Herd immunity, or community immunity, refers to the protection that a high percentage of vaccinated individuals provide to individuals who have never been vaccinated or in whom vaccinations have failed against the spread of infectious diseases. The protection is provided because the disease cannot find new hosts to spread to after a certain percentage of the population is vaccinated.
Here is a good way to think of this: immunized individuals provide a type of firewall, protecting the unvaccinated from the disease. This may be easier to follow with some cartoons depicting the general idea.
Here are pictures:
In a community with many vaccinated individuals, the unvaccinated individual is protected against the spread of disease.
Why is the concept of herd immunity important to understand in the context of vaccinations? Many vaccines are given at specific ages and children who are younger than that age are susceptible to disease. In addition, there is a very small percentage of individuals that vaccines do not provide a protective effect for a variety of reasons. These people often don’t know their routine vaccinations have failed but they and young children are protected when they’re surrounded by a community of individuals who have been vaccinated. The vaccinated people provide a wall of protection.
In communities in which vaccination rates have decreased below the herd threshold, or the percentage of people that must be vaccinated to protect unvaccinated individuals, you have epidemics of measles, whooping cough and other diseases easily prevented by vaccination. Often, in the case of these epidemic outbreaks, the individuals who become ill are both the unvaccinated children as well as children too young for vaccination.
In addition, the proper vaccination of certain groups of people can actually have a protective effect for groups susceptible to disease. A recently published paper in Science magazine detailed that communities with high childhood vaccination rates for the H1N1 influenza vaccine actually had fewer elderly individuals die or become seriously ill from the disease.
I hope this brief overview of herd immunity was informative. If I got something wrong or you have questions, feel free to let me know! I also want to thank all the scientists reading Persephone who volunteered their brains for me to pick ““ I’m certainly going to take you up on it!
My next column will appear in two weeks. Please send me questions or suggestions at AskDorilysAboutScience@persephonemagazine.com, otherwise, you’re going to have to read about what I’m interested in.
Selected sources and interesting reading:
http://www.cdc.gov/vaccines/ – general information on vaccination
http://www.cdc.gov/vaccines/vac-gen/why.htm – why vaccines work
http://www.historyofvaccines.org/content/herd-immunity-0 – this is an excellent resource available to everyone. There are nice videos illustrating the concepts I explained above.
http://www.cdc.gov/vaccines/vac-gen/6mishome.htm – mythbusting misconceptions about vaccinations.
http://www.cdc.gov/vaccines/spec-grps/parents.htm#question ““ a section geared toward parents interested in learning about vaccination.
http://en.wikipedia.org/wiki/Herd_immunity – a decent overview of herd immunity.
Jason M. Glanz et al. (2009). Parental refusal of Pertussis vaccination is associated with increased risk of Pertussis infection in children. Pediatrics: Vol. 123 No. 6 June 2009, pp. 1446-1451.
Jon Cohen. (2004). Immunizing kids against flu may prevent deaths among the elderly. Science: 12 November 2004: 1123.
Joseph N. S. Eisenberg et al.(2009). Protecting the herd from H1N1. Science: 13 November 2009: 934.