best explain what exactly herd immunity is, how it is achieved in a population, how it is lost, and what it means for disease control.