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

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

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 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 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?