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The Principles of Outbreak Epidemiology. By Dr Abhinav Sinha MBBS, MD. Author. Currently doing MD in Community Medicine at the Department of Community Medicine, NSCB Medical College, Jabalpur, India.

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The principles of outbreak epidemiology

The Principles of Outbreak Epidemiology


Dr Abhinav Sinha MBBS, MD



Currently doing MD in Community Medicine at the Department of Community Medicine, NSCB Medical College, Jabalpur, India.

Passed MBBS from the same institute with Gold Medals in Human Physiology and Biochemistry in 2000.

Email- [email protected]

Interest motivation

Interest & Motivation

During the last two and a half years of my postgraduation, I had an opportunity to witness and investigate 3 or 4 of different kinds of outbreaks ranging from Food poisoning to Gastrointestinal to Pyrexia of Unknown Origin to Cholera. What aroused an interest in Outbreak Investigation was the scientific cum artistic way of handling such outbreaks. It is more of an art than a pure science. When I came to know about the Supercourse, I decided to contribute or rather share my little experience with the world in the field of Outbreak Epidemiology. I hope you will enjoy this so called “lecture”.

Learning objectives

Learning Objectives

The main motives behind this lecture are to develop the basic concept in investigating an epidemic, the need to recognize the urgency behind it and to orient the students toward the art and science of outbreak investigation and epidemiology.

Performance objectives

Performance Objectives

After going through this lecture, the students should be able to perform the initial investigation of many smaller outbreaks that occur so much frequently in their vicinity that may often pass unrecognized. Also, they should be able to differentiate the epidemic and the endemic fluctuations in the frequencies of a disease.



Outbreak: Sudden occurrence of an epidemic in relatively limited geographic area. While an outbreak is usually limited to a small focal area, an epidemic covers larger geographical areas & has more than one focal point.

Outbreak Epidemiology: Study of a disease cluster or epidemic in order to control or prevent further spread of the disease in the population.

Field epidemiology

Field Epidemiology

A definition has been proposed by

Goodman. The essential elements are:

The problem is unexpected

An immediate response may be necessary

Epidemiologists must travel to & work on location in the field

The extent of investigation is likely to be limited because of imperative for timely intervention

Objectives of oi

Objectives of OI

Primary- to control the spread of disease

To determine the causes of disease, its source & mode of transmission

To determine who is at risk

To determine what exposures predispose to disease

To know magnitude of the problem

Objectives continued

Objectives continued….

6. To identify new agent

7. To determine the effectiveness of control measures

8. To identify methods for present & future prevention & control

9. Research & training opportunities

10. Public, Political and legal concerns

Unique aspects of oi

Unique aspects of OI

There is a pressure & urgency to conclude the investigations quickly which may lead to hasty decisions.

Data sources are often incomplete & less accurate.

Decreased statistical power due to analysis of small numbers.

Publicity surrounding the investigation – community members may have preconceived ideas.

The pace commitment of oi

The pace & commitment of OI

There is often a strong tendency to collect what is “essential” in the field & then retreat to “home” for analysis. Such premature departure reflects lack of concern by the public, makes any further data collection or direct contact with the study population difficult. Once home, the team loses the urgency & momentum & the sense of relevancy of the epidemic. Don’t leave the field without final results & recommendations.

Trigger events warning signals

Trigger events & Warning Signals

Clustering of cases/deaths in time/space

Unusual increase in cases/deaths

Shift in age distribution of cases

High vector density

Acute hemorrhagic fever or acute fever with renal involvement/altered sensorium

Severe dehydration following diarrhea in patients above 5 years age

Unusual isolate

Diseases requiring investigations

Diseases requiring investigations

Endemic diseases with epidemic potential – malaria, cholera, measles, hepatitis, meningococcal meningitis

Even a single case of diseases for which eradication/elimination goals have been set – polio, guineaworm and yaws

Rare but internationally important diseases with high case fatality rates – yellow fever

Outbreaks of unknown etiology

General lines of action

General lines of action

The basic general lines of action during epidemics include – Preparedness and Interventions (investigations). Success in dealing with an epidemic depends largely on the state of preparedness achieved in advance of any action. It would be an error to consider as an epidemic, a hitherto unrecognized endemic situation or a mere seasonal increase in the incidence of a disease.



Identify a nodal officer at state/district level

Strengthen routine surveillance system

Constitute rapid response teams

Train medical & other health personnel

Prepare a list of laboratories

List the “high risk” pockets

Establish rapid communication network

Undertake IEC activities

Ensure availability of essential supplies

Setup inter-departmental committees



Recognition & response to a request for assistance

Check initial information

Formulate a plan of action

Prepare for field work

Confirm the existence of epidemic

Verify the diagnosis

Identify & count cases/exposed persons

Orient data in terms of person, place & time

Investigations continued

Investigations continued…..

Choose a study design

Collect specimens for lab analysis

Conduct environmental investigations

Formulate & test hypotheses

Implement control measures

Conduct additional studies

Prepare a written report

Communicate the findings

Step 1 recognition response

Step 1 – Recognition & Response

If the local health officials request assistance, the regional epidemiologist should try to acquire as much information about the disease and the population at risk as possible. It is also important to find out why the request is coming – need extra hands?, unable to uncover the details of the disease in question?, share the responsibility?, or legal or ethical issues?

