What is herd immunity?

Vaccination and Herd Immunity

It may sting for a minute, but you're helping to protect Grandma from the flu.
It may sting for a minute, but you're helping to protect Grandma from the flu.
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For many of us, chicken pox was a routine part of childhood. A minor annoyance, but rarely life-threatening. For that reason, a chicken pox vaccine was met with some skepticism when it was introduced in 1995. Still, there was a push in the United States to get children vaccinated, and studies have shown that the effort had tremendous impact on the disease and the costs associated with it. Hospital bills related to chicken pox dropped from $160 million in 1993 to $66 million in 2001, and only one in 80,000 people required hospitalization due to chicken pox in 2001, compared to one in every 20,000 people in 1995 [source: McKay].

Not only does a push for a chicken pox vaccine protect a child and his or her classmates, it can also protect grandparents who may not have had chicken pox. Protecting the elderly is also the idea behind another vaccination that many of us line up for each winter: the flu shot. About 36,000 Americans die of the flu each year, most of them over the age of 65 [source: Allen]. While the elderly are encouraged to get a flu shot, it's actually more effective if the herd around them is vaccinated, including caretakers and visitors that might include germy grandchildren.

The success of the polio vaccine demonstrated the benefits of using immunization to protect a population, and herd immunity can be achieved for a whole host of diseases beyond chicken pox and the flu, including measles, mumps and small pox. And while the term herd immunity seems to imply that the whole herd should be vaccinated, the whole herd is merely protected if a certain percentage is immunized; mathematical models can be used to determine exactly what percentage of the population needs to be vaccinated to prevent a communicable disease. For example, if approximately 80 to 86 percent of a population is immune to polio, then herd immunity is achieved [source: CDC].

However, just because herd immunity is achieved doesn't mean it's completely foolproof. Vaccines themselves aren't 100 percent effective, and diseases can still strike those not immune to them. In some instances, an immunization is only effective for a few years, so the protection may have worn off a majority of the herd at the time of an outbreak. Childhood vaccinations may only delay the age at which outbreaks occur. For example, if a pregnant woman contracts rubella, or German measles, she and the unborn baby face far more complications than a child who endures the disease [source: John, Samuel].

If vaccines don't work or just put off a mass outbreak in the herd, do we even need them at all? Some people don't think a shot in the arm is worth the good of the herd. Read about some criticisms of herd immunity on the next page.