Updated February 03, 2014.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
The Relationship Between Vaccination and Herd Immunity
Vaccines work by teaching your immune system to recognize a potential infection before you are exposed to it in real life. This allows you to more easily fight off an infection, if and when you are exposed. They are an incredibly important tool in public health, and vaccines against common childhood and adult disease have both saved and improved millions of lives.
However, vaccines do not only work by protecting individuals. When large numbers of people are vaccinated against a disease, the protection can spread beyond the people who actually get the vaccine. This effect is known as herd immunity.
Herd immunity occurs when widespread vaccination dramatically decreases the likelihood that an unvaccinated person may come into contact with the disease. That means that even people who can't be vaccinated, for health or other reasons, benefit from vaccine campaigns.
Herd immunity is one reason why it's so important for everyone who can be safely vaccinated against a serious illness to be vaccinated against that illness. Herd immunity only comes into play when most of the population have been vaccinated and any isolated unvaccinated individuals are unlikely to run into each other. When the frequency of vaccination goes down low enough, the risk of infection can jump.
Herd Immunity and the HPV Vaccine
There are not many vaccines available that can protect against sexually transmitted diseases.
Currently, the only vaccine preventable diseases that are often transmitted sexually are hepatitis and HPV.
Vaccinations against these diseases have only become a regular part of the vaccine schedule in recent years. The hepatitis vaccines first became recommended as routine, universal childhood vaccinations in 2005 (Hepatitis B) and 2006 (Hepatitis A). The first HPV vaccine wasn't even approved until 2006, and vaccination is still controversial in some populations.
However, despite the relatively recent availability of the HPV vaccine, herd immunity is already coming into play. By 2012, six years after vaccine introduction, scientists had already begun to observe declines in the prevalence of HPV strains covered by the vaccine -- not just in vaccinated girls but in their unvaccinated counterparts.
Visualizing Herd Immunity
Herd immunity can be hard to think about when you're talking about diseases that spread through the air or through casual contact. However, it's actually pretty easy to understand how herd immunity works in the context of a sexually transmitted disease. Imagine that Mary and John are dating. John is infected with HPV-16, which he gives to Mary. Later on, Mary dates Bill, and she infects him. Then Bill infects his new girlfriend Kathy. If Mary had been vaccinated, she wouldn't have been infected with HPV. She wouldn't have infected Bill, and Bill wouldn't have infected Kathy. Mary's vaccination would have protected both Bill and his future partners.
In other words, the more vaccinated people there are, the less likely that anyone who hasn't been vaccinated will be exposed to someone who is infected. That's herd immunity in action.
Note: Although giving the HPV vaccine to men helps with herd immunity, it also provides direct benefits to vaccinated men. Gardasil, the quadrivalent HPV vaccine, can protect men from several consequences of HPV infection including genital warts, throat cancer, and penile cancer
Sources:
Centers for Disease Control and Prevention (CDC). (2010) Hepatitis A vaccination coverage among U.S. children aged 12-23 months - immunization information system sentinel sites, 2006-2009. MMWR Morb Mortal Wkly Rep. 59(25):776-9.
Kahn JA, Brown DR, Ding L, Widdice LE, Shew ML, Glynn S, Bernstein DI. (2012) Vaccine-type human papillomavirus and evidence of herd protection after vaccine introduction. Pediatrics. 130(2):e249-56.
Mast EE, Margolis HS, Fiore AE, Brink EW, Goldstein ST, Wang SA, Moyer LA, Bell BP, Alter MJ; Advisory Committee on Immunization Practices (ACIP). (2005) A comprehensive immunization strategy to eliminate transmission of hepatitis B virus infection in the United States: recommendations of the Advisory Committee on Immunization Practices (ACIP) part 1: immunization of infants, children, and adolescents. MMWR Recomm Rep. 54(RR-16):1-31.
Sadanand S. (2011) Vaccination: the present and the future. Yale J Biol Med.84(4):353-9.