Step 2 – Check initial information: As soon as the initial information on an outbreak reaches, the regional health coordinator must determine whether the information is correct.

Step 3 – Formulate a plan of action: The plan should be based on situational analysis & taking technical, economical & political factors into account.

Step 4 – Prepare for field work: Identify the team members & assign responsibilities.

Step 5 confirm the existence

Step 5 – Confirm the existence

Are there cases in excess of the baseline rate for that disease & setting? The excess frequency should be found out with Epidemic Threshold Curve. The periodic frequency for previous 3 years is plotted on a graph. Another graph at mean + 2SD level is superimposed on it. Any fluctuations beyond these 2 graphs should be treated as epidemic fluctuations (method of moving averages).

Step 6 verify the diagnosis

Step 6 – Verify the diagnosis

The initial report may be spurious & arise from misinterpretation of the clinical features. This involves a review of available clinical & lab findings that supports the diagnosis. Do not apply newly introduced, experimental or otherwise not broadly recognized confirmatory tests at this stage. 15-20% of the suspected cases may be lab confirmed.

Step 7 identify count cases

Step 7 - Identify & count cases

Identify additional cases not known or reported initially. The case definition must be precise but not too exclusive.. Persons who meet the case definition should be “line-listed”. Also, identify the population at risk or the exposed persons, places where the cases live, work & have traveled to, & the possible exposures that might have lead to the disease.

Search for the source of infection The main purpose here is to eliminate, terminate or isolate the source. The steps involved are – identify the time of disease onset, ascertain the range of incubation periods & look for the source in time interval between the maximum & the minimum IPs. In outbreaks with person-to-person transmission, all the contacts of the index case are to be searched (contact tracing).

Step 8 compile orient data

Step 8 – Compile & Orient data

Identify when patients became ill (time), where patients became ill (place) & what characteristics the patients possess (person). The earlier one can develop such ideas, the more pertinent & accurate data one can collect.

(a) Time: The epi-curvegives the magnitude of outbreak, its mode of spread & the possible duration of the epidemic. The unit of time on X-axis are smaller than the expected incubation period of the disease.

(b) Place: It provides major clues regarding the source of agent and/or nature of exposure. Spot maps show a pattern of distribution of cases.

(c) Person: Examine characters such as age, sex, race, occupation or virtually any other character that may be useful in portraying the uniqueness of case population.

Step 9 choose a study design

Step 9 – Choose a Study Design

The design (Case-control, Cohort & Case-cohort) is chosen based on size & availability of the exposed population, the speed with which the results are needed & the available resources. The study design that is chosen will then dictate the appropriate analysis & hypothesis testing.

Step 10 perform lab analysis

Step 10 – Perform Lab analysis

It consists of collecting & testing appropriate specimens. To identify the etiologic agent, the collection need to be properly timed. Examples of specimens include - food & water, other environmental samples (air settling plates), and clinical (blood, stool, sputum or wound) samples from cases & controls.

Step 11 environmental investigation

Step 11 – Environmental Investigation

A study of environmental conditions & the dynamics of its interaction with the population & etiologic agents will help to formulate the hypothesis on the genesis of the epidemic. Such actions assist in answering How? And Why? questions.

Step 12 formulate test hypothesis

Step 12 – Formulate & Test Hypothesis

As soon as the preliminary data indicate the magnitude & severity of the outbreak, a hypothesis should be made regarding time, place and person; the suspected etiological agent & the mode of transmission. Risk specific attack rates are calculated & compared & relative risk/odds ratio is calculated.

Important points

Important points

1. Rare disease assumption: The OR & RR approximate each other if the attack rates is less than 5% but the attack rates are much higher in outbreaks.

2. To correct for multiple comparisons, the most effective approach is to lower the p-value according to the number of comparisons made.

Step 13 control measures

Step 13 – Control measures

Simultaneous to data collection & hypothesis formation, steps should be taken to contain the epidemic. These measures depend upon knowledge of etiologic agent, mode of transmission & other contributing factors. Protective measures are necessary for patients (isolation & disinfection), their contacts (quarantine) and the community (immunization, etc).

Step 14 additional studies

Step 14 – Additional studies

Because there may be a need to find more patients, to define better the extent of the epidemic, or because a new lab method or case finding method may need to be evaluated, the epidemiologists may want to perform more detailed & carefully executed studies.

Step 15 prepare written report

Step 15 – Prepare Written Report

The final responsibility of the investigative team is to prepare a written report to document the investigations, findings and the recommendations. The written report should be submitted, in a standardized format, to the public health authorities including the ministry of health & remain confidential until it has been given official permission.

Step 16 communicate findings

Step 16 – Communicate findings

Communicating the investigative findings clearly is essential. All public health officers will benefit if the experience acquired by the investigative team is shared by the publication of an account of the outbreak. As a rule, the epidemiologist informs those who reported the first cases of the epidemic first.

Step 17 post epidemic measures

Step 17 – Post-epidemic Measures

The efficacy of control measures should be assessed day by day during the outbreak, a final assessment being made after it has ended. This will provide a logical basis for post-epidemic surveillance & preventive measures aimed at avoiding the repetition of similar outbreaks.



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