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Lab Exercise: Simulating an Influenza Epidemic
OBJECTIVES
1. Describe how immunizations work to cause immunity.
2. Define herd immunity.
3. Define antigenic shift and antigenic drift and their relationship to immunization efficacy.
4. Describe the various types of immunizations. Why is it recommended to get “boosters” of some
immunizations?
BACKGROUND
Epidemiology is the study of disease transmission, and epidemiologists are the scientists within this field
who work to identify the etiologic agent(s) of a particular disease or syndrome. Many epidemiologists study
infectious or communicable diseases, including contagious diseases, which are transmitted easily through a
population. When the spread of these diseases results in the infection of individuals over more than one
continent, the disease is said to have spread to epidemic proportions. It is of particular interest to the
study of epidemiology to understand how a disease is spread and how best to prevent this spread. It is
important to remember that only a few hundred years ago, the Germ Theory of Disease was in its infancy
and without a clear understanding of microorganisms and their relationship to infectious diseases,
scientists lacked a basic understanding of how and why particular diseases spread from one person to
another. Without these, it would be impossible to stop the transmission.
However, the work of Louis Pasteur and Robert Koch, among others, enabled scientists and medical
practitioners to understand and control the spread of disease. Additionally, the work of Louis Pasteur and
Edward Jenner provided a way of protecting individuals from infectious diseases by means of immunization.
The first immunization was administered by Edward Jenner to protect against Small Pox and came in the
form of the less-virulent etiologic agent of Cow Pox, the Vaccinia virus. Jenner called this inoculation a
vaccination as a nod to the Vaccinia virus. Today, we use the term immunization to refer to any inoculation
of an avirulent organism to protect against later infection by a pathogen. Jenner and Pasteur hypothesized
that the body would remember and fight off pathogens better during subsequent exposure. We now know
that Jenner was successful because exposure of the body to Cow Pox produces immunity that the body
uses against the closely-related and far more virulent Small Pox virus.
In modern laboratories, pathogens have been bred to remain immunogenic (i.e. capable of generating an
immune response in the host) while becoming increasingly avirulent- thereby capable of generating the
immunological memory mentioned above, while providing a safe alternative to actual infection. This is
important particularly in cases where the natural pathogen is quite virulent, even unto death or permanent
disability. This was true of Small Pox in the 1700s and it remains true today; Small Pox immunizations are
the same Vaccinia virus vaccination that was administered by Edward Jenner. On the other hand,
immunizations have become quite sophisticated, including attenuated or weakened pathogens, dead
pathogens or even just the genetically-recombined immunogenic subunits of these pathogens (Table 1). For
instance, immunizations for the causative agent of Polio infection, Poliovirus, are given in the form of a dead
virus injection. Protection against the respiratory infection Diptheria is provided in the attenuated toxoid
form of the Corynebacterium diptheriae toxin which causes the symptoms associated with Diptheria
morbidity and mortality; protection against Haemophilus influenzae type B meningitis is provided by a
subunit immunization, where only the immunogenic polysaccharides from the bacterial outer membrane are
administered. These advances have made it possible for immunizations which carry far fewer side-effects
and little to no possibility of disease contraction.
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Table 1: Different types of immunizations
Type of Immunization
Description
dead pathogen
Typically a heat or chemical-killed pathogen is injected into the host for immunological memory.
attenuated
toxoid
subunit
Laboratory culture or directed mutations make the pathogen less virulent to the human host, this
attenuated, or weakened pathogen provides successful immunity so long as the attenuated pathogen
remains similar enough to the virulent form to provide cross immunity.
In infections where disease is caused by the production of exotoxins, toxins are isolated from the
pathogen, chemically or heat inactivated and then injected into the human host.
Particularly immunogenic portions of a given pathogen are recombined into an immunization which
generates immunity but is incapable of causing disease.
Although the discussion above includes pathogens which can cause life-threatening illnesses, not all
immunizations are aimed at pathogens with such severe symptoms. This may be because of the particular
risks to certain subsets of the population (e.g. the elderly, small children and immunocompromised
individuals) or because of the highly contagious nature of the infection. One such example is the Influenza
virus, which causes a disease syndrome commonly known as the Flu.
Figure 1: Diagrammatic representation of the Influenzavirus. Note the 8 single-stranded antisense strands
of RNA, which together make up the viral genome. Also of note are the neuraminidase and hemagglutinin
spikes, which are often the site of both genetic shift and drift.
