EPIDEMIOLOGICAL STUDIES.pptx

TYPES OF EPIDEMIOLOGICAL STUDIES
BY
SIMBEYE T.S (Mr)
LUSAKA. ZAMBIA.
CENTRAL AFRICA

Types of epidemiological studies
• Epidemiologic studies fall into two categories:
• 1. Observational Epidemiological Studies
• 2. Experimental Epidemiological Studies
(intervention)

Observational epidemiological studies
• In an observational study, the epidemiologist
simply observes the exposure and disease
status of each study participant.
• John Snow’s studies of cholera in London were
observational studies.

Observational Epidemiological Studies are
divided into two namely:
i) Descriptive studies
ii) Analytical studies

a) Descriptive Studies
• Case report
• Case series
• Cross sectional study
• Ecological study (correlational studies)

• (b) Analytical Studies
(i) Case control study—Case Reference
(ii) Cohort study—Follow-Up and these are
further divided into:
a) Prospective cohort
b) Retrospective cohort
iii) Case-crossover

• Descriptive epidemiology identifies patterns
among cases and in populations by time, place
and person.
• From these observations, epidemiologists
develop hypotheses about the causes of these
patterns and about the factors that increase risk
of disease.
• Meaning, Epidemiologists can use descriptive
epidemiology to generate hypotheses, but only
rarely to test those hypotheses.

• Descriptive epidemiology identifies patterns
among cases in populations by time, place
and person.
• Distribution of disease is the domain of
descriptive epidemiology which involves the
analysis of disease patterns according to the
characteristics of person, place and time.

• In other words, epidemiologists seek answers
to questions such as:
• A) who is getting the disease?
• B) Where is it occurring?
• C) How is it changing over time?

• Variations in disease occurrence by these three
characteristics (person, place and time) provide
useful information for:
• A) Understanding the health status of a
population.
• B) Formulating hypotheses about the causes of
the disease.
• C) Planning, implementing and evaluating public
health programs to control and prevent adverse
health events.

Elements of descriptive epidemiology
A) Person (age, sex, race, religion,
socioeconomic class)
B) Place (Country, state, city, neighborhood)
C) Time (year, season, month, week, day, hour)

Person
• Personal characteristics that are usually
available for descriptive epidemiology include
age, race, gender and ethnic group,
socioeconomic status, occupation, religion
and marital status.
• These attributes are usually associated with
major variations in disease occurrence.

• Age is probably the most important among
them because it is associated with striking
changes in disease rates.
• The frequency of most diseases increases with
age.
• On the other hand, the incidence of some
diseases declines with age.

• Gender is another personal characteristics
associated with variations in disease
occurrence.
• Certain diseases are more common among
men and others are more prevalent among
women.

• Race and ethnicity also have profound
influence on the disease patterns.
• The rates for many diseases in the USA are
higher among minority groups particularly the
blacks.
• For example, diabetes is twice higher among
blacks and Mexicans than among whites.
• In addition, hypertension is 45% more
prevalent among blacks than among whites.

• These kinds of racial and ethnic differences in
disease rates may stem from differences in
socioeconomic status, health practices,
psychosocial stress and resources,
environmental exposures, discrimination, or
access to health care.

• Socioeconomic status is also a prominent
characteristic by which diseases vary.
• Socioeconomic status is difficult to quantify. It is
made up of many variables such as occupation,
family income, educational achievement, living
conditions, and social standing.
• The sinking of titanic is a historic example of
health disparities between the poor and wealth

• Death rates among passengers of low
socioeconomic status were twice as high as
those among passengers of higher
socioeconomic status because the small
supply of life jackets was preferentially given
to wealth passengers particularly wealthy
women and children.

• The relationship between socioeconomic
deprivation and health is complex because
deprivation is related to race, nutrition, risk
factors such smoking and alcohol use,
environmental and occupational exposures,
and access to and use of healthcare services.

• Religious affiliation also influences disease
rates. Like most of the personal characteristics
described thus far, religion represents a
mixture of factors, including genetic,
environmental, cultural and behavioural
factors.

• It is not surprising that occupation influences
disease patterns, because potent and
sustained exposures to harmful substances
can occur on some jobs.

• Marital status is also known to have an
important effect on the patterns of disease
and death.
• For example, death rates are higher among
people who are unmarried than for those who
are married and living with their exposures.
• The increased rates of death are greatest
among those who never married particularly
never married men.

• The increased rates of death are greatest
among those who never married, particularly
never married men.

