Exploring Infectious Disease Outbreaks and Herd Immunity Through Simulations with a Visual Appeal.

INTRODUCTION

Numerous resources and simulations are available highlighting infectious diseases and vaccination (1–3). The spread of infectious diseases is often simulated using the pH indicator phenolphthalein to trace a basic solution as the infectious agent (http://www.pbs.org/newshour/extra/app/uploads/2014/09/1_Epidemics-NGC.pdf). This activity expands upon this commonly used simulation to include vaccination and herd immunity components, employing a buffered solution to prevent the simulated infection from spreading. This simulation is appropriate for Biology majors and nonscience major undergraduates, as well as for middle school students through to adults in an outreach setting. Follow-up discussion and analysis can be tailored to the audience.

PROCEDURE

Preparation of supplies

For the infectious disease outbreak, label tubes with numbers corresponding to the total number of students (Appendix 1). An even number of participants is required. Add 2 mL of 1 M NaOH to two tubes (which serve as sources of infection). Add 2 mL of dH2O to the remaining tubes (susceptible to infection). The simulation as described is for 16 to 20 participants; add additional infected (NaOH) tube(s) for larger groups. Record which tubes contain NaOH.

For herd immunity, label a corresponding set of tubes using a different color label (to avoid confusion). Add 2 mL of 1 M NaOH to the same numbered tubes. To half of the remaining tubes (50% vaccination rate, can be adjusted), add 2 mL of 0.5 M Tris buffer, pH 6.8. Other buffers have not been employed, though they should be acceptable as long as the buffer and pH indicator range do not overlap. Add 2 mL of dH2O to the remaining (susceptible) tubes.

Each prepared tube requires a plastic transfer pipette. A solution of 1% phenolphthalein (in 50% ethanol) should also be prepared. For safe disposal after the simulation, combine contents of tubes and neutralize with HCl, as necessary, before pouring down the drain with excess water. The use of gloves and eye protection is advised.

Phase I: Spread of infectious disease

Distribute the infectious disease outbreak tubes. Instruct each student to select a partner for their first exchange. An exchange could indicate a handshake, doorknob, sneeze, or sexual partner depending on the selected disease scenario. During an exchange, each student should use their transfer pipette to transfer two to three drops of liquid from their tube into their partner’s tube; liquid remaining in the pipette should be returned to their own tube while pipetting gently to mix. Students should record their partner’s tube number on the board (Fig. 1).

FIGURE 1

Example simulation table to record student data.

Each exchange requires a new partner. Therefore, confirm everyone has a new partner before allowing students to proceed with successive exchanges. Students exchange liquid from their tubes and record their partner’s tube number on the board as described previously. After the third exchange, the instructor adds 2 drops of 1% phenolphthalein to each tube. Students encountering “infected” (NaOH) students during an exchange will have a more basic pH, indicated by the immediate pink color change. Students should indicate their infection status on the board.

Phase II: Herd immunity

Before beginning, confirm that a two- to three-drop addition of NaOH will not alter the pH of the Tris buffer. Students should receive the same numbered tube from the herd immunity set and perform the simulation with the same Phase I partners in the same order as recorded on the board. However, this time 50% of the tubes contain the “immune/vaccinated” buffer that will not change pH after contact with NaOH. After phenolphthalein addition, those students “immunized” with buffer should not get sick and should not be able to spread disease, even if they exchanged with an “infected” student. Tubes that contained water in Phase II may or may not become infected and can provide cases of protection by herd immunity. A comparison of students infected in the infectious disease outbreak simulation versus those infected during the herd immunity simulation should identify any students protected by herd immunity (Appendix 2).

EXTENSIONS AND DISCUSSION

It is at the instructor’s discretion whether to share the expected herd immunity outcome with participants. Students are often engaged and, while surprised by the different simulation outcomes, are able to determine immunity is the reason for the difference. This activity lends itself to the discussion of vaccines and how they provide protection. Vaccine lessons can be tailored to undergraduates or audiences ranging from middle school to public outreach.

University and high school students can serve as epidemiologists to trace the source of infection (index cases) using data on the board (Appendix 2). The process of elimination usually results in identifying the index cases and the first tubes the index cases exchanged with. Individual index cases could be determined by providing students a scenario (sexual behaviors, symptoms, travel history, etc.) that would allow one to eliminate the first exchange partner. Alternatively, students can be asked to represent an epidemiologist or local health department and determine what questions should be asked or which tests performed (e.g., IgM levels) to trace the source of infection.

While a 50% vaccination rate (50% of tubes containing buffer) has been useful in showing protection from infection and in most cases herd immunity in the population, different vaccination rates can be employed for science majors. Different rates allow for discussion of effective coverage rates for various infectious diseases. Appendix 2 illustrates a simulation with a 50% vaccination rate that resulted in effective herd immunity (Example A), while Example B did not. Examining these two scenarios could lead to the discussion of live versus attenuated vaccines; vaccine antigens and design; development and variability of immune responses; and disease transmissibility.

CONCLUSIONS

Since this simulation is performed in two phases, students can see how an infectious disease might spread through a population and how vaccination provides protection. Visualizing the infection (pink color change) has an especially strong impact on non-biology majors, who may be less familiar with vaccine benefits. The activity lends itself to discussions regarding infectious diseases, vaccination, immunity, epidemiology, and public health.

This simulation has been successfully used with various populations (sixth graders, high school students, adults, nonscience majors, and biology majors in an undergraduate Immunology course) and only requires 20 to 30 minutes (minimum). Select nonscience major laboratory sections were asked to indicate the three activities (out of nine) that contributed most to their learning; 22 of 36 (61%) respondents selected the lab experience containing this simulation.

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