Protecting the Herd

From: Emerging and Re-emerging Infectious Diseases by Joseph D. McInerney and Lynda B. Micikus. Written under a contract from the National Institutes of Health, National Institute of Allergy and Infectious Diseases (NIH Publication No 99-4645) 1999. This unit is available free of charge to teachers via http://www.bscs.org

Minor format changes by: Kirstin Bittel

Time: 1 class period
Preparation Time: 15-30 minutes making copies and preparing cards
Materials: Simulation of the Two-Day Disease – Teacher Protocol Sheet (make 1 copy)
Measles Outbreak at Western High and A Little Sleuthing (make 1 copy per student) – These are on the same page; you will need to cut them apart.
Following an Epidemic (make 2 copies per student and 2 transparencies)
Red, pink, and black cards (make 1 of each per student)
Folded pieces of paper labeled “immune” and “susceptible” (make enough of each for half the students)

This activity introduces students to modeling as a scientific exercise. Students learn how models based on observations of disease transmission can be used to predict the likelihood of epidemics and to help public health officers recommend policies to protect the public from infectious diseases. Students use an in-class simulation to explore how infectious diseases spread through a population and discover the phenomenon of herd immunity.

Students will be able:

1. Explain how immunizing a significant proportion of a population against a disease prevents epidemics of that disease (herd immunity).
2. Be able to list factors that affect the proportion of a population that must be immunized to prevent epidemics.
3. Understand how large-scale vaccination programs help control infectious diseases.

National Science Education Standard:
Content Standard A – Science as Inquiry
A variety of technologies, such as hand tools, measuring instruments, and calculators, should be an integral component of scientific investigations. The use of computers for the collection, analysis, and display of data is also a part of this standard. Mathematics plays an essential role in all aspects of an inquiry. For example, measurement is used for posing questions, formulas are used for developing explanations, and charts and graphs are used for communicating results.

Content Standard F- Science in Personal and Social Perspectives
The severity of disease symptoms is dependent on many factors, such as human resistance and the virulence of the disease-producing organism. Many diseases can be prevented, controlled, or cured. Some diseases, such as cancer, result from specific body dysfunctions and cannot be transmitted.

Individuals and society must decide on proposals involving new research and the introduction of new technologies into society. Decisions involve assessment of alternatives, risks, costs, and benefits and consideration of who benefits and who suffers, who pays and gains, and what the risks are and who bears them. Students should understand the appropriateness and value of basic questions--"What can happen?"--"What are the odds?"--and "How do scientists and engineers know what will happen?"

Content Standard G – History and Nature of Science
Because all scientific ideas depend on experimental and observational confirmation, all scientific knowledge is, in principle, subject to change as new evidence becomes available. The core ideas of science such as the conservation of energy or the laws of motion have been subjected to a wide variety of confirmations and are therefore unlikely to change in the areas in which they have been tested. In areas where data or understanding are incomplete, such as the details of human evolution or questions surrounding global warming, new data may well lead to changes in current ideas or resolve current conflicts. In situations where information is still fragmentary, it is normal for scientific ideas to be incomplete, but this is also where the opportunity for making advances may be greatest.

Teacher Background
The re-emergence of some diseases can be explained by the failure to immunize enough individuals, which results in a greater proportion of susceptible individuals in a population and an increased reservoir of the infectious agent. Increases in the number of individuals with compromised immune systems (due to the stress of famine, war, crowding, or disease) also explain increases in the incidence of emerging and re-emerging infectious diseases.

Global vaccination strategies are a cost-effective means of controlling many infectious diseases. Because immunized people do not develop diseases that must be treated with antimicrobial drugs, opportunities for pathogens to evolve and disseminate drug resistance genes are reduced. Thus, mass immunization reduces the need to develop newer and more expensive drugs.

As long as a disease remains endemic in some parts of the world, however, vaccination programs must be maintained everywhere, because an infected individual can travel anywhere in the world within 24 hours. Once global vaccination programs eliminate the infectious agent, (as in the case of the smallpox virus), vaccination is no longer necessary and the expense of those programs is also eliminated. It is estimated that the United States has saved $17 billion so far as a result of the eradication of smallpox (which cost, according to the World Health Organization, $313 million across a 10-year period).

Lapses in vaccination programs explain the re-emergence of some infectious diseases. For example, the diphtheria outbreak in Russia in the early 1990s may have been due to lapses in vaccination programs associated with the breakup of the Soviet Union. Inadequate vaccines and failure to obtain required “booster shots” also explain some disease re-emergence. The dramatic increase in measles cases in the United States during 1989–1991 was likely caused by failure to give a second dose of the vaccine to school-age children. The American Academy of Pediatrics now recommends that all children receive a second dose of the measles vaccine at either ages 4–6 or 11–12.

The activity, Superbugs and Antibiotic Resistance, provide explanations for the re-emergence of some infectious diseases. This activity introduces students to the idea that the re-emergence of some infectious diseases can be explained by a failure to immunize a sufficient proportion of the population. Students learn that epidemics can be prevented by immunizing part of the population, leading to herd immunity.

Prerequisite Student Knowledge
Students should be familiar with how immunization protects individuals from infectious diseases.

