introduction of basic epidemiology

1. Somali International University(SIU)
Faculty Of Health Sciences
Department Of Public Health
Introduction to Basic Epidemiology(1)
Semester 3
Dr.Ahmed Ayna,MS.c PTH-UMST-Sudan
5/10/2021 1

2. At the end of this course, the students should be able to
demonstrate knowledge of:
• Conceptualize the meaning of Epidemiology
• the nature and uses of epidemiology
• the epidemiological approach to defining and measuring the
occurrence of health related states in populations
• Be Familiar with Measuring of Disease Occurrence.
• the contribution of epidemiology to the prevention of disease, the
promotion of health and the development of health policy
• the contribution of epidemiology to good clinical practice and the
role of epidemiology in evaluating the effectiveness and efficiency
of health care.
Learning Objectives
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3. • Originates from the Greek words:
– Prefix – epi (upon, on, befall)+
– Root – demos (people, population, man)+
– Suffix – logy (study of)
• Literal translation: “That which befalls man”
(epidemics)
Epidemiology
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4. • It is the study of the nature, cause, control and
determinants of the frequency and distribution of
disease, disability and death in human
populations. (Timreck, 1994).
• Epidemiology is an investigative method used to
detect the cause or source of diseases, disorders,
syndromes, conditions or perils that cause pain,
illnesses, injury, disability or death in human
populations or groups. (Timreck, 1994)
Definitions of Epidemiology
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5. • It is the study of the distribution and
determinants of disease frequency in human
populations. (MacMahon and Trichopoulos,
1996)
• Epidemiology is a discipline that describes,
quantifies and postulates causal mechanisms
for health phenomena in the population. (Friis
and Sellers, 1996)
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6. • Epidemiology:-A study of the distribution and
determinants of health-related states or
events in specified populations, and the
application of this study to control of health
problems. (Last, 1988).
-Key terms in this definition reflect some of the
important principles of epidemiology.
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7. • Population of interest are human populations
• Describes distribution of health and disease
(by person, place and time variables). Here
described frequency and patterns.
• Identifies determinants of health and disease
(risk factors, causes)
• Health and disease (injury, illness, disability or
death)
Anatomy of the Definition
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8. Hippocrates’ in 400 BC in his “On Airs, Waters
and Place” – the role of the environment in
health and disease among men(Assignment for
further reading)
John Graunt, in 1662, published Natural and
Political Observations Made Upon the Bills of
Mortality which recorded descriptive
characteristics of birth and death data, including
seasonal variations, infant mortality, excess of
male over female deaths, and other findings. He
was the first to employ quantitative methods in
describing population vital statistics.
Highlights in the History of
Epidemiology
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9. John Snow investigated a cholera epidemic in
mid-19th century in London. His work featured
various techniques in epidemiologic inquiry such
as spot map of cases, tabulations of cases and
deaths, development and testing of hypotheses
that contaminated water may be associated with
cholera outbreaks. (Assignment for further
reading and writing)
• Koch in late 1800s espoused the concept that
diseases are caused by living organisms and made
possible more refined classification of disease by
specific causal organisms through his postulates
for disease causation. (Assignment for further
reading and writing postulates)
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10. • The history of epidemiologic methodology is
largely the history of the development of five
ideas:
1) Human disease is related to the environment in
which we live;
2) Counting of natural phenomena may even be more
instructive than just observing them;
3) “natural experiments” can be utilized to investigate
disease etiology;
4) Natural experiments occur more frequently than we
think and reflect the tremendous heterogeneity of
human experience;
5) “true experiments” may be conducted in human
populations in some circumstances.
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11. • Universal political commitment
• Clear and specific goal with precise timetable
• Well-trained and committed staff
• Flexible strategy
• Features of the disease that made its
elimination possible
• Availability of an effective and stable vaccine
Factors that contributed to the success
of eradication:
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12. Elucidate disease etiology
◦ May also identify the determinants of outcomes of
illness
◦ Other outcomes of interest – biologic processes like
growth, multiple pregnancy, intelligence and fertility
Explain local disease patterns
◦ By utilizing what is already known about the etiology of
a particular health problem, epidemiologist may be
able to explain and deal, for example, with a particular
outbreak, and formulate preventive measures suitable
to a specific community
Uses of Epidemiology
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13. Describe the natural history of disease
– To identify factors related to the course of the
disease once the disease is established
– It is useful to know how the duration of a disease
and the probability of various outcomes (recovery,
death, complications) vary by age, gender,
geography, etc.