• Place is defined in terms of geopolitical units such
as countries or states or in terms of natural
geographic features such as mountains or rivers.
• The characteristics of place encompass
numerous aspects of the environment including
the physical environment such as climate, water
and air; biological environment such as flora and
fauna; and social environment such as cultural
traditions.

• For example, malaria occurs in parts of the
world where all these facets of the
environment are conducive to the life cycle of
the anopheles mosquito, the vector that
carries disease from one host to another.

• Physical conditions that are necessary for the
development and survival of the mosquito
include a favourable temperature (20 to 30
degrees Celsius is optimal), adequate
humidity, moderate rainfall and the presence
of standing or gently flowing water.

• Biological factors beneficial to the mosquito
include plants that can collect small pools of
water.

• Social factors that encourage transmission of
the disease include the proximity of homes to
mosquito breeding sites, housing construction
that facilitates mosquito entry and certain
occupations that increase person’s exposures
to mosquitos such as those involving outdoor
work at night.

Time
• Analysis of the changes in disease and death
rates over calendar time provides
epidemiologists with useful information for
causal research and public health planning.
• The scale of time that is examined depends on
the disease and can range from decades or
years to months, weeks, days or hours.

• For example, the death rate from lung cancer
has increased dramatically among women
over the 57 year period from 1950 through
2007.
• Over the same period, there has been a
dramatic decline in deaths from
cerebrovascular diseases.
• Both of these are examples of long term or
secular trends.

• Short term trends are commonly examined for
infectious diseases.
• For example, the 1976 outbreak of
Legionnaires disease at Philadelphia
convention occurred over a 1 month period.
• Short term trends are also relevant for
noninfectious diseases that follow climatic
changes such as heat, waves, hurricans and
pollution episodes.

• For example, the 4 day 1952 smog disaster in
London was associated with an increase in
cardiovascular and respiratory deaths,
particularly among the elderly.
• More recently, temporal elevations in air
pollution levels in Philadelphia were
associated with increases in death rates from
chronic obstructive pulmonary diseases,
pneumonia, heart disease and stroke.

Uses of descriptive epidemiology
• It is used to assess the health status of a
population.
• It is also used to generate hypotheses
about causal factors.
• It is also used for planning, organizing
and evaluating public health programs.

Uses of descriptive epidemiology
• Contributes to research by describing
variations in disease occurrence by time, place
and person

Types of descriptive epidemiology
• Case report
• Case series
• Cross sectional study
• Ecological study (Correlational studies)

• A case report is a detailed description of
disease occurrence in a single person.
• Unusual features of the case may suggest a
new hypothesis about the causes or
mechanisms of disease.
• Example: Acquired Immunodeficiency in an
Infant; Possible Transmission by Means of
Blood Products

• A case series is a report on the characteristics
of a group of subjects who all have a particular
disease or condition.
• Common features among the group may
suggest hypotheses about disease causation.

• However, the chief limitation is that there is
no comparison group.
• Consequently, common features may suggest
hypotheses, but these need to be tested with
some sort of analytical study before an
association can be accepted as valid

• A case series (clinical series) is a medical
research descriptive study that tracks
patients with a known exposure given
similar treatment or examines their
medical records for exposure and
outcome.

Cross-sectional study
• A cross-sectional study studies a whole
population or a randomly selected sample of a
population and collects information about
current disease state (i.e. prevalent disease)
and also measures exposures of interest.

• A cross sectional study is a type of prevalence
study in which exposures and distributions of
disease are determined at the same time,
although it is not imperative for the study to
include both exposure and disease.
• In some instances, the study may base
determination of disease status on a period of
time.

• Cross-sectional studies are relatively
inexpensive and can be conducted within a
short timescale.
• The disadvantage of cross sectional study is
the lack of information on temporality of
disease state and exposures.

• Cross sectional studies are used to formulate
hypotheses that can be followed up in
analytical studies.

Ecological study (Correlational study)
• Ecological studies assesses the overall
frequency of disease in a series of populations
and looks for a correlation with the average
exposure in the populations.
• These studies are unique in that the analysis is
not based on data on individuals.

• Instead, the data points are the average levels
of exposure and the overall frequency of
disease in a series of populations.
• Therefore, the unit of observation is not a
person; rather, it is an entire population or
group.

• In the study conducted in boston univer investigators used
commerce data to compute the overall consumption of
meat by various nations.
• They then calculated the average (per capita) meat
consumption per person by dividing total national meat
consumption by the number of people in a given country.
• There is a clear linear trend; countries with the lowest meat
consumption have the lowest rates of colon cancer, and the
colon cancer rate among these countries progressively
increases as meat consumption increases.

