Epidemiology

Prepared by: Ms. Abeer M. Alshamrni

Modified by: Ms. Abeer Ibrahim
Unit One: Introduction to Epidemiology
 Outline
Unit one ; Introduction to epidemiology

• Definition

• History of epidemiology

• Uses and applications

• The scope of epidemiology

• Descriptive epidemiology

• Analytic epidemiology

• Types of epidemiologic studies (Observational Vs Experimental)

• Case study (calculation of relative risk factors and Odds ratios)

 Objectives
At the end of this unit student will be able to:
• Define epidemiology

• Summarize the historical evolution of epidemiology

• Name some of the key uses of epidemiology

• Identify the core epidemiology functions

• Describe primary applications of epidemiology in public health practice

 Objectives
• List the key features and uses of descriptive epidemiology

• List the key features and uses of analytic epidemiology

• List the three components of the epidemiologic triad

• Describe the different modes of transmission of communicable

disease in a population
History of Epidemiology
Miasmas
john snow / Cholera
outbreak
William Farr

The father of modern vital records, was

another important figure in the
development of epidemiology.

Established the first registry of births and

deaths in the 1830s.

distributing reports that led to public

health interventions.
 Modern Epidemiology

The modern approach in describing an infectious

disease focuses on:

the interaction of the disease agent, the host, and

the environment.
Definition
 Definition
• Epidemiology
• Epi = upon
• Demos =population
• Logos = study of

• Epidemiology is: the study of the distribution and determinants of

health-related states or events in specified populations, and the
application of this study to the control of health problems.

• Key words: Distribution, determinants, health-related states/event,

specified population , application.
Distribution
 Distribution
• Epidemiology is concerned with the frequency and pattern of health events in
a population:

• Frequency; refers not only to the number of health events such as the number
of cases of meningitis or diabetes in a population but also to the relationship of
that number to the size of the population. The resulting rate allows
epidemiologists to compare disease occurrence across different populations.

• Pattern refers to the occurrence of health-related events by time, place, and

person. Time patterns may be annual, seasonal, weekly. Place patterns
include geographic variation, urban/rural differences, and the location of work
sites or schools. Personal characteristics include demographic factors which
may be related to risk of illness, injury, or disability such as age, sex.
 Determinate
• Determinants are the causes and other factors that influence
the occurrence of disease and other health-related events.
Epidemiologists assume that illness does not occur randomly in
a population, but happens only when the right accumulation of
risk factors or determinants exists in an individual.
 Health-related states or
events
Epidemiology was originally focused 3exclusively on
epidemics of communicable diseases but was
subsequently expanded to address endemic
communicable diseases and non-communicable
infectious diseases.
 Specified populations

• The clinician is concerned about the health of an individual; the

epidemiologist is concerned about the collective health of the people in
a community or population. In other words, the clinician’s “patient” is the
individual; the epidemiologist’s “patient” is the community.

• For example, when a patient with diarrheal disease presents, both are
interested in establishing the correct diagnosis. However, while the
clinician usually focuses on treating and caring for the individual, the
epidemiologist focuses on identifying the exposure or source that caused
the illness; the number of other persons who may have been similarly
exposed; the potential for further spread in the community; and
interventions to prevent additional cases or recurrences.
 Application
• Epidemiology is not just “the study of” health in a
population; it also involves applying the knowledge
gained by the studies to community-based practice.

• the epidemiologist uses the scientific methods of

descriptive and analytic epidemiology as well as
experience, epidemiologic judgment, and understanding
of local conditions in “diagnosing” the health of a
community and proposing appropriate, practical, and
acceptable public health interventions to control and
prevent disease in the community.
Uses & Applications
 1. Assessing the community’s
health

• Public health officials responsible for policy development, implementation, and

evaluation use epidemiologic information as a factual framework for decision-
making. To assess the health of a population or community, relevant sources of data
must be identified and analyzed by person, place, and time (descriptive
epidemiology). for example;

• What are the actual and potential health problems in the community?

• Where are they occurring?

• Which populations are at increased risk?

• More detailed data may need to be collected and analyzed to determine whether
health services are available, accessible, effective, and efficient.
 2. Making individual decisions

• Many individuals may not realize that they use epidemiologic information
to make daily decisions affecting their health.