Pre-Lab Preparations
You will need to prepare the following materials before conducting this activity:

  • Measles Outbreak at Western High (make 1 copy per student)
  • A Little Sleuthing (make 1 copy per student)
  • Following an Epidemic (make 2 copies per student and 2 transparencies)
  • Red, pink, and black cards (make 1 of each per student)
  • Folded pieces of paper labeled “immune” and “susceptible” (make enough of each for half the students)



Day 1:
1. Introduce the activity by distributing one copy of Measles Outbreak at Western High, to each student and asking the students to read it.
The scenario described on Measles Outbreak is fictitious, but is based on an outbreak of measles that occurred in Washington State in 1996. An alternate way to introduce the activity is to assign students to make a list of the childhood diseases that they, their parents (or someone from their parents’ generation), and their grandparents (or someone from their grandparents’ generation) had. Explain that “childhood diseases” means diseases that people usually have just once and do not get again (for example, chicken pox). Explain that you do not mean diseases like the flu, strep throat, or colds. On the day you wish to begin the activity, ask students to name some of these diseases, and then ask them to count the number of different diseases each generation in their family had them. Total these numbers across all of the students in the class and ask students to suggest why (in general) their parents and grandparents had more diseases than they did. Students likely will suggest (correctly) that vaccinations against many diseases are now available.

2. After students have read Measles Outbreak, ask them to speculate about what might have happened to cause a sudden outbreak of a disease such as measles that normally, today, is relatively rare in the United States.
Students likely will know that most children in the United States today are vaccinated against measles. They may speculate that the students at Western High were not vaccinated, or that the vaccines did not work in their cases, or even that the pathogen causing this form of measles was somehow able to evade the immune defenses that had been triggered by the vaccinations these children received.

3. Distribute one copy of Master 4.2, A Little Sleuthing, to each student and ask the students to read the story and think about the question that ends it.

4. Point out that despite the success of the measles vaccine, there continues to be small outbreaks of measles in the United States. Explain that the key to understanding why this is true and to answering the question that ends the story about Western High lies in understanding how disease spreads in a population.

5. Explain to students that to help them understand how disease spreads in a population, they will participate in a simulation of the spread of a fictitious disease you will call the “two-day disease.” Distribute two copies of Master 4.3, Following an Epidemic, to each student and display a transparency of this master. Then, direct students to perform two simulations of the spread of the two-day disease, according to the instructions provided on page 77, immediately following the activity.
An “epidemic” is typically defined as “more cases of a disease than is expected for that disease.” Although this is not a very specific definition, it does make it clear that whether scientists call an outbreak of a disease an epidemic depends on the specific disease involved. Though there is no distinct line between an “outbreak” and an “epidemic,” epidemics are generally considered to be larger in scale and longer lasting than outbreaks. Today, five cases of measles within a population could be considered an epidemic because no cases are expected. For this simulation, assume that an epidemic is in progress if 25 percent or more of the population is sick at one time. Observations that students might make about the table and graph that result from the first simulation include:

  • an epidemic occurred because a large portion of the class was sick at the same time;
  • at the beginning of the epidemic, only a few people were sick on the same day; in the middle of the epidemic, a lot of people were sick at the same time; and at the end, only a few people were sick;
  • by the end of the simulation, everyone was immune; and
  • once it started, the disease spread rapidly.

Observations that students might make about the table and graph that result from the second simulation include:

  • only a few people were sick on any one day;
  • no epidemic occurred;
  • at the end of the simulation, some people were still susceptible; and
  • some people in the population never got sick.

Tip from the field test: Do a practice run of several days of the simulation before you do the runs in which you collect data. This will allow you to address any confusion students have about the simulation and will make subsequent runs go much faster. If you have time, you may want to repeat the simulation, in particular the second simulation in which half of the class is immune. In order for students to observe herd immunity, some susceptible students in the population should not get sick. Depending on the arrangement of immune and susceptible students in the class (which is random), this may not happen the first time you run this simulation.

1. Debrief the activity by asking, “Why did an epidemic occur in the first population, but not in the second?” and “Why didn’t all of the susceptible people in the second population get sick?” Introduce the term “herd immunity” and describe it as a phenomenon that occurs when most of the people in a population are immune to an infectious disease. Susceptible people in the population are protected from that disease because the infectious agent cannot be effectively transmitted.
Allow students to discuss their responses to the two questions before you introduce the term herd immunity. Students will likely make comments such as, “Everyone sitting near John was immune, so the disease just died out.” At that point, you can respond by saying, “Yes, what you have just explained is what epidemiologists call herd immunity.” Then you can provide a more complete definition.

2. Ask students to explain, based on their experience in the disease transmission simulation, what would happen if measles vaccinations dropped to a low level in a population.
Students should be able to explain that there would be many susceptible people in the population, so the disease would be transmitted from one to another without dying out. A measles outbreak or epidemic would occur. If students do not mention “re-emergence,” emphasize this point by saying, “Yes, measles would re-emerge in the population.”

Remind students about the measles outbreak story. Ask them to write a final paragraph to the story in which they use the term herd immunity to answer the following questions:

  • Why didn’t the unvaccinated or inadequately vaccinated students and teacher at Western High get measles when they were children rather than as teenagers or adults?
    Students should be able to explain that the unvaccinated or inadequately vaccinated students at Western High were protected by herd immunity when they were younger: Because most of the people around them were immune, the infectious agent could not be transmitted from those people.
  • Why is vaccination not only a personal health issue, but also a public health issue?
    Vaccination is a public health issue because maintaining high levels of immunity in a population prevents epidemics and protects the small percentage of susceptible people from the disease.

Embedded Assessment

As students are discussing their findings, listen to see if students can answer the following questions:

  • Can they explain how immunizing a significant proportion of a population against a disease prevents epidemics of that disease (herd immunity)?
  • Can they list factors that affect the proportion of a population that must be immunized to prevent epidemics?
  • Can they explain how large-scale vaccination programs help control infectious diseases?



PULSE is a project of the Community Outreach and Education Program of the Southwest Environmental Health Sciences Center and is funded by:

NIH/NCRR award #16260-01A1
The Community Outreach and Education Program is part of the Southwest Environmental Health Sciences Center: an NIEHS Award


Supported by NIEHS grant # ES06694

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