Analytical epidemiological studies
• In analytical observational studies, researchers
try to establish an association between
exposure(s) and outcome.
• Analytic epidemiology is concerned with the
search for causes and effects.
• Analytical studies answer the why and the
how questions.

• Epidemiologists use analytic epidemiology to
quantify the association between exposures
and outcomes.
• Analytical studies are used to test hypotheses
about causal relationships.

• Epidemiology provides sufficient evidence to
take appropriate control and preventive
measures.
• The key feature of analytic epidemiology is a
comparison group.

Types of analytical study
• Case control study
• Cohort study
i) Prospective cohort
ii) Retrospective cohort
• Case-crossover

Case–control study
• In this type of study, persons with the disease
of interest are identified (cases) together with
a sample of those without the disease
(controls).
• The cases and controls are then compared
with respect to their exposure to potential risk
factors.

• The case–control approach is particularly
suitable when the disease is rare, and when
the aim is to investigate the effect of many
exposures on one disease.

Cohort study
• A cohort is a group of people who share a
common characteristic within a defined
period (e.g., members of the cohort
underwent a certain medical procedure, were
exposed to a drug, or were born in a certain
period).

• A cohort study is a form of longitudinal study (
a type of observational study) used in
epidemiologic investigations.
• It is an analysis of risk factors undertaken by
following a group of case subjects who do not
have the disease and uses correlations to
determine the absolute risk of a subject
contracting the disease of interest.

• It is one of clinical study designs and should be
compared with a cross-sectional study.
• Cohort studies are generally concerned with
the life histories of segments of populations,
and individual people who constitute these
segments

• A cohort study involves one or more groups of
subjects, defined by their exposure status and
free of the disease of interest, being followed
through time to identify disease onset.
• The purpose of cohort study is to determine
whether initial exposure status influences
risk of subsequent disease.

Types of cohort study
I) Prospective cohort study
Ii) Retrospective cohort study.

• In the prospective approach, cohorts are
identified by their present exposure status and
are followed-up to determine any future onset
of the disease.
• Prospective study determines which exposure
to measure and how.

• Retrospective approach identifies the
exposure status of cohort(s) in the past and in
a parallel sense they are ‘followed-up’ until
the present time, when their disease status is
determined.
• Retrospective approach is quicker and less
expensive, but may not always be appropriate
or possible.

• Case-crossover design is an approach
used to investigate acute triggers that are
potentially causing disease.
• Read and research more on case
cross-over and make your own
notes.

Experimental studies include the following
studies:
• Clinical trials
• Community trials
• Randomized
• Non Randomized

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.
• 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.

• Randomized clinical trials studies the effects
of a treatment in large groups of patients are
the ideal means for identifying appropriate
therapy.

Randomized controlled trials
• In randomized controlled trials researchers
randomly allocate exposures to individuals.
• Participants enter a study and then their
allocation to an arm of the study is determined
randomly, although there may be stratification to
ensure certain key characteristics are balanced
between studies.
• What need to be taken into consideration is that
persons agreeing to take part in a trial may not be
representative of the wider population.

• It is apparent that many questions can be addressed
by several different study designs and thus the decision
has to be made as to the most appropriate design to
answer a particular question.
• The choice is often between feasibility, that is,
obtaining an answer within a required timescale, and
validity (i.e. obtaining the most accurate answer).
• Common sense dictates that one without the other
negates the value of undertaking the study and also
that the two are not mutually exclusive.
• The decision is normally based on an appraisal of both
scientific and logistic considerations.

• 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.

ASSIGNMENT
PLEASE RESEARCH MORE ON
Non randomized trials

MODE OF TRANSMISSION OF DISEASES
• Diseases are transmitted from the reservoir or
source to the host.
• The reservoir is a natural habitat in which the
organism metabolises and replicates.
• A reservoir can be a human being, animal or
environment.

MODE OF TRANSMISSION
• Transmission occurs when the agent leaves its
reservoir through a port of exit and conveyed
by some media of transmission and enters
through the appropriate port of entry and
then infects the susceptible host. This is called
the chain of infection

• Direct transmission is immediate transfer of the
agent from the reservoir to the susceptible host
either by direct contact or droplet.
• Direct contact occurs through kissing, skin to skin,
sexual intercourse etc.
• It also refers to contact with soil or vegetation
harbouring infectious agent

• Indirect transmission- refers to an agent that is
carried from a reservoir to a susceptible host by an
inanimate vehicle.
• Most vectors are mosquitoes, fleas, snails and
ticks. These may carry the agent through
mechanical means.
• The agent that has part of its life cycle in the
intermediate host is called biological transmission

Epidemiology and prevention
• A major use of epidemiologic evidence is to
identify subgroups in the population who are
at high risk for disease.
• if we can identify these high-risk groups, we
can direct preventive efforts, such as
screening programs for early disease
detection.

