Preventing Outbreaks with Herd Immunity

Background Information for Teachers

Community immunity (sometimes called herd immunity) is the concept that people who are immune to an infectious disease help break the chain of transmission of the disease, thus protecting others. This works because people who are immune cannot contract or pass along the disease, so they essentially form a roadblock in the person-to-person disease transmission.

Image Credit: NIAD creative commons 2.0 / Digital Millennium Copyright Act of 1998 (DMCA)

Three scenarios are shown: 1) No one is immunized. This results in a contagious disease spreading through the population. 2) Some of the population is immunized. This results in the contagious disease spreading through some of the population. 3) Most of the population gets immunized. This results in the contagious disease being contained with little to no spread.

Figure 1. This graphic created by NCAID shows how as community immunity increases, so does a population's protection against an outbreak.

Community immunity can be achieved in several ways 1) naturally, through enough people contracting and recovering from the disease, 2) through vaccination 3) biologically, through some people simply not being susceptible to the disease, or 4) a combination of these factors. The more people in a population who are immune, the less chance a disease has of spreading and causing an outbreak. The minimum percentage of a population that needs to be immune to a disease to completely prevent its spread is called the herd immunity threshold.

The herd immunity threshold plays an important role in protecting community members who cannot be effectively vaccinated. For example, elderly people and young infants have weaker immune systems that sometimes do not create protective antibodies, even after vaccination. Similarly, people who are immunocompromised, like cancer patients undergoing chemotherapy, may have immune systems that are so weak they are not able to be given vaccines at all. Incidentally, the same people who cannot be vaccinated effectively or at all, are often the people most at risk of the worst symptoms, or even death, if infected. When immunity is at or above the herd immunity threshold, it is far less likely, statistically, that these vulnerable populations will encounter and contract the disease.

In this lesson, students will use SimPandemic to run simulations of what happens to a population of 100,000 individuals when a few COVID-19-positive individuals are present. As students increase the level of community immunity (i.e. the percent of the population who is immune to COVID-19 at the start of the simulation), they will see that the effective size of the outbreak (the total number of people infected over the course of the simulation) drops. When the community immunity is set at or above the herd immunity threshold (approximately = 66% in this COVID-19 simulation) person-to-person transmission is thwarted to the point that no outbreak occurs. Increasing immunity beyond the herd immunity threshold has little to no value.

If students explore further—on their own or guided by some of the Lesson Plan Variations listed—they will see that not all diseases have the same herd immunity threshold. Infectious diseases have a basic reproduction number (also called R-naught and abbreviated R₀). The basic reproduction number quantifies, on average, how many more people someone who has the disease will infect. The higher R-naught, the higher the herd immunity threshold. In fact, the herd immunity threshold for any disease using this formula:

Students can also see what happens if they try to overwhelm herd immunity with a rapid influx of infected people. While it is theoretically possible to do so, the number of infected individuals required is quite high (often in the thousands) and is, from a practical standpoint, unlikely.

You will see that students obtain slightly different results, even when they have the same inputs for SimPandemic. Many events in the real world and in a simulation of the real world are based on chance. When you become infected in the real world, you often don't know when or where the infection occurred. Perhaps someone sneezed when you were randomly walking by them. A simulation cannot predict that you were going to get infected (except in special cases), but it can predict fairly well that someone would get infected. In SimPandemic there are many events where the simulation literally rolls virtual dice to determine when an infected individual will transmit the disease to another (all within the bounds specified by the input parameters). To better understand the assumptions and parameters involved, read the SimPandemic FAQ.

Additional Background Links

• What is herd immunity?, Microbiology Society
• Herd Immunity: How does it work?, Oxford Vaccine Group

Prep Work (20 minutes)

1. Review the video that students will be watching to learn about the adaptive immune system and vaccines.
   1. How does herd immunity work?
2. Familiarize yourself with the SimPandemic online software. Read through the SimPandemic FAQ and try out the SimPandemic Notebook associated with this lesson plan.

Engage (15 minutes)

Remote learning adaptation: This lesson plan can be conducted remotely. Students can work independently on the Explore section of the lesson plan using the Student Worksheet as a guide. The Engage and Reflect sections can either be dropped entirely, done in writing remotely, or be conducted over a video chat.

1. Start by describing a scenario to your students and asking them for a scientific explanation. Record their answers to revisit at the end of the lesson.

   Imagine this scenario: Two grandparents fly across the country to visit their newborn grandchild. After being there a few days, the grandparents receive a phone call from a contract tracer at the local public health department. The contact tracer tells them that several people on their plane flight were unknowingly contagious with measles. Everyone on board the plane was exposed and there is a chance they might come down with the measles, too. In fact, they may already be contagious and infecting others before the symptoms set in. Knowing that measles is highly contagious (it passes easily from one person to another) and can be very dangerous, or even deadly, for young infants, the grandparents and parents are very worried. They immediately contact the pediatrician. After asking the grandparents a few questions, the pediatrician reassures the family that even though the baby is too young to be vaccinated, she is confident the baby is fine and will not get measles from the grandparents. What do you think the grandparents and pediatrician discussed that made the doctor confident that the baby would not get measles? Give your best science-based guess.

   Students may be puzzled. Encourage them to give their best guess, regardless of whether they are right or wrong. Record all answers to revisit at the end of the lesson.