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14. Provide guidance in the administration and
evaluation of health services(Planning to
Evaluation):-
– Estimation of number of hospital beds required for
patients with specific disease (e.g. mental illness)
or for given segments of the population (e.g.
prematurely born infants, disabled elderly) will
require knowledge of the frequency and natural
history of particular diseases or of all diseases in
the affected segments of the population
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15. – The planning of efficient research (diagnostic,
therapeutic, preventive) will require knowledge of
how many cases of a particular disease are likely
to be found in a given population during a given
period
– Knowledge of the relative frequency of disease in
population subgroups is useful if it enables
intervention programs to target these populations
(e.g. screening programs)
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16. • Causation
• Natural
History
• Description of
health status
of populations
• Evaluation of
intervention
Uses of Epidemiology
Genetic Factors
Good health
Ill health
Environmental factors
(including lifestyle)
Genetic Factors
Subclinical
changes
Death
Clinical Disease
Recovery
Proportion with ill
health, change over
time, change with
age, etc.
Good health
Ill
health
Time
Ill health
Good health
Health promotion
Preventive measures
Public health services
Treatment, Medical care
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17. • Prevention of Disease
• Maintenance of Health
Ultimate Goal of Epidemiology
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18. • It is interdisciplinary
– Draws from biostatistics and other social and
behavioral sciences as well as from medically-
related fields of toxicology, pathology, virology,
genetics, microbiology, parasitology and clinical
medicine.
• Use of population research techniques and
requires quantification of relevant factors
– E.g. the presence or absence (or the amount) of
exposure and the magnitude of the outcome
Features of Epidemiology
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19. • Key Terms in Epidemiology:-
Definition of epidemic:
– “Attacking many people at the same time”, “widely
diffused and rapidly spreading”, “excessive
occurrence of disease”
– The occurrence in a community or region of cases
of illness clearly in excess of normal expectancy
(Benenson, 2000)
– (Outbreak,Endemic,pnadamic and sporadic)
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20. Exposures and Outcomes
The two key elements that we measure in most epidemiological
studies are.
1. The exposure is the process by which an agent comes into
contact with a person or animal in such a way that the person
or animal may develop the relevant outcome, such as a disease
2. The outcome is the disease, or event, or health-related state,
that we are interested in.
3. An exposure can be any factor that may influence the outcome
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21. Exposures and Outcomes continued
• An exposure can be any factor that may influence the
outcome
• The outcome can be any health-related event or state - or it
can be a risk factor for, or a precursor to, a disease.
• The risk factor an aspect of personal behavior or lifestyle, an
environmental exposure, or an inborn or inherited
characteristic, that, on the basis of epidemiological evidence,
is known to be associated with health-related condition(s)
considered important to prevent.
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22. The Role of Epidemiology?
Epidemiology has major functions:
1. to describe patterns of health and disease within
populations
2. to interpret these differences
3. to apply our results to public health practice, and
4. to evaluate the effect of health-related interventions
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23. Observational and Interventional
Epidemiology
• Observational epidemiology (Also called non-experimental
studies). Studies in which the distribution or determinants of
an outcome are examined without any attempt by the
investigator to influence them. Observational epidemiology
includes both descriptive and analytical studies.
• Interventional epidemiology (Also called experimental studies)
Studies designed to test a hypothesis by modifying an exposure
within the study population.
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24. Types of Epidemiological Investigation
Epidemiological study
Observational
Aggregated data
Analytic
Ecological
study
Descriptive
Disease
mapping
Individual based
data
Analytic
Cross-
sectional
study Case-
control
study
Cohort
study
Descriptive
Cross-
sectional
study
Case
report
Case
series
Interventional
RCT Quasi Exp.
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25. Sequence of Epidemiological Studies
Evaluation of Public Health Intervention
Change in Public Health Policy
Intervention Studies
Individual-Based Analysis
Population-Based Analysis
Descriptive Studies
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26. Measures of disease occurrence
and frequency
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27. Objectives
1. Explain how cases and population are defined in
epidemiology
2. Define and calculate measures of occurrence
3. Explain how to select the proper measure of
occurrence
4. Define the assumptions behind the use of these
measure
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28. Seven measure of disease
occurrence and frequency
1. Counts
2. Prevalence
3. Incidence/risk
4. Mean/variance
5. Median
6. Mode
7. Rates
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29. Issues in Defining a Case
Case definition is a set of standardized criteria used
to identify cases and should usually contain
information about the following factors:
1. The method(s) used to identify a case
2. The boundaries of a case
3. The unit of analysis
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30. Case definition
The method(s)
used to identify
a case
• The procedures
or instruments
that have been
used to identify
a case.
The boundaries
of a case
• The dividing line
is drawn
between cases
and non-cases
The unit of
analysis
• Household , a
person, an
episode of an
event, a
diseased organ
or tissue.
