It only takes a small percentage of the population to destroy the herd immunity that we have built up over the decades since the 60's, when mass vaccination programs kicked into high gear.
From Wikipedia:
Vaccination acts as a sort of firebreak or firewall in the spread of the disease, slowing or preventing further transmission of the disease to others.[4] Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter are less likely to contract and transmit the disease between infected and susceptible individuals.[3] Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder, organ transplant recipients, or people with egg allergies.
Herd immunity generally applies only to diseases that are contagious. It does not apply to diseases such as tetanus (which is infectious, but is not contagious), where the vaccine protects only the vaccinated person from disease.[5] Nor does it apply to the IPV poliomyelitis vaccine that protects the individual from viremia and paralytic polio but does not prevent the fecal-oral spread of infection. Herd immunity should not be confused with contact immunity, a related concept wherein a vaccinated individual can 'pass on' the vaccine to another individual through contact.
The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold. Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population.[4] No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.[6] It is the general aim of those involved in public health to establish herd immunity in most populations. Complications arise when widespread vaccination is not possible or when vaccines are rejected by a part of the population. As of 2009, herd immunity is compromised in some areas for some vaccine-preventable diseases, including pertussis and measles and mumps, in part because of parental refusal of vaccination.[7][8][9]
A report by the Centers for Disease Control and Prevention analyzed the gastroenteritis hospitalization rate in the USA before (2000 to 2006) and after (2008 to 2010, the transition year 2007 is excluded) the infant rotavirus vaccination program was introduced in 2006. The analysis shows that not only was the hospitalization rate among infants greatly reduced, but older children and adults who are not usually vaccinated against rotavirus also saw significant declines in hospitalization rates. Since the rotavirus vaccine was introduced, hospitalizations due to rotavirus among children under 5 years old have decreased by 80%, those between 5 to 14 years old by 70%, and people over 65 years old by 14%. The decline in hospitalizations for people between 14 to 65 years of age fell somewhere in between. Rotavirus cases not requiring hospitalization were not measured, but a decline in cases can be reasonably deduced. This report demonstrates that herd immunity incurred by the infant rotavirus vaccination program benefits the rest of the population.[10][11]
