steps in epidemic investigation Prepare for field work Confirm the existence of an outbreak Verify the diagnosis and determine the etiology of the disease. Define the population at risk Develop case definition, start case finding, and collect information on the cases(after choosing study design) Describe person, place and time (by questionnaire) Evaluation of ecological factors Formulate several possible hypothesis hypotheses. Test hypotheses using analytical study Refine hypotheses and carry out additional studies Draw conclusions to explain the causes or determinants of outbreak based on clinical, laboratory, epidemiological & environmental evidence Report and recommend appropriate control measures to concerned authorities at the local/national, and if appropriate at international levels Communication of the findings Follow up of the recommendation to assure implementation of control measures

1. INVESTIGATION OF AN
EPIDEMIC/OUTBREAK
Dr. Yogesh K Singhal, PSM Department,
RNT Medical College, Udaipur (Raj.)
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2. Endemic versus epidemic
 Endemics
 Disease occurring in a population regularly at a usual
level
 Tuberculosis, Malaria
 Epidemics
 Unusual occurrence of the disease in excess of its
normal expectation
 In a geographical location
 At a given point of time
 e.g. Hepatitis E, measles, cholera
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3. Outbreaks versus epidemics
 Occurrence of cases of an illness in excess of expected
numbers
 Scale
 Outbreak
 Limited to a small area, within one district or few blocks
 Epidemic
 Covers larger geographic areas
 Linked to control measures in district/state
 No exact precise threshold: Use a word or the other according to
whether you want to generate or deflect attention
 Be aware of legal implications of the use of the term “Epidemic” in India
(Epidemic disease act, being revised)
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4.  Outbreaks can occur anywhere, from a very remote
area where no health facility exists to nosocomial
outbreaks in a very sophisticated hospitals
 Many of these diseases have seasonal and cyclic
trends which can be discerned through the
surveillance system. These diseases can also cause
outbreaks with the potential to spread rapidly and
cause many deaths.
 Outbreaks cannot always be predicted or prevented,
Outbreaks
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5. Limit the spread of the outbreak by:
 recognition of early warning signals,
 timely investigations and
 application of specific control measures
 An effective surveillance system is essential for
planning, implementation and monitoring of the
diseases.
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6. Epidemiological Triad
Host
Environment Agent
An outbreak comes from a change in the way the host, the
environment and the agent interact: This interaction needs to be
understood to propose recommendations.
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7. Sources of information to detect
outbreaks
 Event-based surveillance
 Rumour register
 To be kept in standardized format in each institution
 Rumours need to be investigated
 Community informants
 Private and public sector
 Media
 Important source of information, not to neglect
 Case-based surveillance
 Review of routine surveillance data and triggers
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8. Information of outbreak:
 Health Personnels
 Nurses and doctors working in any hospital can report the
unexpected number of cases of any particular disease
 Laboratory:
 Every laboratory /network can serve as an excellent source
of outbreak investigation e.g in case of Avian influenza
 Official disease notification system/or surveillance system
 Newspaper or media
 Village health volunteers
 Calls from a Citizen
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9. Early warning signals for an outbreak
 Clustering of cases or deaths
 Increases in cases or deaths
 Single case of disease of epidemic potential
 Acute febrile illness of an unknown etiology
 Two or more linked cases of disease with outbreak potential
 (e.g., Measles, Cholera, Dengue, Japanese encephalitis or plague)
 Unusual isolate (Cholera O 139)
 Shifting in age distribution of cases (Cholera O 139)
 High vector density
 Natural disasters
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10. CHALLENGES (during an epidemic)
 To initiate control measures as quickly as possible
 To study the situation in detail at the same time
 Steps designed to address both the concerns
simultaneously
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11. The first information report
 Filled by the reporting unit
 Submitted to the District Surveillance Officer as
soon as the suspected outbreak is verified
 Sent by the fastest route of information available
 Telephone
 Fax
 E-mail
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12. Investigative team (The rapid response team) includes
 Investigator
 Epidemiologist, clinician and microbiologist, Entomologist when vector-borne disease
 Disease control people e.g sanitary inspector, ANM
 Laboratory technicians
 Specialist in particular areas e.g veterinarian would be very helpful in outbreak of zoonotic
disease
 Public health administration for providing logistic support, mobilizing resources and
providing administrative support.