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31. Issues in Defining a Case continued
Example
In a study of the frequency of acute
diarrhoea in a primary school. A child with
an episode that should last at least 48 hours
but less than 14 days in which reports at
least 3 loose stools per day
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32. Developing Case Definitions
Factors should be
considered
The question
we want to
answer
Comparison Hypothesis
Testing
Disease
Outbreak
Investigation
Lay
Definitions
The
resources
available
Ethical Issues
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33. Issues in Defining the Study population
• The study population is the population of subjects who
potentially could become cases.
• How we define a case, influences how we define the study
population.
• The study population is usually limited by location: e.g. in a
particular hospital, village or district.
• Often the study population is called the population at
risk because it is at risk of developing the outcome of
interest.
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34. Issues in Defining the Study population
continued
Examples
• A study of the uptake of measles vaccination,
in which a case was defined as a child between
the ages of 1 and 5 years who was given at
least one dose of measles vaccine in 1998.“,
Study population?
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35. Issues in Defining the Study population
continued
Examples
• "A study of the frequency of severe
depression in women aged 65 and over, in which
a case was defined as a woman aged 65 and
over who is diagnosed as having severe
depression, according to the WHO ICD-10, in
1998.“, Study population?
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36. Measures of occurrence
• Once we have developed a clear case
definition and defined the study population, we
can go on to quantify the occurrence of the
outcome of interest.
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37. Measures of occurrence
Measures of
occurrence
Prevalence
Point
prevalence
Period
prevalence
Incidence
Incidence
risk
Incidence
rate
Odds
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38. Measures of occurrence
Two main types of measure of occurrence
1. Prevalence is concerned with quantifying the
number of existing cases in a population at
a designated point of time.
2. Incidence is concerned with quantifying the
frequency of occurrence of new cases in a
defined population, arising during a
given time period.
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39. Measures of occurrence continued
Measures of prevalence and incidence are
meaningless unless we define:
1. What we mean by a case
2. The population at risk of becoming cases
3. The time frame, where this is:
a. A specific point in time (for prevalence)
b. An interval of time (for incidence)
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40. Measures of occurrence continued
There are two types of prevalence
1. Point prevalence is the proportion of persons in a
defined population that has the outcome under study
at a specific point in time. So: Point Prevalence =
Number of Cases at time (t)
Study Population at time (t)
• The prevalence depends on the incidence of the
condition and the duration of each case.
• Prevalence provides a useful measure of the state of
health of a community at a particular point in time.
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41. Measures of occurrence continued
2. Period prevalence measures the number
of cases in a population over a defined
period of time. The numerator for period
prevalence includes cases present at the
start, as well as incident cases which
arose during the period of interest.
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42. The two measures of incidence, incidence risk and rate
1. The incidence risk :
is the proportion of new cases which occur in a population
initially free of the condition during a specified period of time.
So, Incidence risk =
Number of new cases in a defined time period
Population at risk at the beginning of the period
• The incidence risk provides information about the risk of
becoming a case in a given time period.
• It is useful for studying the determinants of a condition which
is relatively uncommon in a static population
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43. • Example: in 1980 the annual mortality
rate (risk of dying) was 26 per 1000 in
Malawi and Yemen, 14 per 1000 in Kenya
and India, 10 per 1000 in Belgium and
France, and 4 per 1000 in Costa Rica.
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44. 2. The incidence rate
relates the number of new cases in a specified time period to
the total person-time at risk.
Incidence rate = Number of new cases in a defined time period
Total person-time at risk
• The incidence rate is used for studying the determinants of a
condition that is relatively common and/or occurring in a dynamic
population. Here, the population at risk at the start is not a good
approximation of the total person-time at risk where many people
are likely to become diseased, or where people frequently enter
and/or leave the population
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45. • Example During a follow-up survey
among 350 female inmates in Sao
Paulo, Brazil, the incidence of
pulmonary tuberculosis among HIV
positive women was 9.92 per 100
person-years
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46. Measures of occurrence continued
• The odds of becoming a case is a measure of the number of cases
in a defined population and time period, divided by the number of
people who did not become a case in the same time period.
• Odds = number of cases
number of non-cases
• In a stable population of 100 school children, 20 children had at
least one episode of conjunctivitis during a one year study period.
The odds of developing conjunctivitis in the school during the study
period was:
Odds = cases / non-cases = 20(100-20) = 20 / 80 Odds =0.25
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47. Measures of occurrence continued
• The prevalence (P) is influenced by the
occurrence of new cases (incidence, I)
and the duration of each case (D):
P= I x D.
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48. • Example 1: If all other factors were kept equal,
what would happen to the prevalence of AIDS in a
specified population following the introduction of a
new treatment that prolongs life but does not cure
the disease?
• Example 2: If all other factors were kept equal,
what would happen to the prevalence of asthma in a
specified population following the introduction of a
new treatment that cures the disease?
