The theory of herd immunity was a term applied to the general immunity to a pathogen in a population based on the acquired immunity to it by a high proportion of members over time. It was never intended to describe potential group immunity gained from vaccination. This distinction is important. Vaccination is not synonymous with immunization, although the terms are used interchangeably.
Natural immunization from disease is a complex interactive process involving many bodily organs and systems; it cannot be replicated by the artificial stimulation of antibodies. There is scant evidence to suggest that herd immunity can be applied to vaccines the way the term was meant to be used for contracting a disease. Why? Because in addition to affecting our immune system in separate pathways, contracting a disease gives permanent lifelong immunity, vaccines do not, hence the booster shot in various vaccines.
Research continues to indicate that vaccine acquired herd immunity is a dying myth because many vaccines including measles and pertussis have become ineffective. Fully vaccinated populations still experience epidemics. It is also not very scientific to blame measles or pertussis outbreaks on unvaccinated individuals, although it is currently very popular to do so. Herd immunity is a term that should belong to its original intent of disease contraction and stay out of the vaccination lingo. It just does not fit.
— Dr. Michael Bennese