Explains the steps of investigation of an epidemic.

1. By Dr. Devyani Wanjari
Junior Resident,
Department of Community Medicine,
Dr. Sushila Nayar School of Public Health,
MGIMS, Sevagram, Wardha, Maharashtra, India.

2.  Definitions
 Deciding whether to investigate a possible outbreak
 Significance in public health
 10 Steps of an Outbreak Investigation
 Recommended Preparatory Action
 Roles of NGOs
 Recent Outbreaks
 Conclusion and the way ahead

3.  Endemic: The habitual presence of a
disease within a geographic area.
 Epidemic: The occurrence in a community
or a region of a group of illness of similar
nature clearly in excess of normal
expectancy and derived from a common or
a propagated source.
 Pandemic: Worldwide epidemic.
Source: Gordis Epidemiology

4.  Outbreak: Epidemic limited to
localized increase in the incidence of
a disease, e.g., village, town, or
closed institution.
 Cluster: Aggregation of cases in a
given area over a particular period
without regard to whether the
number of cases is more than
expected.
 Source: CDC investigating an outbreak

5.  Susceptible: At risk of disease
 Immune: Not at risk of disease because:
i. Had disease previously
ii. Have been immunized
 Herd Immunity: defined as the resistance
of a group of people to an attack by a
disease to which large proportion of the
members of the group are immune.

6.  Factors related to the problem itself include:
i. Severity of the illness (serious illness with high risk of hospitalization,
complications, or death)
ii. Number of cases
iii. Source
iv. Mode or ease of transmission
v. control measures
 Example: A single case of gastroenteritis and a single case of botulism.

7. 1. To gain knowledge so as to take urgent steps to contain and control the epidemic
and prevent a recurrence:
a. Define magnitude
b. Particular conditions and factor responsible
c. Cause, Source of infection, modes of transmission
2. Helps to take measures to prevent epidemics elsewhere.

8.  Once the decision has been made, working quickly is essential — as is getting the right
answer. Hence it should be “Quick and Clean” and not “Quick and Dirty”.
 Descriptive steps:
1. Is it an outbreak?
2. Confirm the diagnosis
3. Define, find, count the cases
4. Perform descriptive epidemiology

9.  Analyze:
5. Generate the hypothesis
6. Test the hypothesis
7. Reconsider, refine, and re-evaluate hypotheses
8. Conduct complementary studies
 Synthesis and action:
9. Write a report, communicate
10. Control measure and prevention

10. 1. Scientific and investigative issues:
 must have the appropriate scientific
knowledge, supplies, and equipment to
carry out the investigation.
 consult laboratory staff to ensure that
you take the proper laboratory
material and know the proper
collection, storage, and transportation
techniques.
 what supplies or equipment to bring to
protect yourself.
 plan of action.
2. Management and operational issues:
 A good field investigator must be a
good manager and collaborator as well
as a good epidemiologist.
 Depending on the type of outbreak, the
number of involved agencies may be
quite large and may vary.
 A communications plan must be
established.
 Operational and logistical details are
important.

11.  The excess may not necessarily indicate an outbreak. Look for other factors like:
1. Seasonal variation
2. Notification variation
3. Diagnostic variation (new technique)
4. Diagnostic mistake (false epidemic)

12. 1. Define the “Numerator” (Cases):
a) Clinical features: Is the disease known?
b) What are its serologic or cultural aspects?
c) Are the causes partially understood?
2. Define the “Denominator”: What is the population at risk of developing disease
(i.e. susceptible)?
3. Determine whether the observed number of cases clearly exceeds the expected
number.
4. Calculate the attack rates.

13.  Attack Rate: is defined as-
=
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑎𝑡 𝑟𝑖𝑠𝑘 𝑖𝑛 𝑤ℎ𝑜𝑚 𝑎 𝑐𝑒𝑟𝑡𝑎𝑖𝑛 𝑖𝑙𝑙𝑛𝑒𝑠𝑠 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑠
𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑎𝑡 𝑟𝑖𝑠𝑘
X 100
 The time period is implicit in the attack rate based on about how long after an
exposure most cases develop.
 Primary case: A person who acquires the disease from the exposure.
 Secondary Attack Rate: is defined as the attack rate in susceptible people who were
not exposed to suspected agent who have been exposed to a primary case.
 It is a good measure of person-to-person spread.