2. Tell students that they'll be learning more about how community immunity, sometimes called herd immunity, works and how it can affect the outcomes of an outbreak. At the end of the lesson, they will have a chance to guess what the grandparents and pediatrician discussed based on their new knowledge.
3. See what students already know about community immunity.

   Raise your hand if you have heard of community immunity (i.e. herd immunity) before. What do you think community immunity is?

   Students may or may not know what community immunity is. Listen to students' answers, then show them this video.

https://www.youtube.com/watch?v=8BUCi5Tuzms

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Explore (40 minutes)

1. Distribute either the Student worksheet (PDF) or the Student worksheet (Quiz assignable in any LMS) to each student. Break the class into pairs if they will be collaborating on the worksheet.
2. Set up Part 1 of the lesson by telling students they will be using an online tool called SimPandemic to evaluate the impact of community immunity on a COVID-19 pandemic. Navigate to the SimPandemic Notebook that accompanies this lesson plan and review the information in the first section with students. In particular:
   1. Explain to students that this is what a model of the COVID-19 pandemic looks like if no interventions (no masks, no physical distancing, no shutdowns, etc.) looks like.
   2. Go over the For Simulated Populations of 100,000 outcomes table and how to read the accompanying graph. Make sure every student understands the type of data being presented before continuing. Also make sure that students understand that this graph models the question they were asked about how a vaccine administered before an outbreak can alter the course of an outbreak.
   3. Emphasize that the outcomes are listed per 100,000 individuals to enable direct comparisons between simulations of different scenarios.
   4. Point out the FAQ button. If students get stuck or have questions about the program, the SimPandemic FAQ is likely to have answers.
   5. Explain that if the simulation was run twenty times, each time the results would be slightly different. This is because the simulation mimics real life in that it looks at the chance of getting infected and the chance of having symptoms or dying. In SimPandemic there are many events where the code underlying the simulation literally rolls virtual dice to determine when an infected simulant will transmit the disease to another (all within the bounds specified by the input parameters). Just like rolling dice in a board game, the results can vary within a predictable statistical range.
3. Direct students to work independently, or in pairs, to read through the information in sections 1 and 2 of the SimPandemic Notebook and answer the questions in Part 1 of the Student Worksheet.
4. Set up Part 2 of the lesson by having a student read out loud the first two paragraphs of section 3 of the SimPandemic Notebook. Make sure students understand what the herd immunity threshold is.

   In your own words, what is the herd immunity threshold?

   It's the smallest level of community immunity that makes it impossible for a disease to spread and become an outbreak.

   On a SimPandemic graph, what do you expect to see happen to the blue line representing the number of infected individuals if the COVID-19 herd immunity threshold is met?

   The initial ten people who are infected will not pass the virus on to many more people, so the line will remain relatively flat and then go to zero as time progresses. There will be no hill or mountain made by the line, unlike during a COVID-19 outbreak.

5. Poll your students to see what they think is the herd immunity threshold for COVID-19.

   For COVID-19, how high do you think community immunity needs to be to reach the herd immunity threshold?

6. Tell students that they will have the opportunity to test their predictions by exploring and running their own simulations. They will use the Sandbox in SimPandemic Notebook section 3 to deduce the herd immunity threshold by manipulating the level of community immunity, then running the simulation to see whether or not an outbreak happens. Before letting students explore, consider going over with them:
   1. How to make changes to the settings.
   2. How to save their work in SimPandemic.
   3. Remind them that if they have questions they can always consult the FAQ.

Reflect (10 minutes)

1. Remind students of the scenario you laid out for them about the grandparents who were afraid they may have exposed their infant grandchild to measles (see the Engage section for details). Read back the explanations they originally came up with. Ask students what they now think the scientific explanation is.

   What is your scientific explanation for why the pediatrician is confident the grandparents did not pass measles to their infant grandchild?.

   Based on their new knowledge, students should be able to conclude that the pediatrician asked the grandparents whether they had either had measles before or been vaccinated for it. Both grandparents met one of these criteria (it does not matter which one) and thus both grandparents were immune to measles. Because someone who is immune cannot become infected and pass along the disease, the infant was protected by the grandparents' community immunity.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Epidemiologist

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Career Profile

Epidemiologists work to study, understand, and contain outbreaks. They often work in public health roles to help devise and implement procedures, like vaccination campaigns, that prevent outbreaks. When an outbreak occurs, they are on the front lines of dealing with the outbreak and in communicating life-saving information to the public, as well as calculating important information like the herd immunity threshold for a disease. Read more

Health Educator

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Career Profile

Health educators inform patients and the general public about vaccines and other steps people can take to lower the transmission of an infectious disease. By communicating clearly, they can help the public make informed choices that either prevent an outbreak or diminish the severity of an outbreak. Read more

Lesson Plan Variations

• This lesson plan may be extended by asking students to evaluate how R₀ impacts the herd immunity threshold of a disease.
• Pose the question of whether or not the herd immunity threshold of a disease can be overwhelmed by an influx of contagious individuals. Have students use SimPandemic to model and investigate this question.
• If data is available about the percentage of the population that is immune to COVID-19 or another virus, students can evaluate how far their community is from the herd immunity threshold for that disease and whether natural immunity or another strategy is the best way to reach the herd immunity threshold.
• Several other SimPandemic lesson plans are available.