What is herd immunity?

Herd Immunity and Unvaccinated Children

Unvaccinated children may put their classmates at risk for preventable diseases.
Unvaccinated children may put their classmates at risk for preventable diseases.
Britt Erlanson/The Image Bank/Getty Images

If the word "vaccine" strikes fear in your heart, then you likely came of age not in a time dominated by worries about polio, but in a time when vaccines make news because of side effects ranging from mild irritation to death. The most notable example is probably the debate regarding the claim (not supported by any scientific studies thus far) that the thimerosal in certain childhood vaccines may have led to an increase in autism. These days, in spite of the lack of scientific evidence to support a link between vaccines and autism -- and the fact that thimerosal is no longer used in most childhood vaccines -- much of the public has serious doubts about whether it's worth protecting herd immunity at the cost of exposing children to these supposed risks.

That means that levels of vaccinations have dropped dramatically, particularly in certain parts of the United States where exemption from vaccinations is granted for religious, philosophical or personal reasons. In 2002, the Centers for Disease Control and Prevention estimated that while 90 percent of all American children had received most of the necessary vaccinations, about 1 percent of all children hadn't received any shots [source: McNeil].

Parents who forgo vaccinations may believe they can rely on herd immunity to protect their children: Children who aren't immunized may be protected by the children that did get vaccinations. However, some parents who did vaccinate their children consider this freeloading [source: Ulene]. In addition, as parental fears cause vaccination rates to drop, the safety of the herd, particularly its weakest members, can become compromised.

PubliĀ­c health officials worry that herd immunity may have been too successful for its own good [source: McNeil]. Because all children were immunized in the past, many parents today haven't seen the effects of a widespread polio epidemic or a measles outbreak, diseases that still prevail in countries where vaccines are unaffordable. But what those parents may have seen, however, are reports that detail the side effects of a vaccination, though reports of extreme reactions to vaccines are usually very rare. For example, the polio vaccine is thought to be responsible for eight deaths a year [source: McNeil]. That's frightening, but health officials say that the fear can't compare to the peak of the polio epidemic, when fear of the iron lung was endemic.

While the unvaccinated children represent a threat to herd immunity just by walking around, the refusal of some people to get vaccinated obviously affects the system of supply and demand. Certain vaccinations may not be manufactured if there's a loss of demand for them, meaning that those that truly do benefit from and need their protection might not have access to them anymore. In other words, those of us that mean to get flu shots but never do may inadvertently cause the level of available vaccines to plummet, making it harder to strengthen the herd.

For more information on vaccines and other health-related topics, head on over to the next page.

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