 Public relation person :In certain conditions when the outbreak has caused panic or gained the
intense attention of public, the investigative team should recruit or appoint a person to be in
charge of public relations and press releases. This person should appropriately reassure and
not unduly alarm the public
 Role
 Confirm and investigate outbreaks
 Responsibility
 Assist in the investigation and response
 Primary responsibility rests with local health staff
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13. Objectives of an outbreak investigation
1. Verify
2. Recognize the magnitude
3. Diagnose the agent
4. Identify the source and mode of transmission
5. Identify population at risk
6. Identify risk factors
7. Formulate prevention and control measures
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14. Investigate an Outbreak......Why?
 Stop the epidemic
 Prevent new episodes
 Increase our knowledge
 Evaluate the surveillance system
 Put in place a surveillance system
 Learn to teach
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15. DR YKS
Yes
-> Control measures
Hypothesis fit the facts:
-> Control measures
Hypothesis does not fit the facts:
-> Analytical investigations
Time, place person description
Formulation of hypothesis
No
-> Clinical, microbiological
and epidemiological investigation
Yes
-> Are the source and
modes of transmission
known?
No
Unusual event:
Is this an outbreak?
Investigating an outbreak
15

16. Investigate an Outbreak
 In investigating an outbreak, speed is essential, but
getting the right answer is essential, too. To satisfy
both requirements, Public Health personnel should
approach investigations systematically, using the
following 14 steps:
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17. Steps in outbreak response
1. Prepare for field work
2. Confirm the existence of an outbreak
3. Verify the diagnosis and determine the etiology of the disease.
4. Define the population at risk
5. Develop case definition, start case finding, and collect information on the cases(after
choosing study design)
6. Describe person, place and time (by questionnaire)
7. Evaluation of ecological factors
8. Formulate several possible hypothesis hypotheses.
9. Test hypotheses using analytical study
10. Refine hypotheses and carry out additional studies
11. Draw conclusions to explain the causes or determinants of outbreak based on clinical,
laboratory, epidemiological & environmental evidence
12. Report and recommend appropriate control measures to concerned authorities at the
local/national, and if appropriate at international levels
13. Communication of the findings
14. Follow up of the recommendation to assure implementation of control measures
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18. 1. Prepare for Field Work
(a) Activities to Begin
• Researching the
disease
• Identifying team
members
• Assign duties
• Determine local
contacts
• Activate department
protocol
(b) Equipment for the
Field
• Laptop
• Cell Phone
• Camera
• Specimen Collection
kits
• interview forms
• Infectious Disease
Manual
(c) Possible Team
Members
• Epidemiologist
• Clinicians
• Lab Personnel
• Sanitarians
• Infection Control
Staff
• Administrators
(large outbreaks)
Preparations can be grouped into three categories:
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19. 2. Confirm the existence of an outbreak:
 When reports of illness begin to surface, it is important to determine if an
outbreak is truly occurring. A true outbreak may be occurring or it may be
periodic and unrelated cases of the same disease or even unrelated cases
of similar, but different diseases.
 Step 1: To decide whether the observed number of cases exceeds the
expected numbers we need to first ddetermine the expected number of
cases before deciding if the observed number exceeds the expected
 Step 2: Collect data of expected values using surveillance records, hospital
discharge data, disease registries, mortality statistics, lab data, other
agencies, health care providers & community surveys.
 Compare the current number of cases with the number from the previous
few weeks or months or from a comparable period during the previous few
years
 Before launching a full investigation, verify signs, symptoms & test
results DR YKS
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20. 3. Verify the diagnosis and determine the etiology
 Verification of diagnosis
 clinical findings
 laboratory results
 Investigator should visit several patients with the disease. In case if a sound
clinical background is not known than a qualified physician can be taken for
the investigation.
 Certain questions can be asked to the patients to reach to a
diagnosis
 What were their exposures before becoming ill?
 What do they think caused their illness?
 Do they know anyone else with the disease?
 Do they have anything in common with others who have the disease?
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21. 4. Define the population at risk
 Defining whole population which is exposed
 Census data
 Local health centre data
 Voter’s list
 Actual counting of the population
 The latest map of the area is obtained with the
important landmarks and divisions
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22. 5. Develop case definition, start case finding, and
collect information on cases
 Confirmed cases– have a positive laboratory result (isolation of the
causative agent or positive serological test).
e.g outbreak of bloody diarrhoea E.coli 0157:H7 is isolated from a stool
culture
 Probable cases – have the typical clinical features of the illness but
without laboratory confirmation.
e.g bloody diarrhoea or hemolytic uremia syndrome without
microbiological confirmation
 Possible case: have fewer or atypical clinical features.
e.g. non-bloody diarrhoea without microbiological confirmation
Continue……………..DR YKS
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23. Steps:
 Active surveillance
-peripheral health personnel,
- personnel from other government departments,
-NGOs and community representatives
 Valuable information can be obtained by contacting
community representatives, especially if the outbreak is focal.