Epidemiology and prevention
• If we can identify high-risk groups, we may be
able to identify the specific factors or
characteristics that put them at high risk and
then try to modify those factors.

Levels of prevention
1. Primary level of prevention
2. Secondary level of prevention
3. Tertiary level of prevention

• Primary level of prevention denotes an action
taken to prevent the development of a disease
in a person who is well and does not (yet)
have the disease in question.

• For example, we can immunize a person
against certain diseases so that the disease
never develops or, if a disease is
environmentally induced, we can prevent a
person’s exposure to the environmental
factor involved and thereby prevent the
development of the disease.

• For example, we know that most lung cancers
are preventable. If we can stop people from
smoking, we can eliminate 80% to 90% of lung
cancer in human beings.

• Secondary level of prevention involves
identifying people in whom a disease process
has already begun but who have not yet
developed clinical signs and symptoms of the
illness.

• This period in the natural history of a disease
is called the preclinical phase of the illness.
• Early detection of the disease makes
treatment easier and more effective and
prevents complications and mortality.
• Several recent studies indicate that routine
testing of the stool for occult blood can detect
treatable colon cancer early in its natural
history.

• Tertiary level of prevention denotes
preventing complications in those who have
already developed signs and symptoms of an
illness and have been diagnosed—that is,
people who are in the clinical phase of their
illness.

• This is achieved through prompt and
appropriate treatment of the illness combined
with ancillary approaches such as physical
therapy that are designed to prevent
complications such as joint contractures

Two approaches to prevention
• Two possible approaches to prevention are a
1. Population-based approach.
2. High-risk approach.

• In the population-based approach, a
preventive measure is widely applied to an
entire population.
• For example, prudent dietary advice for
preventing coronary disease or advice against
smoking may be provided to an entire
population.

• Measures that are applied to an entire
population are relatively inexpensive and
noninvasive.
• Population based approaches are usually
considered public health approaches.

• In High-risk approach, preventive measures
are directed at high-risk groups.
• For example, screening for cholesterol in
children might be restricted to children from
high-risk families.
• A measure that is applied on a high-risk
subgroup of the population is usually more
expensive and is often more invasive or
inconvenient.

• High-risk approaches usually require a clinical
action to identify the high risk group to target
with intervention.

DISEASE SURVEILLANCE AND CONTROL
• Surveillance is defined as the continuous or
ongoing scrutiny of the factors that
determine the occurrence and distribution of
diseases and other health related events
through a systematic collection of data
• Surveillance is ongoing scrutiny, generally
using methods distinguished by their
practicability, uniformity and frequently their
rapidity rather than by complete accuracy.

DISEASE SURVEILLANCE AND
CONTROL
• Public health surveillance is defined an ongoing,
systematic collection, analysis and interpretation
of health-related data essential to the planning,
implementation and evaluation of public health
practice closely integrated with the timely
dissemination of these data to those responsible
for prevention and control.

DISEASE SURVEILLANCE
AND CONTROL
• Surveillance is information for action.
• Also Surveillance is the bridge between what
we think is happening and what actually is
happening.

TYPES OF SURVEILLANCE
Passive surveillance
• Passive surveillance often gathers disease data
from all potential reporting health care
workers.
• Health authorities do not stimulate reporting
by reminding health care workers to report
disease nor providing feedback to individual
health workers (London School of Hygiene and
Tropical Medicine, 2009).

• Passive surveillance is the most common type
of surveillance in humanitarian emergencies.
• Most surveillance for communicable diseases
is passive.
• The surveillance coordinator may provide
training to health workers in how to complete
the surveillance forms, and may even send
someone to periodically collect forms from
health facilities.

• Nonetheless, passive surveillance is often
incomplete because there are few incentives
for health workers to report.

Active surveillance
• An active surveillance system provides
stimulus to health care workers in the form of
individual feedback or other incentives
(London School of Hygiene and Tropical
Medicine, 2009).

• Often reporting frequency by individual health
workers is monitored; health workers who
consistently fail to report or complete the
forms incorrectly are provided specific
feedback to improve their performance.
• There may also be incentives provided for
complete reporting.

• Active surveillance requires substantially more
time and resources and is therefore less
commonly used in emergencies.
• But it is often more complete than passive
surveillance.