• When persons decide to quit smoking, climb the stairs rather than wait
for an elevator, eat a salad rather than a cheeseburger with fries for
lunch, they may be influenced, consciously or unconsciously, by
epidemiologists’ assessment of risk.

• In the 1950s, epidemiologists reported the increased risk of lung cancer

among smokers. In the 1970s, epidemiologists documented the role of
exercise and proper diet in reducing the risk of heart disease.
 3. Completing the clinical picture

• When investigating a disease outbreak, epidemiologists rely on health-

care providers and laboratorians to establish the proper diagnosis of
individual patients. But epidemiologists also contribute to physicians’
understanding of the clinical picture and natural history of disease.

• For example, in late 1989, a physician saw three patients with

unexplained eosinophilia (an increase in the number of a specific type of
white blood cell called an eosinophil) and myalgias (severe muscle
pains). Although the physician could not make a definitive diagnosis, he
notified public health authorities. Within weeks, epidemiologists had
identified enough other cases to characterize the spectrum and course of
the illness that came to be known as eosinophilia-myalgia syndrome.
 4. Searching for causes

Much epidemiologic research is devoted to searching for causal

factors that influence one’s risk of disease. Ideally, the goal is
to identify a cause so that appropriate public health action
might be taken
Scope of Epidemiology
 Core of Epidemiologic Functions

• In the mid-1980s, five major tasks

of epidemiology in public health
practice were identified:

• public health surveillance,

• field investigation,
• analytic studies,
• evaluation,
• and linkages.
• A sixth task, policy development,
was recently added.
 1. Publics Health Surveillance

• Public health surveillance is the ongoing, systematic collection,

analysis, interpretation, and dissemination of health data to help
guide public health decision-making and action. Surveillance is
equivalent to monitoring the pulse of the community.

• The purpose of public health surveillance,

18 which is sometimes
called “information for action,” is to portray the ongoing patterns of
disease occurrence and disease potential so that investigation,
control, and prevention measures can be applied efficiently and
effectively.

• Reports like; Morbidity & Mortality

• Reported by : healthcare providers, infection control practitioners, or
laboratories
 2. Field investigation

• surveillance provides information for action. One of the first

actions that result from a surveillance case report or report
of a cluster is: investigation by the public health
department.

• The investigation may be as limited as a phone call to the

health- care provider to confirm or clarify the circumstances
of the reported case, or it may involve a field investigation
requiring the coordinated efforts of dozens of people to
characterize the extent of an epidemic and to identify its
cause.
 Field investigation

• The objectives of such investigations also vary. Investigations

often lead to the identification of additional unreported or
unrecognized ill persons who might otherwise continue to
spread infection to others.

• For example, one of the hallmarks of investigations of

persons with sexually transmitted disease is the identification
of sexual partners or contacts of patients. When interviewed,
many of these contacts are found to be infected without
knowing it, and are given treatment they did not realize they
needed
 3. Analytic studies

• Surveillance and field investigations are usually

sufficient to identify causes, modes of transmission,
and appropriate control and prevention measures.

• it involves the study of disease incidence and

distribution by time, place, and person. It includes
the calculation of rates and identification of parts of
the population at higher risk than others.
 4. Evaluation

• Evaluation is the process of determining, as systematically and

objectively as possible, the relevance, effectiveness, efficiency, and
impact of activities with respect to established goals.

• Evaluation of an immunization program, for example, might assess

the efficiency of the operations, the proportion of the target
population immunized, and the apparent impact of the program on
the incidence of vaccine-preventable diseases
 5. Linkages

• Epidemiologists working in public health settings rarely act

in isolation. In fact, field epidemiology is often said to be a
“team sport.” During an investigation an epidemiologist
usually participates as either a member or the leader of a
multidisciplinary team.

• Other team members may be laboratorians, sanitarians,

infection control personnel, nurses or other clinical staff,
and, increasingly, computer information specialists.
 6. Policy development

• Epidemiologists who understand a problem and the population in

which it occurs are often in a uniquely qualified position to
recommend appropriate interventions.