 Passive surveillance
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24. Collect the following type of information about every
case
 Identifying information: name, address, and telephone
number
 Demographic information: age, sex, race, and occupation
 Clinical information:
 Risk factor information:
 Investigation of hepatitis A, information regarding
exposure to food and water
 Reporter information:
 Line listing: Collected information is described on a standard
case report form known as line listing.
In a line listing each column represents an important variable
such as name or identification number ,age ,sex ,case
classification etc. while each row represents a different case
.New cases are added to a line listing as they are identifiedDR YKS
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25.  Do not wait for laboratory results before starting
treatment and control activities
POINT TO
REMEMBER
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26. 6. Describe time, person, place and generate
hypotheses.
 Distribution by time:
 The onset of illness of the cases should be graphed(histogram)
by hours, days, weeks or months. This graph is known as
epidemic curve
Continue……………..
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27. 6. Describe person, place and time and generate hypotheses.
 Epidemic curve
 confirming the existence of an epidemic
 forecasting of the further evolution of the epidemic
 identifying the mode of transmission
 determining the possible period of exposure and/or the
incubation period of the disease under investigation
 identifying outliers in terms of onset of illness, which might
provide important clues as to the source.
Two types of outbreaks
1. Common source 2. Propagated source
Continue……………..
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28. Continue……………..
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29. 1.Common source outbreak: This outbreak occurs when people
gets the infection by exposure to the same source of infection.
These are two types
 A. Point common source: When there is single source that
exists for the short time and all cases have common exposure to
it in that same particular period.
Continue……………..DR YKS29

30.  B. Continuous common source: The epidemic curve shows an
abrupt increase in number of cases but instead of having a
peak decline within incubation period new cases persist for a
longer time with a plateau shape instead of peak before
decreasing.
 If there are many peaks or irregular jagged curves this
suggest an intermittent common source
Continue……………..
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31.  2. Propagated source: This type of outbreak is caused by a
transmission from one person to another person which requires
direct contact such as touching, bite, sexual contact
 Slow increase in number of cases with progressive peaks
approximately one incubation period apart. The span of 1st and
last case will also last longer than several incubation period e.g
HIV
Continue……………..
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32. Distribution by person
 Index case: The first case on epidemic curve is index case. It is
important with the possibility that he/she brought the infection to
the community.
 Outlier case: The case which appears at the beginning and at the
end of the curve are known as outlier case. They provide
information about the source and the way disease is spreading in
that territory.
Continue……………..
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33. DISTRIBUTION BY PLACE
 The investigator can calculate the attack rate of case by
different place. This can be place of residence, place of
work and place of exposure and soon plot a spot map
showing the source of infection or contamination
 If cases are scattered in many place, investigator should
explore the secular pattern of the case over time. This
will indicate about the spread of outbreak from one
area to another area or whether people living in
different place had a common exposure
Continue……………..
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34. Distribution by place
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35. 7. Evaluation of ecological factors
 Ecological factors which have made the epidemic
possible should be investigated such as sanitary
status of eating establishments, water and milk
supply, movement of human population, humidity,
air pollution, population dynamics of insects, animal
reservoirs.
 Relate the disease to environmental factors to know
the source of infection, reservoirs and modes of
transmission.
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36. 8. Generation of hypothesis :
Hypothesis can be generated by 2ways
1)First- Consider what you know about the disease itself?
-What is the agent’s usual reservoir?
-It’s transmission
-What vehicles are commonly implicated?
-What are the know risk factors?
2)Second- talk to few case patients by visiting to their
homes to search for common exposures.
-Local staff community will provide information
Continue……………..
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37. 8. Generation of hypothesis :
 Descriptive epidemiology provides some information
 If the epidemic curve points to a narrow period of
exposure
 What events occurred around that time?
 Why do the people living in a particular area have the
highest attack rates?
 Why are some groups with particular age, sex & other
person characteristics, at greatest risk than other group
with different person characteristics
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38. 9. Test hypotheses using analytical studies
 Hypothesis can be tested by comparing the hypothesis with the
established facts or by using analytical epidemiology to quantify
relationship & explore the role of chance.
