Herd Immunity:Another Good Reason to Vaccinate

October 2, 2009

There are 2 major reasons to vaccinate your child.

There are 2 major reasons to vaccinate your child. First and foremost, vaccination leads to the development of your child's own immunity and protects him or her against potentially serious infections. A second reason is that vaccinating your child against an infection helps reduce the chances that this infection will affect others in your family and your community- even those who may not have been vaccinated. This second benefit, the indirect protection that vaccination confers on other people who perhaps are too sick or frail to be vaccinated, is called herd immunity. Ultimately, if enough people are vaccinated and herd immunity becomes pervasive enough, vaccination can lead to the complete eradication of an infection-as happened with polio in the United States and with smallpox worldwide.

What is herd immunity?

The term “herd immunity” has been defined in various ways. Here we use the term to refer to the shield of protection that persons with immunity to an infection provide to those around them who are not protected against it.

How does herd immunity work?

Vaccine-preventable infections continue to occur in society because of the presence of persons who are not immune, called “susceptible persons.” A person may be susceptible for many reasons. These include:

•Inability to receive vaccines or to respond adequately to them because of being too old or too young or having an underlying medical condition.
•Vaccine failure.
•Decision not to be vaccinated.

Susceptible persons may be family members (for example, a new baby or an uncle receiving chemotherapy), friends, neighbors, schoolmates, or the stranger who sits next to you on a bus or plane. If susceptible persons interact with others who are infected, they will become infected themselves. Once they become infected, they can, in turn, interact with other susceptible persons and infect them, thereby continuing the spread of the infection.

However, when an infected person comes in contact with an immunized person, the infection cannot spread to that person. If an infected person comes into contact only with people who are immune, the organisms causing the infection are likely to die before they are able to take up residence in another susceptible “host” (Figure). Thus, if enough persons are immunized, the spread of the infection is stopped. That is the shield of herd immunity.

In order for herd immunity to be effective, the vast majority of the population must be vaccinated-ie, the number of susceptible persons must be minimized and the number of immune persons maximized. By tipping the balance strongly in favor of immunity, the chance that an infectious disease can spread is substantially reduced. 1 When a child is vaccinated and produces an adequate immune response, he becomes part of the shield of herd immunity and helps prevent the spread of infections to those who are still susceptible.

What infections are involved?
Herd immunity is important for infections that are spread from person to person. Most vaccinepreventable infections-including measles, mumps, chickenpox, diphtheria, whooping cough, polio, and influenza-fall into this category. A few vaccinepreventable diseases are not spread between humans. For example, tetanus is caused by coming into contact with bacteria that are found in the soil and elsewhere in the environment; thus, herd immunity is not a factor in the control of this infection.2 Other, less common vaccine-preventable infections that are not spread from person to person are rabies, yellow fever, and Japanese B encephalitis.

What are the benefits of herd immunity? Herd immunity greatly reduces or eliminates transmission of disease for everyone. Many historical observations and mathematical models support the helpful effect of herd immunity.3,4 An example of a positive effect of herd immunity is the protection of the elderly that results when schoolchildren are immunized with the influenza vaccine; many elderly persons have been spared the serious consequences that they might have experienced had they contracted influenza.5

What is the parents' role in herd immunity?
Most vaccine-preventable diseases are not seen frequently in the United States. However, this is because of our country's high vaccination rates. It is easy to become complacent about the fact that these diseases are rare in this country. However, vaccine-preventable illnesses occur more often in other parts of the world and thus may be merely a plane ride away, as was seen in one recent mumps outbreak, which apparently was initiated by a visitor from Great Britain.6

By having their children vaccinated, parents play a crucial role in ensuring that vaccine-preventable infections remain rare in the United States. The importance of vaccination for all eligible children is underscored by the fact that the Supreme Court has ruled school immunization requirements to be constitutional.7

What are the consequences of vaccine refusal?
Refusing to have your child vaccinated can diminish herd immunity. If there are large numbers of unvaccinated people, infections such as measles or whooping cough can spread and infect large numbers of people. 3,7-11 In order to maximally protect all children, it is important that all vaccine-eligible children be vaccinated. One family's refusal to vaccinate their own child to avoid potential adverse effects can result in other children being placed at greater risk for infections.

Of course, many children whose families refuse to have them vaccinated will never contract the infections that the vaccines are intended to prevent. However, that is likely because these children are benefiting- as “free riders,” so to speak-from the herd immunity created by children whose parents chose to have them vaccinated.12

Also, when a healthy child who has not been vaccinated does contract a vaccine-preventable infection, his risk of serious disease may well be low. However, a child with an illness such as leukemia-who could not be vaccinated because of his underlying illness- might be at greater risk. A simple infection such as chickenpox, spread to a child with underlying illness by a “healthy” unvaccinated child, could become life-threatening.

References:

REFERENCES:


1.

Anderson RM, May RM. Vaccination and herd immunity to infectious diseases.

Nature

. 1985;318(6044):323-329.

2.

Fair E, Murphy TV, Golaz A, Wharton M. Philosophic objection to vaccinationas a risk for tetanus among children younger than 15 years.

Pediatrics.

2002;109:E2.

3.

Fine PE. Herd immunity: history, theory, practice. Epidemiol Rev. 1993;15:265-302.

4.

Fox JP, Elveback L, Scott W, et al. Herd immunity: basic concept and relevanceto public health immunization practices. 1971.

Am J Epidemiol

. 1995;141:185-197.

5.

Reichert TA, Sugaya N, Fedson DS, et al. The Japanese experience withvaccinating schoolchildren against influenza.

N Engl J Med

. 2001;344:889-896.

6.

Centers for Disease Control and Prevention (CDC). Mumps outbreak at asummer camp-New York, 2005. MMWR. 2006;55:175-177.

7.

Omer SB, Salmon DA, Orenstein WA, et al. Vaccine refusal, mandatoryimmunization, and the risks of vaccine-preventable diseases.

N Engl J Med

.2009;360:1981-1988.

8.

Centers for Disease Control and Prevention (CDC). Some common misconceptionsabout vaccination and how to respond to them. May 29, 2007.

http://www.cdc.gov/vaccines/vac-gen/6mishome.htm

. Accessed August 31,2009.

9.

Omer SB, Pan WK, Halsey NA, et al. Nonmedical exemptions to schoolimmunization requirement: secular trends and association of state policies withpertussis incidence.

JAMA

. 2006;296:1757-1763.

10.

Feikin DR, Lezotte DR, Hamman RF, et al. Individual and community risksof measles and pertussis associated with personal exemptions to immunization.

JAMA

. 2000;284:3145-3150.

11.

Glanz JM, McClure DL, Magid DJ, et al. Parental refusal of pertussisvaccination is associated with an increased risk of pertussis infection in children.

Pediatrics

. 2009;123:1446-1451.

12.

Diekema DS; American Academy of Pediatrics Committee on Bioethics.Responding to parental refusals of immunization of children.

Pediatrics

. 2005;115:1428-1431.