Saturday, May 19, 2012

Community immunity (herd immunity)

Pardon my hiatus. I made a lateral transfer to epidemiology and I'm still getting used to my new work hours.
Craig and Mary, April 23, 2005
Father and mother of the bride

I had planned to post the following thread on Mother's Day. Instead, my father-in-law and I accompanied Mary, my mother-in-law, to an emergency department where she had a head CT and then a brain MRI which disclosed a glioblastoma – the worst type of brain tumor.

Holly and I met in 2003 while I was taking care of Seth, who had sustained a severe traumatic brain injury in a motor vehicle accident. Seth has remains in a minimally conscious state. Despite her rheumatoid arthritis, Mary has cared for Seth at home for nearly seven years. Now, Mary is in the care of some of the same nurses who cared for Seth eight and a half years ago and the best neurosurgeon I have known in my career as a neuro nurse. She will have surgery on Monday.

Mary and Craig welcomed me into their home as their son seven years ago. Mary has been an inspiration to everyone who knows her. Please be in prayer for her.

Community immunity

Another study on the incidence of measles was published shortly after my last post: Assessment of the 2010 global measles mortality reduction goal. The investigators found that the number of deaths from measles worldwide has decreased by 74% between 2000 and 2010. Measles outbreaks in Africa and immunization program delays in India prevented the goal of 90% reduction in measles deaths from being achieved. Forty-seven percent of the estimated 139,300 measles deaths in 2010 occurred in India and 36% occurred in Africa.

In previous posts I've talked about some ways that vaccines indirectly protect people other than the recipient. Maternal antibodies are transported across the placenta, protecting newborn babies from neonatal tetanus and may protect babies against pertussis. Cocooning is another strategy to protect babies against pertussis.

Communicable diseases are diseases that are transmitted from one person to another. For a communicable disease to be transmitted, a susceptible person must be exposed to the infecting agent. Most vaccine-preventable diseases are transmitted by contact with someone who is infected with a virus or bacterium (Yellow fever, which is transmitted by mosquitoes, is one exception that comes to mind. Tetanus is a vaccine-preventable disease that is not communicable). The probability that a susceptible person will be exposed to an infected person decreases with the proportion of immune people in a population. Therefore, susceptible individuals are protected by other people's immunity. This is known as "community immunity" or "herd immunity."

Last year there were 222 cases of measles in the U.S. Seventy-two cases were imported from outside of the U.S., and 128 cases were known to be associated with the imported case (CDC, 2012b). The basic reproductive number (R0, or "R naught") for measles is 14, which means that in a susceptible population, a person with measles will transmit the virus to 14 other people, who will each transmit it to 14 people until there are no longer enough susceptible people in the population to sustain transmission. The size of the measles outbreaks in U.S. last year were limited by the small number of people in the country who are susceptible to measles; that is, most people in the U.S. are immune to measles. The national average for receipt of at least one dose of measles-containing vaccine (MCV) is 90% (CDC, 2012a).

Examples of indirect vaccine protection:

A recent study that demonstrated a protective effect of vaccines on people other than the recipients was the 2010 Hutterite study. During the 2008-2009 influenza season the study investigators randomized 46 Hutterite colonies in western Canada. Children ages 3 to 15 years in 22 of the colonies received seasonal influenza vaccine and children in 24 of the colonies received hepatitis A vaccine as a control (rather than a placebo). During the flu season, the numbers of cases of influenza were counted in all of the participating colonies. At the end of the flu season, the numbers of cases of influenza in the colonies in which children received influenza vaccine and colonies in which children received hepatitis A vaccine were compared. The study investigators found that the seasonal influenza vaccine given to children was 61% effective in preventing flu in colony members who did not receive the vaccine (Loeb et al., 2010).

From 1998 to 2003, 7-valent pneumococcal conjugate vaccine dramatically reduced the incidence of invasive pneumococcal disease (IPD) in children less than 5 years of age who received the vaccine. At the same time, the incidence of IPD also decreased in all age groups, with the largest rate of reduction in people aged 65 years and older. The decrease in incidence was seen only in seven pneumococcal serotypes included in the vaccine, so it is unlikely that the decreased incidence was caused by some other factor. Overall, 69% of the protective effect of pneumococcal vaccine was seen in people who had not received the vaccine (CDC, 2005).

One more example is in the near elimination of deaths due to chickenpox (yes, people die from chickenpox) after the introduction of chickenpox vaccine. The reduction in the number of deaths due to chickenpox was seen in all age groups, not just those who had received the vaccine (Marin, Zhang, & Seward, 2011).

In a previous post, I talked about diseases that have been eliminated from the U.S. by vaccines. Next, I'd like to discuss disease eradication.

More information:


Centers for Disease Control and Prevention. (2005). Direct and indirect effects of routine vaccination of children with 7-valent pneumococcal conjugate vaccine on incidence of invasive pneumococcal disease – United States, 1998-2003. Morbidity and Mortality Weekly Report, 54(36), 893-897.

Centers for Disease Control and Prevention. (2012). Epidemiology and prevention of vaccine-preventable diseases (12th Ed.).

Centers for Disease Control and Prevention. (2012). Measles – United States, 2011. Morbidity and Mortality Weekly Report, 61(15), 253-257.