• As a result, epidemiologists working in public health regularly

provide input, testimony, and recommendations regarding disease
control strategies, reportable disease regulations, and health-care
policy.
 The Epidemiological Approach

• The practice of epidemiology relies on a systematic approach. In very

simple terms, the epidemiologist:

• Counts cases or health events, and describes them in terms of time,

place, and person;

• Divides the number of cases by an appropriate denominator to

calculate rates; and

• Compares these rates over time or for different groups of people.

Descriptive Epidemiology
• Three essentials characteristics of the
disease that we look for in descriptive
studies

• Person

• Place

• Time
 Person
• Age, gender, ethnic group

• Genetic predisposition

• Concurrent disease, diet, physical activity, smoking

• Risk taking behavior

• Education, occupation
• In Figure 1.15, infant mortality rates for 2002 are shown by race
and Hispanic origin of the mother.
 Place
• presence of agents or vectors

• climate

• geology

• population density

• economic development nutritional practices,medical practices

• place data can be shown in a table such as Table 1.4, a map provides a
more striking visual display of place data. On a map, different numbers or
rates of disease can be depicted using different shadings, colors, or line
patterns, as in Figure 1.11.
 Time
• Calendar time

• Time since an event

• Physiologic cycles

• Age (time since birth)

• Seasonality

• Temporal trends
• Time data are usually displayed with a two-dimensional graph. The
vertical or y-axis usually shows the number or rate of cases; the
horizontal or x-axis shows the time periods such as years, months, or
days. The number or rate of cases is plotted over time. Graphs of
disease occurrence over time are usually plotted as histograms (Figure
1.5).
Analytical Epidemiology
Analytical epidemiology studies require information to ...

• know where to look

• know what to control for

• develop viable hypotheses

 The Epidemiologic Triangle: three characteristics
that are examined to study the cause(s) for disease
in analytic epidemiology
• Host

• Agent

• Environment
 Epidemiological
Triangle
Host
personal traits
behaviors
genetic
predisposition
immunologic factors
 Epidemiological
Triangle
• Agents
biological
physical
chemical

influence the chance

for disease or its severity
 Epidemiological
Triangle
• Environment

external conditions
physical/biological/s
ocial

contribute to the
disease process
Types of epidemiological studies

(Observational Vs. Experimental)

 Experimental Studies
• In an experimental study, the investigator determines through a
controlled process the exposure for each individual (clinical
trial) or community (community trial), and then tracks the
individuals or communities over time to detect the effects of the
exposure.
 Example for experimental
studies
• For example, in a clinical trial of a new vaccine, the investigator may
randomly assign some of the participants to receive the new vaccine,
while others receive a placebo shot. The investigator then tracks all
participants, observes who gets the disease that the new vaccine is
intended to prevent, and compares the two groups (new vaccine vs.
placebo) to see whether the vaccine group has a lower rate of disease.

• Similarly, in a trial to prevent onset of diabetes among high-risk

individuals, investigators randomly assigned enrollees to one of three
groups — placebo, an anti-diabetes drug, or lifestyle intervention. At the
end of the follow-up period, investigators found the lowest incidence of
diabetes in the lifestyle intervention group, the next lowest in the anti-
diabetic drug group, and the highest in the placebo group.39
 Observational studies
• In an observational study, the epidemiologist simply observes
the exposure and disease status of each study participant.

• The two most common types of observational studies are cohort

studies and case-control studies; a third type is cross-sectional
studies.
 Example of Observational
Studies
• John Snow’s studies of cholera in London were observational
studies.
 Cohort Study
• In a cohort study the epidemiologist records whether each study
participant is exposed or not, and then tracks the participants to see if
they develop the disease of interest. Note that this differs from an
experimental study because, in a cohort study, the investigator
observes rather than determines the participants’ exposure status.
After a period of time, the investigator compares the disease rate in
the exposed group with the disease rate in the unexposed group.

• If the disease rate is substantively different in the exposed group

compared to the unexposed group, the exposure is said to be
associated with illness.

