Transcript Epidemiology: The Study of Disease, Injury, and Death in the

Chapter 3
Epidemiology: The
Study of Disease,
Injury, and Death in the
Community
Introduction
• Epidemiology = population medicine
• Epidemiologists concerned with course of
disease in a population
• Collect information about disease status of a
community
• How many people are sick? Who is sick? When
did they become sick? Where do they live?
• Data can be used to prevent disease outbreaks
or determine effectiveness of prevention effort
Definitions
• Epidemiology: study of distribution and
determinants of health-related states or events in
specified populations
• Epidemic: unexpectedly large number of cases of
an illness, specific health-related behavior or
event, in a particular population
• Endemic: disease that occurs regularly in a
population as a matter of course
• Pandemic: outbreak over wide geographic area
History of Epidemiology
• Concept dates back to Hippocrates, 300 B.C.
• Few advances made after fall of classical
civilizations of Greece and Rome
• Subsequent waves of infectious disease
epidemics
• Plague, yellow fever, cholera
• Example of how epidemiological methods can
be used to limit disease and deaths: John Snow
The Importance of Rates
• Rates allow for comparison of outbreaks at
different times or in different places
• Cases: people afflicted (those who are sick)
• Rates: number of events in a given population
over a given period of time or given point in time
• Natality (birth), morbidity (sickness), mortality or
fatality (death) rates
• Population at Risk: those susceptible to particular
disease or condition
Morbidity Rates
• Incidence rate: number of new health-related events
or cases of a disease in a population exposed to that
risk during a particular period of time, divided by
total number in same population
• Important in study of acute diseases – diseases in
which the peak severity of symptoms occurs and
subsides within days or weeks
• Attack rate: incidence rate calculated for a particular
population for a single disease outbreak; expressed as
a percentage
Morbidity Rates
• Prevalence rate: number of new and old cases in a
given period of time, divided by total number in that
population
• Useful in study of chronic disease – diseases that
usually last three months or longer
Incidence, Prevalence, and Attack
Rates
Crude and Age-Adjusted Rates
• Incidence and prevalence rates expressed as crude or
specific
• Crude rates: denominator includes the total
population
• Crude birth rate: number of live births in given year,
divided by midyear population
• Crude death rate: number of deaths in given year from all
causes, divided by midyear population
• Age-adjusted rates: used to make comparisons of
relative risks across groups and over time when
groups differ in age structure
Crude Rates
Specific Rates
• Measure morbidity and mortality for particular
populations or diseases
• Cause-specific mortality rate: measures death
rate for a specific disease
• Case fatality rate: percentage of cases of a
particular disease that result in death
• Proportionate mortality ratio: percentage of
overall mortality in a population that is
attributable to a particular cause
Specific Rates
Reporting of Birth, Deaths, and
Diseases
• Physicians, clinics, and hospitals required to
report births, deaths, and notifiable diseases
• Notifiable diseases: infectious diseases in
which health officials request or require
reporting; can become epidemics
• Reported to CDC via National Electronic
Telecommunications System (NETS)
• Various challenges to maintaining accurate data
Notifiable Disease Scheme
Standardized Measurements of Health
Status of Populations
• Mortality statistics: most reliable measure of
population health status
• Life expectancy: average number of years a
person from a specific cohort is projected to
live from a given point in time
Life Expectancy
Years of Potential Life Lost
• Years of potential life lost (YPLL): number of years
lost when death occurs before one’s life expectancy
• Subtract person’s age at death from his or her life
expectancy
• Difficult to determine because life expectancy
changes at different ages
• Age 75 is often used in calculations
• Weighs death of young person as counting more than
death of old
Other Measures
• Disability-Adjusted Life Years (DALYs)
• Measures burden of disease
• One DALY=one lost year of healthy life
• Health-Adjusted Life Expectancy (HALE)
• Number of years of healthy life expected, on
average, in a given population or region of the
world
Sources of Secondary Data
• Secondary data – data collected by someone
else, possibly for another purpose
• Useful in planning of public heath programs
and facilities
• U.S. Census
• Enumeration of the population
• Taken every 10 years
• Gathers data on race, age, income,
employment, education, dwelling type, other
Sources of Standardized Data (ctd)
• Statistical Abstract of the United States
• Book published annually by Bureau of Census
• Summary of statistics on social, political, and economic
organization of the United States
• Monthly Vital Statistics Report
• Vital statistics are summaries of records of major life
events: birth, death, marriage, divorce
• Published by National Center for Health Statistics under
the CDC
• Also calculates death rates by race and age
Sources of Standardized Data (ctd)
• Morbidity and Mortality Weekly Report (MMWR)
• Prepared by CDC from state health department
reports
• Reports morbidity and mortality data by state and
region of U.S.
• Reports outbreaks of disease, environmental
hazards, unusual cases, or other public health
problems
National Health Surveys
• National Health Survey Act of 1956 authorized
continuing survey of amount, distribution, and effects
of illness and disability in the U.S.
• Three types of surveys
• Health interviews of people
• Clinical tests, measurements, and physical
examinations
• Surveys of places where people receive medical
care
Some National Health Surveys
• National Health Interview Survey (NHIS)
• Conducted by NCHS
• Questions respondents about their health
• National Health and Nutrition Examination
Survey (NHANES)
• Assesses health and nutrition status through
mobile examination center
• Behavioral Risk Factor Surveillance System
(BRFSS); National Health Care Survey (NHCS)
Epidemiological Studies
• Investigations carried out when disease or
death occurs in unexpected or unacceptable
numbers
• Descriptive studies
• Describe epidemics with respect to person,
place, and time
• Analytic studies
• Aimed at testing hypotheses
Descriptive Studies
• Describe epidemics with respect to person, place,
and time
• Who?
• Case count, followed by who is ill (children, men,
women, race, etc.)
• When?
• Time of onset for each case
• Epidemic curves created
• Where?
• Determine residential address and travel history
Epidemic Curves
• Graphic display of the cases of disease according to
the time or date of onset of symptoms
• Single epidemic curves
• Can be used to calculate incubation period – period
of time between exposure to an infectious agent
and onset of symptoms
• Common source epidemic curve
• Point source epidemic curve
• Continuous source epidemic curve
• Propagated epidemic curve
Analytic Studies
• Test hypotheses about relationships between health
problems and possible risk factors
• Observational studies: investigator observes
natural course of events, noting exposed vs.
unexposed and disease development
• Case/control studies
• Cohort studies
• Experimental studies: investigator allocates
exposure and follows development of disease
Case/Control Studies
• Case/Control
• Compares those with disease to those without
but with similar background and/or with prior
exposure to certain risk factors
• Aimed at identifying factors more common in
case than control group
Cohort Studies
• Cohort is classified by exposure to one or more
risk factors and observed to determine rate of
disease development
• Cohort: group of people who share important
demographic characteristic
• Odds ratio
• Relative risk
Experimental Studies
• Carried out to identify cause of disease or
determine effectiveness of vaccine, drug, or
procedure
• Control for variables
• Control groups
• Randomization
• Blinding
• Placebo: blank treatment
Criteria of Causation
• Questions exposure causing development of
disease
• Criteria
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Strength
Consistency
Specificity
Temporality
Biological plausibility
Discussion Questions
• How can data collection for notifiable diseases
be improved?
• Why is tracking vital statistics so important?
• How does calculating Years of Potential Life
Lost change the way we think about
community health efforts?