14.  Verifying the diagnosis is important:
a) to ensure that the disease has been properly identified, since control measures
are often disease-specific.
b) to rule out laboratory error as the basis for the increase in reported cases.
 Simultaneous occurrence of different conditions can also be confusing.
 To ensure this:
1. Review the clinical findings and laboratory results
2. Visit one or more patients with the disease to verify the diagnosis
3. Summarize the clinical features using frequency distributions

15. A. Define:
 Case : is a standard set of criteria for deciding whether an individual should be
classified as having the health condition of interest.
 A case definition includes clinical criteria restricted by time, place, and person.
 The case definition must not include the exposure or risk factor you are interested
in evaluating.
 The clinical criteria should be based on simple and objective measures.
 Eg. Meningococcal disease:
An illness with sudden onset of fever (>38.5°C rectal or >38.0°C axillary) and one or
more of the following: neck stiffness, altered consciousness, other meningeal sign or
petechial or puerperal rash.

16. Case Categories
1.Suspected
has fewer of the typical
clinical features
2.Probable
has typical clinical
features of the disease
without laboratory
confirmation
3.Confirmed
must have laboratory
verification

17. B) Find and count:
 Surveillance is the systematic collection, analysis and dissemination of health data
for the planning, implementation and evaluation of public health programs.
1. Passive Surveillance: Diseases are reported by health care providers
2. Active Surveillance: Health agencies contact health providers seeking reports
 Line listing of the collected information
Surveillance
Passive Active

18.  The process, in which the outbreak is characterized by time, place, and person, is
called Descriptive Epidemiology.
 Importance of the step:
i. Provides a comprehensive characterization of the outbreak
ii. Can identify or infer the population at risk
iii. Helps in formation of testable hypotheses
iv. Allowing you to begin intervention and prevention measures
v. Enabling you to identify and correct errors and missing values

19.  Traditionally, a histogram is used to
depict the time course of an epidemic.
 If we draw a line connecting the tops
of the bars, it is called the Epidemic
Curve or Epi Curve.
 It helps to find out:
i. Magnitude of the epidemic
ii. Pattern of spread
iii. Where you are in the course , trends
iv. For evaluation
v. Outliers- Important clues
vi. A probable time of exposure
Source: Gordis Epidemiology

20. When?
 3 critical variables:
1. When did the exposure take place?
2. When did the disease begin?
3. What was the incubation period for the
disease?
 If we know any 2 of these, we can
calculate third.
 Incubation Period: The interval from the
point of infection to the onset of each
case.

21.  Interpreting an Epidemic Curve:
1. Point-source epidemic: An epidemic curve
that has a steep upslope and a more gradual
down slope.
2. Continuous common-source epidemic: In a
point-source epidemic, if the duration of
exposure is prolonged all the cases occur
within one incubation period.
3. Intermittent common-source epidemic:
epidemic curve reflecting the intermittence
and duration of exposure and the number of
persons exposed.
4. Propagated epidemic: one spread from
person-to-person with increasing numbers of
cases in each generation.
A single peak
Plateau
Progressively taller
peaks
Irregularly jagged
curve

22. Where?
 Provides information on:
1. Geographic extent of a problem
2. Demonstrate clusters or patterns
that provide important etiologic
clues.
 Spot Map: illustrating where cases
live, work, or may have been
exposed.
 Drawback: It do not take the size of
the underlying population into
account. Spot map of deaths from cholera in Golden Square area,
London, 1854 (redrawn from original)
Source: CDC Investigating an outbreak

23.  Solution-
Area Map: Shows area specific rates.
 Helps to compare incidence between
different areas with different
population densities.
COVID-19 cases per 100,000 people by state, as of April 23.
Source: www.mygov.in/covid-19

24. Who?
 provides a description of whom the case-patients are and who is at risk.
 Includes:
1. Host characteristics (age, race, sex, and medical status)
2. Possible exposures (occupation, leisure activities, and use of medications,
tobacco, and drugs)
 Eg. AIDS, Hepatitis B.
D) Summarizing by time, place, and person.

25.  The hypotheses should be testable, since evaluating hypotheses is the next step in
the investigation.
 “Round up the usual suspects.”
 In an outbreak context, hypotheses are generated in a variety of ways:
i. What you know
ii. Case-patients
iii. The local staff
iv. The descriptive epidemiology
v. When the epidemiology does not fit the natural pattern, think unnatural, i.e.,
intentional- Bioterrorism.

