What Is Herd Immunity?

June 2016, Vol 4, No 6 - Inside Infectious Disease
Linh B. Van, PharmD, BCPS-AQID

According to the National Institutes of Health, community immunity (ie, herd immunity) occurs when a significant portion of a community is immunized against a contagious disease, and most members are protected against that disease because of the few opportunities for an outbreak.1 Successful vaccination reduces infection or infectiousness in certain people, and can decrease risk in other susceptible individuals in the population.2 A vaccine’s effect on transmission creates herd immunity with indirect protection. The magnitude of this indirect effect depends on several factors: the infection transmissibility, nature of the vaccine-induced immunity, vaccine distribution, and immunity in the population. The herd immunity threshold—or immunity level required in the population to interrupt transmission—is calculated using (R0 – 1)/R0, with R0 denoting reproduction rates.3 These reproduction rates represent the number of other people that an average infected person can contaminate.

Herd Immunity in the Context of Measles

Measles, for example, is a highly contagious disease caused by an enveloped, single-stranded, ribonucleic acid virus.4 The disease has a long incubation period of approximately 10 days, followed by 3 to 5 days of illness characterized by fever, cough, conjunctivitis, and Koplik’s spots4; an erythematous maculopapular rash precedes recovery.4 Patients become infectious towards the end of the incubation period, patients become infectious, and remain so throughout the appearance of the rash.4 Complications of measles include months-long immunosuppression.4,5 Measles has an R0 of 12 to 18, which means that an infected person could contaminate 12 to 18 susceptible people.3 Interestingly, anecdotal evidence suggests that an R0 of >200 is possible under conditions of close spatial confinement.4 Herd immunity >95% is needed to prevent sporadic viral outbreaks in a population,4 but cannot be achieved with just 1 dose of the measles, mumps, and rubella (MMR) vaccine.4 Current practice recommends 2 doses of the MMR vaccine, with the first dose administered when the recipient is aged 12 through 15 months.6 Herd immunity of 92% to 94% is required to prevent sustained spreading of the virus, and is a threshold much higher than that of almost all other vaccine-preventable diseases.3

In the United States, the Centers for Disease Control and Prevention declared the elimination of measles in 2000,6 and endemic rubella in 2001.7 However, vaccine-preventable diseases, such as measles, can still occur with the introduction of imported cases (eg, the 2015 measles outbreak linked to an amusement park in California).8 In addition, an increasing number of parents in the United States opt to not have their children vaccinated, resulting in an accumulation of susceptible individuals who can become infected and maintain transmission.3

Adversity to Vaccinations and Herd Immunity

Growth of anti-vaccine sentiment is a complicated issue.2 Success of vaccinations in reducing disease occurrence adds to vaccine hesitancy, and makes vaccinations seem unnecessary.9 Valid concerns about MMR vaccination safety include live viral vaccines posing a risk of disseminated disease in immunocompromised persons, or the small risk for anaphylaxis in healthy people. Invalid concerns about vaccination safety stem from a false report linking the trivalent MMR vaccine to autism.4 Although this specific report was formally retracted—with subsequent analyses revealing no credible connection between autism and vaccination4—vaccine hesitancy persists, affecting between one-quarter and one-third of parents in the United States.9 Herd immunity in people who vaccinate may protect those who delay or forego vaccination to some degree,9 but clustering of disease susceptibility caused by vaccine hesitancy can threaten herd immunity.8,9

There is some hope, however, for maintaining herd immunity for vaccine-preventable diseases. On June 30, 2015, Jerry Brown, governor of California, signed and approved Senate Bill No. 277, thereby removing personal belief-based vaccine exemptions for children attending daycares and public and private schools, with regard to existing immunization requirements.10 The new law becomes effective on July 1, 2016 and requires new students or those advancing to the 7th grade level to be fully immunized in order to participate in classroom-based instruction. We may, however, also need to take a different approach to vaccine messaging to improve parents’ intents to immunize their infants; it appears as though emphasizing societal benefits does not necessary translate to immunizations—at least with the MMR vaccine—and a national survey suggests that providers should emphasize benefits that vaccines have on the patient directly.11


  1. National Institute of Allergy and Infectious Diseases. Community immunity ("herd" immunity). www.niaid.nih.gov/topics/pages/communityimmunity.aspx. Updated October 21, 2010. Accessed June 1, 2016.
  2. Fine P, Eames K, Heymann DL. “Herd immunity”: a rough guide. Clin Infect Dis. 2011;52:911-916.
  3. Orenstein W, Seib K. Mounting a good offense against measles. N Engl J Med. 2014;371:1661-1663.
  4. Plemper RK, Hammond AL. Will synergizing vaccination with therapeutics boost measles virus eradication? Expert Opin Drug Discov. 2014;9:201-214.
  5. Holzmann H, Hengel H, Tenbusch M, Doerr HW. Eradication of measles: remaining challenges. Med Microbiol Immunol. 2016;205:201-208.
  6. Center for Disease Control and Prevention. Measles vaccination. www.cdc.gov/measles/vaccination.html. Updated July 1, 2015. Accessed June 1, 2016.
  7. Center for Disease Control and Prevention. Chapter 3: infectious diseases related to travel. Rubella. http://wwwnc.cdc.gov/travel/yellowbook/2016/infectious-diseases-related-to-travel/rubella. Updated July 10, 2015. Accessed June 1, 2016.
  8. Liu F, Enanoria WTA, Zipprich J, et al. The role of vaccination coverage, individual behaviors, and the public health response in the control of measles epidemics: an agent-based simulation for California. BMC Public Health. 2015;15:447.
  9. Jacobson RM, St Sauver JL, Finney Rutten LJ. Vaccine hesitancy. Mayo Clin Proc. 2015;90:1562-1568.
  10. California Legislative Information. SB-277 public health: vaccinations. http://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=201520160SB277. Published June 30, 2015. Accessed June 1, 2016.
  11. Hendrix KS, Finnell SM, Zimet GD, et al. Vaccine message framing and parents' intent to immunize their infants for MMR. Pediatrics. 2014;134:e675-e683.
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