• It is often used if an outbreak has begun or is
suspected to keep close track of the number
of cases.
• Community health workers may be asked to
do active case finding in the community in
order to detect those patients who may not
come to health facilities for treatment.

Sentinel surveillance
• Instead of attempting to gather surveillance data
from all health care workers, a sentinel surveillance
system selects, either randomly or intentionally, a
small group of health workers from whom to
gather data.
• These health workers then receive greater
attention from health authorities than would be
possible with universal surveillance(London School
of Hygiene and Tropical Medicine, 2009).

• Sentinel surveillance also requires more time
and resources, but can often produce more
detailed data on cases of illness because the
health care workers have agreed to participate
and may receive incentives.
• It may be the best type of surveillance if more
intensive investigation of each case is
necessary to collect the necessary data.

PURPOSE OF SURVEILLANCE
• Its main purpose is to detect changes in trends
or distribution in order to initiate investigative
or control measures.

• To provide scientific baseline data and
information for priority setting, planning,
implementing and evaluating disease
control program for both communicable
and non-communicable health problems.
.

• To define the magnitude and distribution of
diseases by time, person and place dimension.

• To be able to identify diseases, injuries,
hazards and other health related factors as
early as possible, i.e. prediction and early
detection of outbreaks.

Epidemiology and health expenditure
• Health expenditure consists of all
expenditures or outlays for medical care,
prevention, promotion, rehabilitation,
community health activities, health
administration and regulation and capital
formation with the predominant objective
of improving health.
(www.fhb.gov.hk/statistics)

Epidemiology and health
expenditure cont…
• Health spending consists of health and
health-related expenditures.
• Health-related expenditures include
expenditures on health-related functions
such as medical education and training,
and research and development.

expenditure cont…
• Health includes both the health of
individuals as well as of groups of
individuals or population.
• Private health expenditure includes direct
household(out of pocket) spending,
private insurance, charitable donations
and direct service payments by private
co-operations

expenditure cont…
• Public health expenditure consist of
recurrent and capital spending from
government (central and local) budgets,
external borrowing and grants (including
donations from international agencies and
nongovernmental organizations) and
social (or compulsory) health insurance
funds

expenditure cont…
• Total health expenditure is the sum of
public and private health expenditure. It
covers the provision of health
services(preventive and curative) family
planning activities, nutrition activities
and emergency and designated for health
but does not include provision of water
and sanitation

expenditure cont…
• Main outcome measures; proportion of
healthcare budget spent on each category
of disease and cost of health care.
• Analysis of cost effective and efficient
health intervention policies and strategies

expenditure cont…
PRINCIPLES OF ECONOMICS
 Microeconomics applied to health care
looks at the behaviors of individuals and
organizations that result from tradeoffs in
utility and budget limits
• Both supply and demand are affected by
price.

FACTORS THAT INFLUENCE
HEALTH CARE COSTS
Demographics affecting health care:
 Population size
 Population density
 Fertility rate
 Mortality rate
 Population growth rate

Demographics affecting health care:
• Age distribution
• Migration
• Vital statistics
• Socio-economic conditions

HEALTH CARE COSTS Cont…
Technology and intensity
 The introduction of new technology
enhances the delivery of care, but it also
has the potential to increase the cost of
care.
 The adopting of new technology demand
investment in personnel, equipment, and
facilities

Chronic illness:
 Chronic medical conditions were
identified by those costing the most, the
number of bed days, work loss days, and
activity impairements.

The uninsured:
 The uninsured workers cannot afford to
purchase health insurance or their
employers may not offer health insurance
as a benefit.

The poor:
 Socioeconomic status is inversely related
to mortality and morbidity for almost
every disease.

Access to care
 The primary reasons for delay, difficulty
or failure to access care include lack of
transportation, communication problems,
lack of information and long travel
distances.

Rationing health care
 Health care services in Zambia are
distributed according to the level of the
providing health institution e.g. health
center, first level hospital. This approach
helps to control and reduce costs.

DISEASE SURVEILLANCE AND
CONTROL
• Surveillance: is ongoing scrutiny, generally
using methods distinguished by their
practicability, uniformity and frequently their
rapidity rather than by complete accuracy.
• Its main purpose is to detect changes in
trends or distribution in order to initiate
investigative or control measures.

REFERENCES
• Stanhope, M., and Lancaster. (2004).
Community and Public Health Nursing. 6th
edition, Mosby.
• Last, J.M. (2001). A Dictionary of
epidemiology. 4th edition. New York. Oxford
University

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