Fine, P. E. M. & Mulholland, K. (2008). Community immunity. In S. A. Plotkin, W. A. Orenstein, & P. A. Offit (Eds.) Vaccines (5th Ed.) [Electronic version]

Heyman, D. L. (2011). Disease eradication and control. In R. L. Guerrant, D. H. Walker, & P. F. Weller (Eds.). Tropical infectious diseases: principles, pathogens and practice (3rd Ed.). [Electronic version].

Loeb, M., Russell, M. L., Moss, L., Fonesca, K., Fox, J., Earn, D. J. D. et al. (2010). Effect of influenza vaccination of children on infection rated in Hutterite communities. JAMA, 303(10), 943-950.

Marin, M., Zhang, J. X., & Seward, J. F. (2011). Near elimination of varicella deaths in the US after implementation of the vaccination program. Pediatrics, 128(2), 214-220.

National Institute of Allergy and Infectious Diseases. (2010). Community immunity ("herd" immunity).

Reinberg, S. (2012). Measles deaths falling worldwide. HealthDay News.

Simons, E., Ferrari, M., Fricks, J., Wannemuehler, K., Anand, A., Burton, A. et al. (2012). Assessment of the 2010 global measles mortality reduction goal: results from a model of surveillance data. Lancet, DOI:10.1016/S0140-6736(12)60522-4.


  1. Hi Matthew, I have a couple questions about herd immunity. I'll start by letting you know where my bias is - I'm pro-vaccine, so not surprising I've been having conversations with anti-vax folks who have directed me to rather bias articles stating that either 1) herd immunity "does not apply" to vaccines or 2) that the logic behind herd immunity is flawed because the vaccines we receive as children wear off after some amount of time and most adults never go back to get vaccinated again, except maybe if they step on a nail, yearly flu shots, or travel outside the country. (Here is an example of one such article that I felt likely to be anti-vax biased - So, what is the kernel of truth (is there one?) that these statements are based on? Do vaccines wear off and should adults redo vaccinations on a similar schedule as their children after a certain age? When calculations are done, in the case of measles, for example, that X state (say, California) is close to or going over their 5% of unvaccinated that weakens herd immunity, are adults who may not have been vaccinated since the were children included in the 5% or the 95%? Or is it, perhaps, that by the time of adulthood, those who were vaccinated as children are less likely to catch measles anyway because they are over the age of greatest risk, assuming they have no auto-immune issues, and that keeps herd immunity intact? You can probably tell by my question that I only have a high level understanding of herd immunity and am not, at this point, able to intelligently refute the idea that weakened "herd immunity" is something "real."

    1. Joy,

      I've heard that argument before. Let me start by saying that all vaccines are not created equal.

      For most people, immunity to measles from MMR appears to be life-long. MMR is a live attenuated (weakened) virus vaccine. The vaccine viruses behave like wild-type viruses without causing disease. That means that our immune responses to live virus vaccines are identical to the responses to natural infection. Specifically, live virus vaccines induce both antibody-mediated and cell-mediated immune responses. Serum antibody titers to measles wane over time but cell-mediated immune memory is much more persistent. Most vaccinated people who are exposed to measles virus (vaccine or wild-type) have an anamnestic response – their serum antibody titers shoot up almost immediately.

      Some people mistake the second dose of MMR for a "booster." It's not. Around 92% of people develop immunity to measles with one dose of MMR. Around 98% develop immunity to two doses. The second dose is given to increase the number of people who are immune to measles, not to "boost" the antibody titers of those who are already immune to the disease.

      The fact that both measles and rubella have been eliminated from the U.S. is evidence of herd immunity. Yes, there are primary and secondary vaccine failures, but measles transmission in this country is not sustained.

      Immunity to bacterial diseases is different. Immunity to pertussis is not permanent. People who get pertussis can get is again. On the other hand, humans do not develop immunity to tetanus or diphtheria from natural infection. Tetanus and diphtheria toxoids (vaccines) produce immunity to those diseases that nature doesn't give us. Nevertheless, immunity from DTaP/Tdap wanes.

      Pneumococcal, meningococcal, and Haemophilus influenzae type B (Hib) vaccines target polysaccharides (sugars) on the surfaces of those bacteria. Those polysaccharides are poorly immunogenic; that is, they don’t stimulate a strong immune response. Not only that, but there are a lot of different types of pneumococcal, meningococcal, and Haemophilus influenzae polysaccharide capsules and they are not cross-reactive: immunity to one doesn't confer immunity to others. Conjugated pneumococcal, meningococcal, and Hib vaccines use bacterial polysaccharides that are attached (conjugated) to another immunogenic material like tetanus toxoid or diphtheria toxoid. This stimulates both cell-mediated and antibody-mediated immunity. There are a number of studies that have demonstrated reductions in disease in non-immunized adults after the introduction of childhood immunization with conjugate vaccines – herd immunity.

      Influenza is a different animal because the antigens on the surface of influenza A viruses mutate almost continuously. Nevertheless, there was a study in Canada that showed that immunizing children against influenza protected adults who were not vaccinated which, by definition, is herd immunity.

      In short, what is true for one vaccine is not true of all vaccines. I've heard people say that most people who get measles have been vaccinated. That's not at all true. People who say that are confusing measles with pertussis (please see my post on attack ratios:

      I hope this helps.


    2. Thank you, Matthew. This helps a lot. I'm sure I will be directing a lot of folks to this response!

  2. *I meant to say "...that weakened "herd immunity" is *not* 'real.'"

  3. Also, thank you for this blog. So many great information laid out here!

    1. Thanks for reading it and thank you very much for your comments and questions!