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Lecture 3 - Outbreak Investigations
Terms in this set (14)
Common source outbreak
Definition: direct or indirect exposure to the SAME agent
>> multiple people exposed to one agent, but agent can be...
-- INFECTIOUS or ENVIRONMENTAL
-- INCUBATION period is NOT part of the transmission cycle
>> incubation period: from time of infection to signs and symptoms
2 Types of Common Source Outbreak
1. Point source = exposure during the SAME brief period of time
2. Continuing (or intermittent) source = exposure GREATER THAN a brief period of time
2 Types of Outbreaks
1. Common Source Outbreak
>> incubation period NOT part of transmission: means that once you get the disease, you IMMEDIATELY show symptoms
2. Propagated Outbreak
>> incubation period IS part of transmission
Definition: transmission of infectious agent from one susceptible host to another
>> agent is transferred from host to host
-- INDEX -- initial case that starts the outbreak
-- PRIMARY -- cases from index
-- and SECONDARY cases -- cases from primary
-- incubation period IS part of the transmission
-- propagated outbreak is CONTINUOUS until no more susceptible people (or contact is reduced)
How we examine the cause of the outbreak
ARR (attack rate ratios)
a) ARR > 1 = causative
-- rate of disease in exposed was higher than in unexposed
-- positive association; that agent most likely caused the outbreak
b) ARR = 1 = exposure does NOT appear to be associated with disease
c) ARR < 1 = preventive
-- negative association
Template for ARR conclusions
In the exposed, the rate of developing disease X was _____ (ARR number) times the rate of those who were NOT exposed
What is a KEY FEATURE of a COMMON SOURCE OUTBREAK?
the INCUBATION PERIOD is NOT part of the transmission cycle
-- this means that once you get the disease, you immediately show signs and symptoms
How do we determine whether we have a POINT SOURCE or a CONTINUING SOURCE?
plot CASES over TIME
-- cases: y-axis
-- time: x-axis
In a plot of cases over time, what part of the plot indicates the INCUBATION PERIOD?
distance between peaks
What question does INCIDENCE RATE/DENSITY answer?
how long were these people at risk for?
-- acknowledges that not all individuals were at risk for the same amount of time
What is the assumption in CUMULATIVE INCIDENCE?
that the entire population was at risk during the entire period
Top 3 Deaths in US
Stroke or chronic respiratory disease
Relationship between CUMULATIVE INCIDENCE and INCIDENCE RATE/DENSITY
cumulative is an UNDERESTIMATE of incidence rate/density
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