What is herd immunity?

Can a person win the immunity prize if the people around are vaccinated? Get more health tips with staying healthy pictures.
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In the 2000 book "Bowling Alone," political scientist Robert D. Putnam argued that social capital in America was declining. As one way to support that point, Putnam pointed to statistics involving membership in community organizations. People simply saw each other less, according to Putnam. There was no chance to meet the neighbors down the street, socialize with other members of the community or get to know anyone outside of your own house. And that was if you even saw the people in your own house -- Putnam believed that technology such as television and the Internet had completely negated the need to speak to anybody.

Putnam's point was summed up in the very title of the book: People in the United States were bowling more and more, but they were heading to the local alley by themselves. The old days of joining a league and fraternizing with the same group of people every week were over. Now, people were shut off from all social connections and bowling alone.

But beyond the societal problems that Putnam believes can arise from declining social capital, an "every man for himself" approach can have tremendous effects on public health. Keeping populations of a community free from viral disease rests in part on the success of herd immunity. Herd immunity rests on the principle of safety in numbers; if more people are immune to a certain virus, either through vaccination or through already having the disease, then more people in the population, even if they themselves aren't immune, are protected from the disease.

­To illustrate the point, let's return to that bowling alley where people are bowling by themselves. Let's say the guy on the first lane contracts influenza, and he passes it along to the woman on the second lane. If that woman isn't immune to influenza, then the disease will likely continue its path lane by lane until every person in the bowling alley is suffering. But if that woman is immune, then the disease stops with her, because the virus has nowhere else to go (assuming that the guy in our example didn't have contact with anyone else). By her immunity, she protected all the people on subsequent lanes, even if they didn't get a flu shot that year.

Sounds simple, right? Read more about herd immunity -- and why some people don't believe in the wisdom of crowds -- on the next page.

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.

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.
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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.

Publi­c 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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More Great Links


  • Allen, Arthur. "For the Good of the Herd." New York Times. Jan. 25, 2007. (Nov. 18, 2008)http://www.nytimes.com/2007/01/25/opinion/25allen.html?scp=2&sq=herd+immunity&st=nyt
  • "History and Epidemiology of Global Smallpox Eradication." Centers for Disease Control and Prevention. (Nov. 18, 2008)http://www.bt.cdc.gov/agent/smallpox/training/overview/pdf/eradicationhistory.pdf
  • Hoffman, Jan. "Vaccinating Boys for Girls' Sake?" New York Times. Feb. 24, 2008. (Nov. 18, 2008)http://www.nytimes.com/2008/02/24/fashion/24virus.html?sq=herd%20immunity&st=nyt&scp=3&pagewanted=all
  • John, T. Jacob and Reuben Samuel. "Herd immunity and herd effect: new insights and definitions." European Journal of Epidemiology. 2000.
  • McKay, Betsy. "Chicken Pox Vaccine Cuts Cost for Hospital Care, Study Shows." Wall Street Journal. Sept. 7, 2004.
  • McNeil, Donald G. Jr. "When Parents Say No To Child Vaccinations." New York Times. Nov. 30, 2002. (Nov. 18, 2008)http://query.nytimes.com/gst/fullpage.html?res=9F00E3DB1038F933A05752C1A9649C8B63&scp=8&sq=herd+immunity&st=nyt
  • PLoS Medicine. "When It Comes to Flue, the More Who Get Vaccine, the Better." Washington Post. Oct. 28, 2008. (Nov. 18, 2008)http://www.washingtonpost.com/wp-dyn/content/article/2008/10/28/AR2008102801295.html
  • Ulene, Valerie. "Vigilance with vaccines: The decision of some parents not to immunize their children raises the risk for everyone. If too many make the same choice, an outbreak of measles or mumps becomes possible." Los Angeles Times. Feb. 7, 2005.
  • Wanjek, Christopher. "Vaccines Need a Shot in the Arm." LiveScience. Aug. 15, 2006. (Nov. 18, 2008)http://www.livescience.com/health/060815_bad_shots.html