 Analytical studies like-cohort &case-control studies.
Do necessary environmental or other studies to supplement the
epidemiological study:
Although an analytical study might be able to confirm the
hypothesis ,the investigator still needs to find environment or
other evidence to support and explain the epidemiological
evidence
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39. 10. Refine Hypotheses and Carry Out Additional
Studies
 When analytic epidemiological studies do not confirm your
hypotheses, you need to reconsider your hypotheses and look for new
vehicles or modes of transmission. This is the time to meet with case-
patients to look for common links and to visit their homes.
 Even when your analytic study identifies an association between an
exposure and a disease, you need to obtain more specific exposure
histories or a more specific control group.
 When an outbreak occurs, whether it is routine or unusual, you should
consider what questions remain unanswered about the disease and
what kind of study you might use in the particular setting to answer
some of these questions. The circumstances may allow you to learn more
about the disease, its modes of transmission, the characteristics of the
agent, and host factors.
DR YKS
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40. 11. Draw conclusions to explain the causes or
determinants of outbreak based on clinical, laboratory,
epidemiological& environmental evidence
 The investigator must identify the cause of outbreak based on the
agreement of following piece of evidence
 Laboratory
 Clinical
 Environmental
 Epidemiological
 The epidemiological evidence found by the descriptive and analytical
study should explain
 Pattern of spread as described by epidemic curve
 Statistical strength of association between exposure an developing the
disease. The dose response relationship which demonstrates high
strength by association when exposure is increased
 Exposure should precede illness DR YKS
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41. 12. Report and recommend appropriate control
measures to concerned authorities at the
local/national/international levels:
 The investigator should timely report the findings to
the responsible individuals at local, national and
international levels, so that appropriate action should
be taken
 Don’t wait the end of the investigation :
— General measures at beginning
— Specific measures according to the results
 Two task should be completed before leaving the field
1) Complete analysis and data interpretation
2) Present main finding with recommendation
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42.  " The art of epidemiological reasoning is to
make some reasonable conclusions starting
from imperfect data”
George VIL Comstock
 But try to have soma data almost perfect. .. it
is easier.
an epidemiologist
DR YKS
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43. 13. Communicate Findings
 Your final task in an investigation is to communicate your findings to others who need to
know. This communication usually takes two forms: 1) an oral briefing for local health
authorities and 2) a written report.
 Your oral briefing should be attended by the local health authorities and people
responsible for implementing control and prevention measures. This presentation is an
opportunity for you to describe what you did, what you found, and what you think should
be done about it. You should present your findings in scientifically objective fashion, and
you should be able to defend your conclusions and recommendations.
 You should also provide a written report that follows the usual scientific format of
introduction, background, methods, results, discussion, and recommendations. By
formally presenting recommendations, the report provides a blueprint for action. It also
serves as a record of performance, a document for potential legal issues, and a reference
if the health department encounters a similar situation in the future. Finally, a report that
finds its way into the public health literature serves the broader purpose of contributing to
the scientific knowledge base of epidemiology and public health.DR YKS
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44. 14. Follow up of the recommendation to assure
implementation of control measures
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45. • Don’t forget ! (last but not least)
 — Ethical aspects, (animaIs ????)
 — Respect the participants (obtain their agreement)
 — Iocal debriefing
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46. Outbreak detection & late Response
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47. Outbreak detection & quick Response:
Importance of timely action
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48. Year 2015 : Outbreaks in Udaipur
 Swine flu : Feb, March, April
 Acute diarrheal cases: April, may, June
 Acute Viral fever: July, August
 Malaria
 Dengue Going on
 Scrub typhus
DR YKS
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49. Exercise 1 :
 During the previous year, nine residents of a
community died from the same type of cancer.
List some reasons that might justify an
investigation.
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50. Exercise Answer 1 :
 One reason to investigate is simply to determine how many cases you would expect in the community. In a
large community, for instance, nine cases of a common cancer (e.g., lung, breast, or colon cancer) would not
be unusual. In a very small community, nine cases of even a common cancer may seem unusual. If the
particular cancer is rare, then nine cases even in a large community may be unusual.
 If the number of cases turns out to be high for that community, we might pursue the investigation further. Our
motive might be research—perhaps we will identify a new risk factor (workers exposed to a particular
chemical) or predisposition (people with a particular genetic marker) for the cancer. Control and prevention
may also be a justification. If we find a risk factor, control and prevention measures could be developed.
