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PDF Files for PrintingActivity 4 - Protecting the Herd

At a Glance

Focus: Students use in-class and Internet-based simulations of the spread of an infectious disease through a population to discover the phenomenon of herd immunity.

Major Concepts: 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.

Objectives: After completing this activity, students will

. be able to explain how immunizing a significant proportion of a population against a disease prevents epidemics of that disease (herd immunity),
. be able to list factors that affect the proportion of a population that must be immunized to prevent epidemics, and
. understand how large-scale vaccination programs help control infectious diseases.

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

Basic Science-Public Health Connection: 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.

Introduction

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 age 4-6 or 11-12.

This activity and Activity 3, Superbugs: An Evolving Concern, both provide explanations for the re-emergence of some infectious diseases. Activity 3 explained that some re-emerging diseases are due to the evolution of antibiotic resistance among pathogens. Activity 4, Protecting the Herd, introduces students to the idea that the re-emergence of other infectious diseases can be explained by a failure to immunize a sufficient proportion of the population. On the first day of the activity, students learn that epidemics can be prevented by immunizing part of the population, leading to herd immunity. The concept of herd immunity is elaborated in the optional, second day of the activity. Here, students learn that the threshold level of immunity required to establish herd immunity (and thus prevent epidemics) varies depending on the transmissibility of the disease, the length of the infectious period, the population density, and other factors.

Materials and Preparation

You will need to prepare the following materials before conducting this activity:

If you do Day 2 of the activity, you will also need the following materials:

Note to teachers: If you do not have enough computers equipped with Internet access, you will not be able to conduct the optional Day 2 of this activity.

Procedure

DAY 1

1. Introduce the activity by distributing one copy of Master 4.1, 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, and colds. On the day you wish to begin the activity, ask students to name some of these diseases, then ask them to count the number of different diseases each generation in their family had. 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 vaccination against many diseases is now available.

Möbius StripThis is an opportunity to point out that research in microbiology and related disciplines in the last 50 years has led to the development of many vaccines in addition to the measles vaccine. Children of the 1990s who receive recommended vaccinations are protected from many infectious diseases that plagued children in the past, including diphtheria, whooping cough, measles, hepatitis B, and chicken pox.

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 vaccine didn't 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 continue 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 two-day disease, according to the instructions provided, 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:

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

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.

6. 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.

puzzle pieceThis step takes students to the major concept of the activity: The re-emergence of some diseases can be explained by immunity levels that are below the level required for herd immunity.

7. 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."

8. 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:

assessmentCollect and review students' paragraphs to assess their understanding of the major concept of the activity. Address common misunderstandings in the next class session and read two or three of the best paragraphs to the class.

. 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.

DAY 2 (Optional)

1. Open the activity by reminding students about two-day disease and the simulation that they completed. Then ask them what characteristics may vary between two-day disease and other diseases. Point out that differences in these characteristics affect the likelihood that an epidemic of a particular disease will occur and the percentage of the population that must be immune to that disease to achieve herd immunity.

Expect students to suggest that people who are sick may contact more than one person per day, may be sick (and infectious) for more than two days, may die from the disease, and may not get sick from just one contact. Students also may point out that the disease may require "intimate" rather than casual contact or it may not require person-to-person contact.

2. Ask students to predict what the results of the simulation would be if they varied each of four characteristics of the disease: virulence (the likelihood of dying from the disease), duration of infection, rate of transmission (how contagious the disease is), and level of immunity in the population. Insist that students provide some rationale for their predictions. Write their predictions on the board or a blank transparency.

To help students think about this, you may wish to ask questions such as, "Do you think there would have been an epidemic of two-day disease if people sometimes died from the disease? If so, do you think it would have been a more or less severe epidemic?"

Virulence, duration of infection, rate of transmission, and level of immunity are the four parameters that the computer simulation will allow students to vary. Students may make predictions such as, "The more virulent a disease is, the greater the likelihood of an epidemic," or "The higher the immunity level of a population, the less likely it is that an epidemic will occur."

3. Tell students that they will use a computer simulation to investigate the likelihood of an epidemic when they vary one of the four characteristics they just discussed. Distribute one copy of Master 4.5, Disease Transmission Simulation Record, to each student and ask students to organize into their teams. Assign each team one of the four characteristics to investigate and direct students to circle this characteristic on the master.

Tell students that because a larger population size is used in the computer simulation, an epidemic is defined as an outbreak of disease in which 10 percent or more of the population is sick at one time.

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