In healthy adults, the flu is manifested as a mild to moderate upper respiratory infection. However, it can
cause severe illness and at times can lead to death. In the United States up to 20% of the population gets
the flu annually. Severe flu symptoms lead to the hospitalization of 200,000 people and the deaths of
36,000 annually. Additionally, Influenza pandemics have been responsible for tens of millions of deaths in
the 20th century alone (Table 2).
Table 2: Mortality data for the Influenza pandemics of the late 19th and 20th centuries.
Pandemic
Dates
Number of Deaths
Russian Flu
1889-90
1 million
Spanish Flu
1918-20
40 million
Asian Flu
1957-58
1 to 1.5 million
Hong Kong Flu
1968-69
0.75 to 1 million
Immunization to Influenza is provided as an injection of attenuated Influenza viruses; however, flu
immunizations (like flu infections) do not provide lifelong immunity against the flu. There are two major
reasons for this: antigenic drift and antigenic shift which both rely on the highly mutable nature of the 8
single-stranded antisense RNA Influenza genome. The changes in the RNA genome are beneficial to the
virus when they lead to changes in the surface antigen proteins, Neuraminidase and Hemagglutinin (Figure
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2). The viral RNA-dependent-RNA-polymerase which replicates this genome has no proof-reading
mechanism, causing the high error rate known as antigenic drift. Additionally, antigenic shift refers to the
co-infection with multiple strains of Influenza can cause a recombination of the RNA fragments into a new
variety (Figure 3). Additionally, there is evidence to suggest that Influenza pandemics are caused as the
fixed set of viral strains interact with the constantly changing population of human immunity (i.e. when the
resistant members of a population die, the population becomes susceptible and pandemic infections can
ensue.). In fact, Flu strains are named for the combinations of these proteins on viral envelopes For
example, strain H1N1 is thought to have caused the Spanish Flu of 1918.
Figure 2: Diagrammatic representation of the effect of both genetic drift and shift on the surface proteins
of the Influenzavirus.
Figure 3: Diagrammatic representation of antigenic shift, where two strains of Influenzavirus co-infect a
single host. Recombination of the two genomes results in an entirely new strain of flu.
Because of the high mutation rate associated with Influenzavirus, immunizations quickly become useless as
the viral proteins change to avoid detection by the immune system. Thus, effective flu immunization
programs must include yearly immunizations of subunit immunizations which are comprised of the antigenic
fragments of the Flu strains most likely to cause infection that year.
INTRODUCTION
In this lab, you will be part of a simulated epidemic. You will examine the communicability of your “disease,”
examine the effect of specific risk factors and high risk groups, and finally the effect of immunization in
creating herd immunity- whereby the immunity of certain individuals stops the transmission of an infectious
disease and protects the susceptible members of a population. For instance, infants under the age of 6
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months cannot receive the flu immunization, but these members of the population are particularly
susceptible to severe flu infection. Thus, parents often are immunized in order to protect their children.
The experiment will proceed in two “rounds,” examining the spread of influenza through two populations:
one with an actual class data used for the immunization rate and a second with an immunization rate of
70%. You will play a character in these populations, with assigned characteristics (
Figure 4).
Age 25 years
Risk Factors: none
Complications: none
Conclusion: experienced fever, myalgia, headache, malaise, cough,
sore throat, and rhinitis. Recovery occurred within 1 week.
Vaccine efficacy: 80% (range 1-80)
If you are exposed and were vaccinated and the last two digits of
your SSN fall within the range of numbers 1-80, then assume that
the vaccine was effective and you did not become infected with
flu.
Figure 4: Sample character card for the simulated epidemic. This card will be assigned to you, and will be
used throughout the experiment.
Your characteristics will include:
1. Age: which determines the effectiveness of your immunization; whether or not you can be
immunized (the immunization is not recommended for children under 6 months) and the
complications the character will experience if he/she is infected with influenza.
2. Risk Factors: factors including diabetes or heart disease. Risk factors will influence how the
disease affects the individual and could affect the efficacy of the vaccine.
3. Conclusion: describes what would hypothetically happen to that character if he/she was
infected with influenza.
4. Immunization Efficacy: which represents the effectiveness of the immunization upon exposure
to influenza
If it is determined that you have been immunized, you will collect a second card, labeled “Vaccinated.” If
you become infected through the course of the experiment, you will obtain a third card, labeled “Infected.”
LAB EXERCISE
Protocol:
I. Experimental Population Experiment One: Actual Immunization Rate
1. Obtain a character card from your instructor.
2. As a class, complete Table 3, row 1.
3. If you actually received the flu immunization this year, then obtain a “vaccinated” card and recall
the last two digits of your social security number. As a class, complete Table 3, row 2.