26.  hypotheses in a field investigation are evaluated using a combination of
environmental evidence, laboratory science, and epidemiology.
 Hypotheses are evaluated in one of two ways:
1. by comparing with the established facts
2. by analytic epidemiology to assess causal relationships
Analytic Epidemiology
Case Control Study
Odd’s Ratio
Retrospective Cohort Study
Relative Risk and
Attributable Risk
2 by 2
table

27.  Factors that should also be considered while evaluating:
1. Testing statistical significance
2. Consistency with other studies
3. Temporality
4. Biological plausibility

28.  When analytic epidemiology is unrevealing, rethink your hypotheses.
 Even when an analytic study identifies an association between an exposure and
disease, the hypothesis may need to be honed.
 Sometimes a more specific control group is needed to test a more specific
hypothesis.
 One reason to investigate outbreaks is research: “Experiment of nature”

29.  While epidemiology can implicate vehicles and guide appropriate public health
action, laboratory evidence can confirm the findings.
 Environmental studies are often helpful in explaining why an outbreak occurred.
 Eg. Outbreak of E. coli O157:H7

30.  The final task is to summarize the investigation, its findings, and its outcome in a
report, and to communicate this report in an effective manner.
 two forms:
1. An oral briefing for local authorities: Oral briefing should be attended by the
local health authorities and persons responsible for implementing control and
prevention measures- Easy to understand.
2. A written report:
 follows the usual scientific format of introduction, background, methods, results,
discussion, and recommendations.
 a blueprint for action
 a document for potential legal issues
 a reference for future

31.  In most outbreak investigations, the primary goal is control of the outbreak and prevention
of additional cases.
 Confidentiality is an important issue in implementing control measures.
Eg. TB patient- loss of both patient as well as health care worker
 Control measures are usually directed against one or more segments in the chain of
transmission (agent, source, mode of transmission, portal of entry, or host) that are
susceptible to intervention.
 Some interventions aim to increase a host’s defenses. Eg. Vaccination.
 Initiate or maintain surveillance.

32.  Strengthen routine surveillance system
 Identify a nodal officer at the state and district levels
 Train medical and other health personnel
 List and prepare laboratories and hospitals
 List high risk pockets
 Establish a rapid communication network
 Ensure availability of essential supplies
 Set up an inter-departmental committee, including NGOs for inter-sectoral co-
ordination: Task Force

33. Task Force:
 Should be established by the Prime minister.
 a task force is established with the best brains – experienced and competent.
 The task force members should be dedicated full-time to this one epidemic, work
full-time and make daily decisions.
 Functions:
1. Must guide immediate crisis management
2. Design short-, medium- and long-term strategies for control of infection and
community outbreaks
3. Disseminate authentic information on outbreaks
4. Squash fake news

34.  Plays crucial roles in various forms of emergencies.
 May be International or Local.
 Strengths:
1. Ability to respond fast
2. Flexibility
3. Creating Public Awareness
4. Great knowledge of local situations (Local NGOs)

35.  2019- Covid 19
 2015- Indian Swine Flu Outbreak
 2014- Ebola in West Africa
 2014- Encephalitis in India
 2012- MERS (Middle East Respiratory
Syndrome) in the Middle East
 2009/10- Swine Flu (H1N1) worldwide
 Epidemics and pandemics have killed
people far more than wars!

36.  Gathering of real-time information for immediate action, and not merely summary
reports on statistics, is the function of true public health surveillance.
 Opportunities to gain new knowledge of diseases and to discover weaknesses of the
current public health practices and systems.
 Co-ordination between different sectors is important.
 Circulation of report is necessary to spread the information so that no such episodes
occur in other areas in future.

37.  Health management must be supported by research, which is both curiosity-based
and problem-solving.
 a health management system is complete only when both arms of modern medicine
– universal healthcare and public health – are functional.
 India’s healthcare: We need to work together to make it-
1. Not patchy and improperly distributed but universal
2. Not based on only reactive responses but with task force in charge and national
strategy
3. With functional public health and public health surveillance
4. With strengthening of healthcare services in all districts

38. -Rudyard Kipling
Source: Gordis Epidemiology 6th Edition

39.  CDC Investigating an Outbreak
https://www.cdc.gov/csels/dsepd/ss1978/lesson6/section2.html
 Gordis Epidemiology: 6th Edition
 Sathe’s Epidemiology and management for Health Care: 5th Edition
 Park
 Oxford book of public health
 www.mygov.in/covid-19
 https://taskforce.org/outbreak-response/
 https://www.nhp.gov.in/outbreak-investigation_pg
 https://www.physio-pedia.com/Endemics,_Epidemics_and_Pandemics
 https://science.thewire.in/health/coronavirus-covid-19-community-transmission-
national-task-force-influenza-endemic-pandemic/