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August 2007
Protecting the "Herd"
Why Vaccinations Still Matter

by Steven F. Hirsch, M.D.

The routine use of vaccines is arguably one of the greatest medical interventions of all time. Childhood disease and death rates have dramatically declined as a direct result of mass vaccinations. Parents, doctors and scientists alike should be proud of the tremendous contribution that vaccines have made to the health of our children and our community.

However, it is not evident to most parents why their children still need to be vaccinated against diseases so rare that you (and I for that matter) have most likely never seen a case. Typically, the only reason given to parents to vaccinate against rare diseases, such as polio, is "to protect their children from an exposure in the community."

While that argument is true, it omits the fundamental scientific reasoning behind mass vaccination, known as the principle of herd immunity. As long as a disease still exists in any part of the world, protecting each individual is necessary to give protection to the rest of the population or the "herd."

The principle of herd immunity states that if you reach a critical percentage of community immunity, typically between 85 - 95 percent, the herd effect will protect the remaining 10-15 percent without immunity. This 10 - 15 percent includes children who are too young to receive a vaccine, those who are unable to receive certain vaccinations due to contraindications (i.e., cancer, HIV) and those who unknowingly have a vaccine failure. A vaccine failure occurs when a person who is given a vaccine does not have an appropriate immune response to it, and thus is still unknowingly susceptible to the disease.

For each virus, statisticians are able to calculate the minimum percentage of community immunity necessary to achieve herd immunity and prevent an outbreak. Though we only need about 85 percent of the community to have immunity to rubella, smallpox and diphtheria to prevent an outbreak, diseases such as whooping cough (pertussis) and measles require at least 94 percent immunity.

To illustrate the concept of herd immunity, consider three diseases that are highly contagious but preventable through vaccination: smallpox, polio and measles.

Smallpox

In the case of smallpox, scientists calculated that if the critical herd immunity percentage could be maintained in enough locations for a long enough period of time, smallpox would be eradicated forever. In 1967, a year in which smallpox infected an estimated 15 million people leading to 2 million deaths, the World Health Organization launched an intensive campaign to eradicate smallpox through vaccination.

In a tremendous display of international cooperation and vaccine distribution, the global eradication of smallpox was achieved in 1977. Though the last case of smallpox in the United States was recorded in 1949, in order to maintain herd immunity in this country, routine vaccination continued until 1971.

If more people had chosen not to vaccinate their children, as the disease became less common, our population could have dropped below the critical herd immunity percentage. Thus, the global eradication initiative would have been in jeopardy, and there would be a good chance our children would still need a smallpox vaccine today. Instead, in 2007, the only remaining sign of smallpox is the small scar on the upper arms of many who received the smallpox vaccine decades ago.

Polio

Polio was the next disease targeted for eradication. About 1 - 2 percent of those who have polio get meningitis and about 1 percent become paralyzed. Due to aggressive vaccination programs, polio was thought to be eliminated in the United States in 1979. Unfortunately, we are still at risk of an epidemic because there are large pockets of communities in this country that are not vaccinated against polio and thus remain below the herd immunity percentage.

In 2005, five unvaccinated Amish children in a Minnesota town unexpectedly tested positive for the polio virus. Since the majority of polio infections have minimal or no symptoms, most people carrying the virus do so unknowingly and therefore unwittingly transmit the disease to others. In this case, because the surrounding communities had a high vaccination rate (and maintained herd immunity), the virus was contained and did not lead to a widespread epidemic.

Measles

In 2003, measles caused an estimated 530,000 deaths in developing countries. This is especially tragic because a simple vaccination can prevent the disease. From 1990 to 2005, successful vaccination programs have decreased the number of cases in the Western Hemisphere by more than 99 percent.

In 2003, a measles outbreak occurred in the Republic of the Marshall Islands. Though the population is only 56,000, because the immunization rate of 75 percent is below the necessary herd immunity rate of 94 percent, the epidemic resulted in 703 cases of measles and 3 deaths. Compare this to a 2003 measles outbreak in Mexico where the measles immunization rate was over 95 percent, well above the herd immunity threshold. In Mexico, with a population of 100,000,000, only 41 cases were diagnosed before the disease was contained.

Though your doctor, school system and government may require you to fully vaccinate your child, as a parent, the decision is ultimately yours. Your child can be exempt from vaccines for religious or philosophical reasons.

However, if you are considering not fully vaccinating your child, consider the increased risk to him if he is exposed to the disease, and evaluate the public health risks. Remember that, through the principle of herd immunity, vaccines not only protect your child, but also provide additional protection to the 10 - 15 percent of the population who have no immunity and are thus the most vulnerable.

Sustaining herd immunity to these diseases has led to the global eradication of smallpox, the Western Hemisphere eradication of polio and extremely low rates of measles, diphtheria and pertussis. With continued aggressive vaccination programs around the world, we could see the global eradication of both polio and measles in our lifetime.


Steven F. Hirsch is a pediatrician in solo practice in Rockville. Visit his website, www.hirschpediatrics.com.

Members of the Montgomery County Medical Society will frequently share articles about health and medicine with readers of Washington Parent.


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