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This is a sample of the instructor resources for Managerial Epidemiology: Concepts and
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Fleming Instructor Resources
Case Study 2.2. Outbreak of Influenza in a Kentucky Nursing Home
Assume that an outbreak of Influenza A occurred among 400 residents of a New York
Nursing Home during December 2006 and January 2007, despite the vaccination of 375 of them
between mid-October and mid-November of 2006. The residents, 70% of whom were female,
had a mean age of 85 years and shared common recreational and dining areas.
QUESTIONS
1. If 75 of the residents developed influenza-like illness (ILI), what proportion of the
residents became sick?
2. Of those with ILI, 40 developed pneumonia, 25 required hospitalization, and two died.
What proportion of those with ILI developed pneumonia? What percent of those with ILI
and pneumonia were hospitalized? What proportion of those with ILI died?
3. Of the 375 residents who were vaccinated, 60 developed ILI. Of the 25 residents who
were not vaccinated, 20 developed ILI. What percent of vaccinated residents developed
ILI? What percent of unvaccinated residents developed ILI? How many more times
higher is the rate of ILI among those who were unvaccinated compared to those who
were vaccinated?
4. Of the 375 vaccinated residents, 35 developed pneumonia following ILI compared to 15
residents among the 25 who were not vaccinated. What percent of vaccinated residents
developed pneumonia following ILI? What percent of unvaccinated residents developed
pneumonia following ILI? How many more times higher is the pneumonia following ILI
among those who were unvaccinated compared to those who were vaccinated?
5. What was the vaccine efficacy for preventing LIL and pneumonia?
ANSWERS
1. The incidence rate (Chapter 3) of ILI is (75/400)*100 = 18.8%.
2. The incidence rate of pneumonia among those with ILI is (40/75)*100 = 53.3%. The
hospitalization rate among those with ILI and pneumonia is (25/40)*100 = 62.5%. Of
those with ILI, the proportion who died (case-fatality rate, Chapter 6), is (2/75)*100 =
2.7%.
3. Of those who were vaccinated, (60/375)*100 = 16.0% developed ILI. Of those who were
not vaccinated, (20/25)*100 = 80.0% developed ILI. The ratio of the two rates, 80.0/16.0
= 5.00 is called a relative risk (Chapter 12) and indicates that the unvaccinated group was
26
5.00 times as likely to develop ILI compared to the vaccinated group.
4. Of those who were vaccinated, (35/375)*100 =9.3% developed pneumonia following ILI.
Of those who were not vaccinated, (15/25)*100 = 60.0% developed pneumonia following
ILI. The ratio of the two rates, 60.0/9.3 = 6.45 indicates that the unvaccinated group was
6.45 times as likely to develop pneumonia following ILI compared to the vaccinated
group.
5. The vaccine efficacy is calculated as the difference in illness rates (unvaccinated –
vaccinated) divided by the unvaccinated illness rate. In this case, that would be (0.800.16)/0.8 = 0.8 (80.0%) for vaccine efficacy in preventing ILI, and (0.6-0.093)/0.6 = .845
(84.5%) for vaccine efficacy in preventing pneumonia.