• retrospective/ prospective or follow up

 Case-Control Study
• In a case-control study, investigators start by enrolling a group
of people with disease. As a comparison group, the investigator
then enrolls a group of people without disease (controls).
Investigators then compare previous exposures between the two
groups

• The key in a case-control study is to identify an appropriate

control group, comparable to the case (comparison) group in
most respects, in order to provide a reasonable estimate of the
baseline or expected exposure.
 Cross-Sectional Study
• Cross-sectional studies (also known as prevalence or correlational studies)
examine relationships between potential causal factors and disease at a point
in time.

• In this third type of observational study, a sample of persons from a

population is enrolled and their exposures and health outcomes are measured
simultaneously.

• The cross-sectional study tends to assess the presence (prevalence) of the

health outcome at that point of time without regard to duration.

• For example, in a cross-sectional study of diabetes, some of the enrollees

with diabetes may have lived with their diabetes for many years, while
others may have been recently diagnosed
 Cross-Sectional Study
• On the other hand, a cross-sectional study is a perfectly fine tool
for descriptive epidemiology purposes.

• Cross sectional studies are used routinely to document the

prevalence in a community of health behaviors (prevalence of
smoking), health states (prevalence of vaccination against
measles), and health outcomes, particularly chronic conditions
(hypertension, diabetes).
Calculation of Risk ratio
& Odds ratio
• The measures of association described in the following
section compare disease occurrence among one group
with disease occurrence in another group. Examples of
measures of association include

• risk ratio (relative risk),

• rate ratio,

• odds ratio (case control study)

• and proportionate mortality ratio.

 Risk ratio( RR)
Risk ratio( RR)

• The group of primary interest (with disease) is labeled the exposed group, and the
comparison group is labeled the unexposed group.

• It does so by dividing the risk (incidence proportion, attack rate) in group 1 by the
risk (incidence proportion, attack rate) in group 2 . The two groups are typically
differentiated by such demographic factors as sex (e.g., males versus females) or by
exposure to a suspected risk factor (e.g., did or did not eat potato salad).

• Method for Calculating risk ratio/

The formula for risk ratio (RR) is:

Risk of disease (incidence proportion, attack rate) in group of primary interest

Risk of disease (incidence proportion, attack rate) in comparison group
• A risk ratio of 1.0 indicates identical risk among the two groups.
A risk ratio greater than 1.0 indicates an increased risk for the
group in the numerator, usually the exposed group. A risk ratio
less than 1.0 indicates a decreased risk for the exposed group,
indicating that perhaps exposure actually protects against
disease occurrence.

RR if = 1.0 if > 1.0 if < 1.0

the incidence in Increase risk

the exposed group exposed groups decrease risk of
is the same with than non exposed exposure group
Interpretation of
the non exposed group (decrease risk,
risk ration
group (increase risk, negative
(no risk , no positive association)
association) association)
 Odds Ratio
• An odds ratio (OR) is another measure of association that
quantifies the relationship between an exposure with two
categories and health outcome.
Odds Ratio interpretation

OR if = 1.0 if > 1.0 if < 1.0

exposure is exposure
exposure not
Interpretation of positively negatively
associated with
Odds ratio associated with associated with
disease
disease disease
• Exercise 1.7 Classify each of the following studies as:
• A. Experimental
• B. Observational cohort
• C. Observational case - control
• D. Observational cross - sectional
• E. Not an analytical or epidemiologic study
• _____ 1. Representative sample of residents were telephoned and asked
how much they exercise each week and whether they currently have (have
ever been diagnosed with) heart disease.
• _____ 2. Occurrence of cancer was identified between April 1991 and July
2002 for 50,000 troops who served in the first Gulf War (ended April 1991)
and 50,000 troops who served elsewhere during the same period.
• _____ 3. Persons diagnosed with new-onset Lyme disease were asked how
often they walk through woods, use insect repellant, wear short sleeves and
pants, etc. Twice as many patients without Lyme disease from the same
physician’s practice were asked the same questions, and the responses in
the two groups were compared.
• _____ 4. Subjects were children enrolled in a health maintenance
organization. At 2 months, each child was randomly given one of two types
of a new vaccine against rotavirus infection. Parents were called by a nurse
two weeks later and asked whether the children had experienced any of a
list of side-effects.
Thank you