Epidemiology: The Study of Disease,

Injury, and Death in the Community

Chapter 3

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

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 # in same population

• Prevalence rate: number of new and old cases in a given period of time, divided by total # in that population

• Attack rate: incidence rate calculated for a particular population for a single disease outbreak; expressed as a percentage

• Incidence rates important to study of acute diseases• Acute disease: lasts three months or less

• Prevalence rates useful for study of chronic diseases• Chronic disease: last longer than three months

• Incidence and prevalence rates expressed as crude or specific

Incidence, Prevalence, and Attack Rates

Incidence, Prevalence, and Attack Rates

Crude and Age-Adjusted Rates

• Crude rates: denominator includes the total population• Crude birth rate: # of live births in given year,

divided by midyear population

• Crude death rate: # 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

Crude and Age-Adjusted Rates

Specific Rates

• Measure morbidity and mortality for particular populations or diseases

• 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

Important Rates in Epidemiology

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

• Health officials maintain weekly records

• 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• Easier to track death than illness

• Life expectancy: average number of years a person from a specific cohort is projected to live from a given point in time

• Years of potential life lost (YPLL): number of years lost when death occurs before one’s life expectancy

Life Expectancy Figures

Years of Potential Life Lost

• Subtract person’s age at death from his or her life expectancy

• Difficult to determine because life expectancy changes at different ages

• Weighs death of young person as counting more than death of old

Sources of Standardized Data

• Various valid sources have specific value and usefulness to those in public health• U.S. Census• Taken every 10 years

• Enumeration of the population

• More complex now

• Gathers data on race, age, income, employment, education, and other social indicators

• 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

Sources of Standardized Data (ctd)

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

• 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

• Secular, seasonal, and single epidemic curves

• Single epidemic curves• Point source epidemic curve

• Propagated epidemic curve

Secular Epidemic Curve

Seasonal Epidemic Curve

Point 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• 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?