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49. Measures of occurrence continued
• The prevalence used to evaluate the impact of
preventative measures aimed at reducing the
burden of a disease or condition in a
community.
• Any comparative results need to be
interpreted with caution because of the
number of factors that influence prevalence
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50. • The incidence risk is also used for evaluating the
impact of preventative measures that aim to reduce
the number of new cases of a condition.
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51. Tuberculosis in Benadir districts
 Tuberculosis is a reportable condition
 All diagnosed cases must be reported to the
department of health.
 In 2011, there were 689 new cases of
tuberculosis in Benadir districts
51
Is this
information
useful?
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52. 1. Counts
 Provide an absolute number of the burden of disease
 However counts has limited utility for two reasons
 The burden of disease in the population is very
different if the population size is 100,000 versus
1,000,000
 Some people are not at risk for developing a new
onset of tuberculosis in 2011 (due to pre-existing
infection), thus we need to know not only the size of
the total population, but the size of the total
population at risk
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53. Incidence and prevalence
 Two measures overcome many of the limitations
of a simple count of cases - incidence and
prevalence
 Prevalence tells us about the proportion of cases
among the total population at any given time
 Incidence tells us the probability of a new onset
of disease among those at risk for developing the
illness
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54. 2. Prevalence
The proportion of people who have the disease
(existing cases plus new cases) over the total
population for a given time period
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55. Disease occurrence in a sample of Benadir districts over time
Year 1, 5 individuals
developed the outcome
Year 2, an additional 7
people developed the
outcome
Year 3, an additional 4
people developed the
outcome
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56. What is the prevalence of disease in Year 2?
 What is the numerator?
5 cases in Year 1 + 7 cases in Year 2 = 12
 What is the denominator?
Total sample size = 30
 Prevalence = 12/30 = 0.4
The prevalence of disease in Year 2 is 40%
56
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57.  What is the numerator?
5 cases in Year 1 + 7 cases in Year 2 + 4 cases in Year
3 = 16
 What is the denominator?
Total sample size = 30
 Prevalence = 16/30 = 0.533
 The prevalence of disease in Year 3 is 53.3%
57
What is the prevalence of disease in Year 3?
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58. Summary: Prevalence
 For prevalence, we need a numerator (number of
existing cases), and denominator (total sample
size), and a time period of interest
 The time period should be specified as much as
possible
 For example, when we say “in Year 2” we mean
over the duration of time that spanned up to Year
2
58
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59. 3. Incidence
 Perhaps the most widely used tool in
epidemiology
 Goes by many names - most common alternative
name is “risk,” and less commonly, “incidence
proportion”
 Numerator = number of new cases
 Denominator = population at risk of becoming a
new case
 Specified over a specific time period
59
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60. What is the incidence of disease in Year 2?
 What is the numerator?
7 new cases in Year 2
 What is the denominator?
25 people at risk (5 people already developed
the disease in Year 1 and are thus not at risk)
 Incidence = 7/25 = 0.28
The incidence (risk) of disease in Year 2 is 28%
60
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61. What is the incidence of disease in Years 2 and 3?
 What is the numerator?
7 new cases in Year 2 + 4 new cases in Year 3 = 11
 What is the denominator?
 25 people at risk (5 people already developed the
disease in Year 1 and are thus not at risk)
 Incidence = 11/25 = 0.44
The incidence (risk) of disease in Years 2 and 3 is
44%
61
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62. Summary: Incidence
 For incidence, we need a numerator (number
of new cases), and denominator (total sample
size at risk), and a time period of interest
 The time period should again be specified as
much as possible
62
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63. The relation between
incidence and prevalence
 For incidence, we need a numerator (number
of new cases), and denominator (total sample
size at risk), and a time period of interest
 The time period should again be specified as
much as possible
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64. Understanding incidence and
prevalence: the bathtub example
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65. Examples of the relation between
incidence and prevalence
 High incidence, steady prevalence
 Example: highly contagious infectious disease
with very short duration or a high case-fatality
 Low incidence, high prevalence
 Examples: diseases with long duration such as
arthritis, diabetes, Crohn’s disease, and other
chronic illnesses
65
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66. Examples of the relation between
incidence and prevalence
Impact of a new treatment that prolongs life with the disease but does not cure it
New HIV Infections
People Living with HIV
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67. Summary, incidence, prevalence
 Prevalence is affected by incidence and duration
 If a disease has short duration,
 Prevalence ~= incidence*
 If a disease has long duration, in general,
 Prevalence > incidence
* Assumes that incidence is constant over time
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68. 5/10/2021 68
Populatio
n at risk
Existing
cases
Deaths,
cures,
etc.

69. Assignment
Outline comparison between
Prevalence and Incidence by giving
examples
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