Have you heard of herd immunity?                  

            It is more than a half century since polio has crippled American children. A disease that once sent parents into a panic was eliminated by the Salk and Sabin vaccines. Some readers will remember that schools, public facilities and swimming pools were shut down for fear of polio and many of us had relatives or classmates who were its victims.

            A few countries are not as fortunate as ours. Afghanistan, Pakistan, Nigeria and Syria, ripped apart by war, have not maintained their polio immunization programs, if they ever had one. Indeed, healthcare volunteers from other countries have been killed while dispensing vaccines because tribal leaders claim that the polio vaccine is a threat, not a lifesaver. Children in those countries are being crippled and killed by a preventable disease.

            Since the polio vaccine came into wide use the virus doesn’t have many available targets left. When a large majority of a population, 85 or 90 percent, has been immunized against a disease, there are too few susceptible individuals for the virus to gain a foothold and to cause an outbreak. Such herd immunity has protected First World populations from the devastation of measles, certain forms of meningitis, mumps, diphtheria and other diseases even though the germs themselves are still present. The only major disease that has been completely eradicated by aggressive vaccination is smallpox. For the others, only herd immunity will protect those persons who have not received the vaccine – until something happens.

            That “something” is usually the arrival of an infected person from a country that does not maintain herd immunity, and who circulates among unvaccinated, susceptible individuals. When a visitor, possibly from India, carried the polio virus into a religious community in the Netherlands that did not believe in immunization, 59 persons developed paralytic poliomyelitis and 2 died. Among the rest of the population of the Netherlands, where 97 percent of children have received the vaccine, there was not a single case of polio.

            A growing number of parents in the United States have chosen not to immunize their children because of fear of complications of vaccines or the feeling that childhood diseases have disappeared. Vaccines are not perfect but they are not nearly as deadly as the natural diseases that they protect us against. Recent outbreaks of measles and whooping cough in states where vaccine refusal is increasing are evidence of that.