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Herd immunity is often referred to as "magic". Indeed, it may offer the unusual opportunity to get something from nothing: vaccination of one part of the population may protect the unimmunised part as well.

Nevertheless, there is also a risk: because of the phenomenon of herd immunity, under certain circumstances, a vaccination programme can do more harm than good on a population basis.

This might arise if a/ the target disease has more severe clinical consequences in adults than in children, and b/ the vaccination coverage achieved is below a certain threshold (the "sorcerer’s apprentice" situation).

Such diseases include rubella, chickenpox, mumps, hepatitis A.