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Magill’s Medical Guide, 9th Edition

Herd immunity

by Tyler Biscontini

Category: Biology

Also known as: community immunity

Specialties and related fields: epidemiology, immunology, infectious diseases

Definition: a form of protection against the spread of infectious diseases that requires many individuals in within a community to be immune to the disease, usually through vaccination.

Key terms:

pathogen: A disease-causing agent such as a virus, bacterium, fungus, or parasite.

HOW HERD IMMUNITY WORKS

Infectious diseases spread when an infected person directly or indirectly is exposed to an uninfected person. Disease can spread through person-to-person contact, airborne, waterborne, or foodborne transfer of pathogens, or physical contact with an infected surface. Contracting a contagious disease can be avoided by vaccinations, past exposure to similar pathogens, and, occasionally, by inherited resistance to the disease. The most common of these methods is vaccination.

Herd immunity occurs when the immunity of a large segment of the population prevents contagious diseases from affecting those in the population who are not immune. The percentage of individuals who must be immune for this to happen varies with the infectiousness of the disease. Measles, for example is so contagious that if one person with measles is in a room for one hour with 100 non-immune individuals, 90 of them will contract measles. For herd immunity against measles to be effective, about 93 percent of individuals in the community must be immune to the disease. When a large percentage of a community is immune to a disease, the percentage of individuals who can bring that disease into the community is reduced. If the percentage of immune individuals is high enough, infected individuals may never interact with other vulnerable individuals, thus stopping the disease from spreading through the community or herd.

THREATS TO HERD IMMUNITY

Potentially deadly diseases such as measles, mumps, and polio have been mostly eliminated in developed countries through mass vaccination campaigns. Because a large percentage of the population continues to be vaccinated, some anti-vaccinationists will refuse vaccination based on their belief that herd immunity will protect them. As of 2017, about 2 percent of parents rejected all vaccines for their children, and about 30 percent rejected at least one vaccine or delayed its administration. Vaccine refusal or delay reduces the percentage of immune individuals in the population, making the entire population more vulnerable to disease and decreasing the effectiveness of herd immunity. Herd immunity is especially important to individuals who cannot be vaccinated for medical reasons. Many people who are immunocompromised (e.g., HIV positive, receiving chemotherapy or immunosuppressant drugs) cannot safely receive certain vaccines and must depend on herd immunity which is often weak.

Anti-vaccination movements threaten the effectiveness of herd immunity because when fewer individuals in the community are vaccinated, diseases such as measles or pertussis that have essentially been eliminated can make a resurgence. Often these diseases are brought into a country by travelers form an area where the disease is common. These travelers can be infectious but not yet show symptoms of the disease. In 2015, the United States experienced a measles outbreak when an infected traveler from outside the country visited a California amusement park. One hundred and eighty-nine people in twenty-four states contracted measles, which had been eliminated in the United States in 2000. Most the cases in the 2015 outbreak were among people who were not vaccinated. In 2016, 6,353 cases of mumps, another disease that has effectively been eliminated, were reported in the United States. Again, most cases were in unvaccinated individuals who failed to be protected by herd immunity.

For Further Information:

1 

College of Physicians of Philadelphia “Herd Immunity” History of Vaccines. www.historyofvaccines.org/content/herd-immunity-0 (accessed July 31, 2017).

2 

“Herd Immunity” In: Davidson, Tish. Vaccines: History, Issues, and Science. Santa Barbara, CA: Greenwood, 2017; 16.

3 

Kim, Tae Hyong, et al. “Vaccine Herd Effect.” Scandinavian Journal of Infectious Diseases 43 no. 9 (September 2011): 683-89. www.ncbi.nlm.nih.gov/pmc/articles/PMC3171704 (accessed July 31, 2017).

4 

McGinty, Jo Craven. “How Anti-Vaccination Trends Vex Herd Immunity.” Wall Street Journal. Dow Jones, February 6, 2015. www.wsj.com/articles/how-anti-vaccination-trends-vex-herd-immunity-1423241871 (accessed July 31, 2017).

Citation Types

Type
Format
MLA 9th
Biscontini, Tyler. "Herd Immunity." Magill’s Medical Guide, 9th Edition, edited by Anubhav Agarwal,, Salem Press, 2022. Salem Online, online.salempress.com/articleDetails.do?articleName=MMG2022_0652.
APA 7th
Biscontini, T. (2022). Herd immunity. In A. Agarwal, (Ed.), Magill’s Medical Guide, 9th Edition. Salem Press. online.salempress.com.
CMOS 17th
Biscontini, Tyler. "Herd Immunity." Edited by Anubhav Agarwal,. Magill’s Medical Guide, 9th Edition. Hackensack: Salem Press, 2022. Accessed October 22, 2025. online.salempress.com.