27
CHAPTER 2—INFECTIOUS DISEASE
EPIDEMIOLOGY
1
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HISTORY
o
o
o
o
Evidence of disease in very old bones
Early human had limited contact with others;
reduced spread
p
of disease
As travel increases, more disease and epidemics
occur
Impact of the plague—wipes out entire towns
and villages
2
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AGENT HOST ENVIRONMENT
AGENT-HOST-ENVIRONMENT
Agent disease-causing factor
Agent—disease-causing
o Host—person or animal in whom disease
is caused
o Environment—conditions under which
h t is
host
i exposedd to
t the
th agentt
o
3
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4
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5
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6
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DISEASE TERMS
o
o
o
o
o
o
o
Pathogens—disease-causing organisms
Pathogenesis—development of disease
Infective diseases—pathogen/agent capable of entering,
multiplying, surviving in host
Virulence—extent of pathogenicity, strength
Invasiveness—ability of pathogen to enter/grow in host
Communicability—f(environment, susceptible hosts, fomites,
vectors)
Etiology—factors
Etiology
factors that cause disease; science of causes
(Last 2000)
7
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DISEASE CATEGORIES
o
o
o
Acute disease—relatively severe, short duration,
treatable, recovery or death
Subacute disease—intermediate in
severity/duration, may result in recovery
Chronic disease—less severe, long duration, may
not result in recovery
8
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COMPARE CHRONIC/ACUTE DISEASE
Duration
CHRONIC
Long term
ACUTE
Short term
Agent
Environmental,
lifestyle, behavior,
chemicals
Pathogens
Treatment
Treat symptoms,
pain; minimal cure
Antibiotics
kill agents
Course of disease
Intervention in pathogen
pathogenesis minimal; disease
not reversible
Intervention usually
effective, disease
reversible
Goal of care
Retard advancement
Total cure of disease,,
control, rehab
Duration of care
Long term
Short term
9
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CHARACTERISTICS OF INFECTIOUS AGENTS
o
Infectivity—ability to lodge in and infect/multiply in
host
o
Measure by infection or attack rate
o
o
Number of persons with antibody response/number exposed
Secondar attack rate
Secondary
o
o
o
More useful method, why? No serology
Number exposed to developing disease within one incubation period/total
number exposed to primary case
P th
Pathogenicity—ability
i it
bilit off agentt to
t produce
d
“disease”
“di
”
o
Depends on
o
o
o
o
Rapidity/extent of agent multiplication
Extent of disease damage
Whether agent produces toxin
Measure by pathogenicity rate
o
Number of infections with clinical disease/total number infected
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CHARACTERISTICS OF
INFECTIOUS AGENTS
o
o
Virulence—severity of disease
o Extent of pathogenicity or strength
o Measure by case-fatality rate
o Number of fatal cases/number of cases
P d
Produce
di
disease in
i host
h t by
b
o Toxin (e.g., staph poisoning in 3–6 hours)
o Invasion/infection (e.g.,
(e g salmonella,
salmonella 12
12–24
24 hours)
o Production of immune response in host that produces
disease (e.g., AIDS)
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CHARACTERISTICS OF AGENTS
AGENTS—
NONINFECTIOUS
Chemical—concentration, toxicity
o Physical—size,
Physical size shape,
shape intensity
o Psychological—chronicity, suddenness
o Genetic—homo/heterozygosity
G
i h
/h
i
o
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TRANSMISSION OF DISEASE
o
o
o
o
Reservoir—where
Reservoir
where infectious agent normally
lives and multiplies
Vector—any
Vector
any living nonhuman carrier of disease,
e.g., mosquito
Fomites—articles that transmit infection, e.g.,
doorknobs
Carrier—person who spreads infectious agent
13
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TYPES OF CARRIERS
o
Carrier spreads infectious agent
Carrier—spreads
o Active/healthy/passive carrier—exposed to
agent,
g , can spread
p
disease,, no symptoms
y p
o Convalescent carrier—infectious but
recoveringg
o Incubatory carrier—infectious, beginning
phase of disease
o Intermittent carrier—spreads disease
intermittently
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PORTALS OF ENTRY
o
o
o
o
o
o
o
o
o
o
o
Conjunctiva of eye
Skin breaks
Respiratory tract
GI tract
Reproductive tract
Oral
Urinary tract
Cardiovascular
Percutaneous (trauma,
(trauma injection)
Transplacental
Organ
g transplantation
p
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HOW ARE DISEASES TRANSMITTED?
o
Incubation period—interval
period interval between time of contact
with agent and onset of illness
o Difference between disease and illness
o Multiplication of infectious agent Æ symptoms
o Analogy with chemical agent?