4. If you have been chosen by your instructor to be infected, obtain an “infected” card.
5. Proceed through the experiment:
a. Those with “infected” cards should randomly select two people to be “exposed.”
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b. If you are “exposed” and you are not immunized, obtain an “infected” card.
c. If you are exposed and are immunized, look at the Vaccine Efficacy Range on your character
card. Determine whether or not your immunization was effective.
d. All students who now have an “infected” card must reach out and randomly contact two people
sitting near them to be exposed (at least one of those individuals should not already be
infected).
e. Repeat steps b through c.
f. Repeat steps a through d two more times.
6. After the fourth round, read the Conclusion for your character. As a class, complete Table 3, row
2 and
Table 4.
Experimental Population Experiment Two: Increased Immunization Rate
1. If you have been chosen by your instructor to be immunized, obtain a “vaccinated” card.
2. If you have been chosen by your instructor to be infected, obtain an “infected” card.
3. Proceed through the experiment:
a. Those with “infected” cards should randomly select two people to be “exposed.”
b. If you are “exposed” and you are not immunized, obtain an “infected” card.
c. If you are exposed and are immunized, look at the Vaccine Efficacy Range on your character
card. Determine whether or not your immunization was effective.
d. All students who now have an “infected” card must reach out and randomly contact two
people sitting near them to be exposed (at least one of those individuals should not already
be infected).
e. Repeat steps b through c.
f. Repeat steps a through d two more times.
4. After the fourth round, read the Conclusion for your character. Use these data to complete Table
6.
DATA AND OBSERVATIONS
I. Experimental Population Experiment One: Actual Immunization Rate
Experimental Population Statistics
Table 3: Population data for Experimental Population Experiment One
No. of students
Number of students in the population
Total number of students that have been immunized for Influenzavirus
Total number of students that were infected with Influenzavirus
Experimental Data Collection
1. When prompted by your instructor, answer the following questions by raising your hand. While
you are completing
Table 4 in your lab notebook, your instructor will be filling out a similar
table on the board.
1. Raise your hand if you were 0 to 6 months old.
2. Keep your hand up if you were infected.
3. Keep your hand up if you had complications.
4. Keep your hand up if you died.
2. Repeat step 1 for all age classes.
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Table 4: Data collected for Experimental Population Experiment One
number with
age groups
total number
number infected
complications
number of deaths
recovery time
0–6 months
6–12 months
2–15 years
16–60 years
>60 years
Risk Data Collection
1. When prompted by your instructor, answer the following questions by raising your hand. While
you are completing Table 5 in your lab notebook, your instructor will be filling out a similar table
on the board.
1. Raise your hand if you had a risk factor.
2. Keep your hand up if you were infected.
3. Keep your hand up if you experienced complications.
4. Keep your hand up if you died.
5. What was your recovery time?
2. Repeat questions 1–5 for members of the population who do not have risk factors.
Table 5: Risk data collection table
with risk factors
without risk
factors
total number
number infected
number infected with complications
number of deaths
average recovery time
II. Experimental Population Experiment Two: Increased Immunization Rate
1. When prompted by your instructor, answer the following questions by raising your hand. While
you are completing Table 6 in your lab notebook, your instructor will be filling out a similar table
on the board.
1. Raise your hand if you were immunized.
2. Raise your hand if you were infected.
3. Keep your hand up if you had complications.
4. Keep your hand up if you died.
Table 6: Data collected for Experimental Population Experiment Two
Total number in the population
number immunized
number infected
number of infected who experienced
complications
number of infected who died
DISCUSSION
Experimental Population Experiment One: Actual Immunization Rate
1. What percentage of the class received the immunization? (Divide the number of immunized individuals
by the total number of students in the class and multiply by 100.)
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2. What percentage of the class became infected?
Experimental Population Experiment Two: Increased Immunization Rate
1. What percentage of the class received the immunization? (Divide the number of immunized individuals
by the total number of students in the class and multiply by 100.)
2. What percentage of the class became infected?
3. What percentage of the infected individuals experienced complications? (Divide the number of
infected individuals who experienced complications by the total number of infected individuals and
multiply by 100.)
4. What percentage of infected individuals died? (Divide the number of infected individuals who died by
the total number of infected individuals and multiply by 100.)
5. What is herd immunity? Was herd immunity achieved in either of the experimental populations? If
not, what effect does an increased immunization rate have on a population?
6. Describe how antigenic shift happens in the influenza virus. How does the effect the efficacy of the
immunization?
7. Why are yearly influenza immunizations recommended?
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