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:

• National Institute of Allergy and Infectious Diseases: Community Immunity ("Herd" Immunity)
• The College of Physicians of Philadelphia: Herd Immunity

References:

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. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5436a1.htm.

Centers for Disease Control and Prevention. (2012). Epidemiology and prevention of vaccine-preventable diseases (12^th Ed.). http://www.cdc.gov/vaccines/pubs/pinkbook/index.html.

Centers for Disease Control and Prevention. (2012). Measles – United States, 2011. Morbidity and Mortality Weekly Report, 61(15), 253-257. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6115a1.htm.

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 (3^rd 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. http://jama.ama-assn.org/content/303/10/943.full.

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. http://pediatrics.aappublications.org/content/early/2011/07/21/peds.2010-3385.full.pdf.

National Institute of Allergy and Infectious Diseases. (2010). Community immunity ("herd" immunity). http://www.niaid.nih.gov/topics/pages/communityimmunity.aspx.

Reinberg, S. (2012). Measles deaths falling worldwide. HealthDay News. http://consumer.healthday.com/Article.asp?AID=664027.

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. http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(12)60522-4/fulltext.