o Noninfectious, e.g., years of stress Æ high blood
p essu e
pressure
o A statistical distribution
17
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INCUBATION PERIODS
o
o
o
o
o
o
o
o
o
o
o
o
Botulism
Common cold
Conjunctivitis
Influenza
Bacterial/viral
i l/ i l pneumonia
i
Gonorrhea
Meningitis
Herpes simplex
Measles
Dysentery
y
y
Epstein-Barr
Serum hepatitis
12–36 hours
12 72 hours (usually 24)
12–72
1–3 days
1–3 days
1–3 days
d
2–5 days
2–10 days
up to 2 weeks
12–26 days
2–4 weeks
4–7 weeks
45–160 days
19
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STAGES OF INFECTION
o
o
o
o
o
Incubatory (subclinical)—before signs and
symptoms
Prodrome stage—later part of incubation period
o Generally nonspecific signs and symptoms
(“tired”)
Acute phase—signs and symptoms obvious
Resolution phase—patient begins to improve
Convalescent period—signs and symptoms
disappear
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KINDS OF PREVENTION
o
o
o
Primary prevention—stop
prevention stop disease before it starts
o Immunization
o Sanitation
Secondary prevention—reduce illness onset,
duration, or transmission, or detect disease early
(screening)
Tertiaryy pprevention—cure illness or limit degree
g
of disability, suffering, or complications
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HERD IMMUNITY
o
o
o
Community resistance
Decreased probability of epidemic
How to measure herd immunity?
o
o
o
Proportion of immune in population
Herd immunity and chance of carrier contact
How much herd immunityy is enough?
g
o
o
o
How infectious is disease?
What is incubation period?
What is the size and social behavior of group?
23
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INFECTION CONTROL IN
HEALTHCARE FACILITIES
o
o
Ensure staff is immunized
Ensure facility is sanitized (clean beds, rooms,
equipment)
q p
)
Well-trained cleaning staff
o Infection control policies
o Special
S i l attention
tt ti to
t rooms with
ith invasive
i
i procedures
d
o
o
o
o
o
Personal hygiene
Personal protective equipment (gowns)
Hand washing
Food ppreparation
p
and storage
g
24
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EPIDEMICS
o
o
o
Definition of epidemic: Occurrence of disease in excess of normal
expectancy (Last 2000)
Kinds of epidemics
o Common vehicle—agent transmitted by water, food, air,
inoculation
o Point source—if single source like food
o Serial transfer from host to host
o Respiratory, oral, sexual
Epidemic terms
o Pandemic—widespread epidemic
o Holoepidemic—highly prevalent disease acquired early in life,
e g chicken pox
e.g.,
o Hyperepidemic—persistent level above prevalence, e.g.,
pneumonia in nursing homes
o Endemic—usual prevalence of disease, e.g., colds, flu
25
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INCIDENCE FOR FOODBORNE
DISEASE
Crude attack rate—number of people ill
× 100 /number attending event
o Food-specific
p
attack rate—number of
people who ate food and got ill × 100/total
number who ate food
o
26
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STEPS IN OUTBREAK INVESTIGATION
1.
2.
3.
4.
5.
6.
7
7.
8.
9.
Establish the existence of an outbreak (epidemic)
Verify the reported diagnosis
Prepare a case definition and count cases
Prepare for fieldwork
Compile descriptive epidemiology
Develop hypotheses
E
Evaluate
l t hypotheses
h
th
Reconsider and refine hypotheses
Plan and implement additional necessary studies (clinical,
environmental, epidemiological, laboratory)
10. Implement control and prevention actions
11. Communicate findings
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27
TYPES OF CASES
o
o
o
o
o
o
o
Case—person diagnosed with disease
Primary case—first case in population in
epidemic with disease
Index case—first
fi case recognized
i d in
i epidemic
id i
Secondary cases—got disease from primary
case
Recurring case—person gets disease again
New case
case—part
part of incidence calculations
Suspect case—signs/symptoms but undiagnosed
28
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30
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31
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32
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HOW TO PREVENT/CONTROL DISEASE
o
Destroy
y agent
g
o
o
Pasteurization, chlorination, radiation of meat, heat
Break cycle of transmission
Eliminate means of transmission (e.g., drain swamp, sewage
treatment)
o Reduce direct contact (e.g., wear condoms)
o
o
Increase immunization/resistance
o
o
Reduce risk factors
o
o
o
Immunization programs,
g
g
good health habits
Behaviors, diet, etc.
Good hygiene, food handling, protective clothing, repellants
I l t infectious
Isolate
i f ti
cases
o
o
Isolation—separate infectious cases during communicability
Quarantine—limit freedom/movement of exposed well people
34
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