Alternatively, if the cancer is generally treatable when found early and a screening test is available, then
we might try to determine not why these people developed the disease, but why they died from it. For
instance, if the problem were cancer of the cervix, detectable by Pap smear and generally treatable if
caught early, we might find (1) problems with access to health care, or (2) physicians not following the
recommendations to screen women at the appropriate intervals, or (3) laboratory error in reading or
reporting the test results. We could then develop measures to correct the problems we found (public
screening clinics, education of physicians, or laboratory quality assurance).
 If new staff need to gain experience on a cluster investigation, training may be a reason to investigate. If
there is public concern, it may generate political pressure. Perhaps one of the people affected is a member
of the mayor's family. A health department must respond to such concerns, but does not usually need to
conduct a full-blown investigation. Finally, legal concerns may prompt an investigation, especially if a
particular site in the community is implicated.
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51. Exercise 2 :
 During August, district health department received
reports of 12 new cases of tuberculosis and 12
new cases of aseptic meningitis. Tuberculosis does
not have a striking seasonal distribution; however,
aseptic meningitis, which is caused primarily by a
viral infection, is highly seasonal and peaks from
August–October. What additional information is
needed to determine whether either of these
groups of cases is an outbreak?
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52. Exercise Answer 2
 We need to know how many cases of each of these diseases
usually occurs in this area during August. Because tuberculosis is
not seasonal, the number of cases could be compared with (a)
the numbers reported during the preceding several months and
(b) the numbers reported during August of the preceding few
years.
 However, since aseptic meningitis is seasonal and peaks from
August–October, the number of cases during August is
expected to be higher than the number reported during the
preceding several months, so you would need to compare with
the numbers reported during August of the preceding few
years.
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53. Exercise 3 : Review the six case report forms in the Appendix
and create a line listing based on the information.
Exercise Answer 4
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54. Exercise 4 :
 You are called to help investigate a cluster of
17 men who developed leukemia in a
community. Some of them worked as electrical
repair men, and others were ham radio
operators. Which study design would you
choose to investigate a possible association
between exposure to electromagnetic fields
and leukemia?
 OPTION (A) case control (B) cohort
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55. Exercise Answer 4
 Because the total population at risk is not well defined,
you would use a case-control study. You would begin by
enrolling the 17 people already identified with leukemia
as the case group. You would also need to determine
what group might serve as an appropriate comparison,
or control, group. Neighbours might be used for the
control group, for example. In your case-control study,
you would determine whether each case-patient and
each control had been exposed to electromagnetic
fields (however you defined that exposure). Finally, you
would compare the exposures of case-patients and
controls.
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56. Exercise 5 :
The manager of a grocery store has reported a rash
illness among the store’s workers. What type of study would
you use to determine the source of the outbreak? Why?
What is the appropriate measure of association? After
reviewing the table showing the data on exposure to celery
for these workers, calculate the measure of association and
interpret your results.
Rash No Rash
Exposed to celery(56) 25 31
Not exposed to celery(70) 5 65
DR YKS
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57. Exercise Answer 5
 You would use a cohort study because the outbreak is small and confined. The
appropriate measure of association for a cohort study is relative risk, which is
calculated in this case as the attack rate for workers exposed to celery divided
by the attack rate for those who were not exposed.
 The attack rate for exposed workers is 25 / 56, or 44.6%. The attack rate for
workers who were not exposed is 5 / 70, or 7.1%. Thus, the relative risk for
exposure to celery is 44.6 / 7.1, or 6.3. This means that workers who were
exposed to celery were 6.3 times more likely to develop the rash illness than
those who were not exposed, and it is therefore likely that celery was the source
of the outbreak. However, before you could draw this conclusion, you would
need to compare the relative risk for celery with that for other vegetables and
fruits to see if the implication is stronger for any of them.
 Then, to test the likelihood of your findings, you would need to calculate a test
of statistical significance such as chi-square for the item with the highest relative
risk and look up the corresponding p-value in a table of p-values. If the p-value
was below .05, your findings would be considered statistically significant.
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58. DR YKS
Take home messages
1. Do not ignore or hide outbreaks
2. Respond to early warning signals
3. Investigate outbreaks to:
 Control the current outbreak
 Learn about the disease in the local setting
4. Outbreaks require an organized, well managed
response
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59. Thanks Thanks Thanks Thanks
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ks Thanks Thanks
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s
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Thanks Thanks Thanks Thanks Than
ks Thanks Thanks
DR YKS
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