C5

Etymology and Fundamental Definition of Epidemiology

• The word epidemiology is derived from three Greek roots:
  • Epi: Meaning on or upon.
  • Demos: Meaning people.
  • Logos: Meaning the study of.
• At its core, epidemiology is the study of what befalls a population.
• The comprehensive definition of epidemiology is: The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.

Key Principles of Epidemiology

• Study: Epidemiology is a scientific discipline grounded in sound methods of scientific inquiry. It is data-driven, relying on a systematic and unbiased approach to the collection, analysis, and interpretation of data. Methods involve careful observation and the use of valid comparison groups to assess whether observed frequencies (e.g., cases of disease or exposure) differ from expected values. It incorporates methods from biostatistics, informatics, and the biological, economic, social, and behavioral sciences.
• Distribution: This involves the frequency and pattern of health events:
  • Frequency: Refers to the number of health events (e.g., cases of meningitis or diabetes) in relation to the size of the population. This resulting rate allows for the comparison of disease occurrence across different populations.
  • Pattern: Refers to the occurrence of health-related events by time, place, and person.
    • Time patterns: May be annual, seasonal, weekly, daily, hourly, or distinguish between weekday and weekend.
    • Place patterns: Include geographic variation, urban/rural differences, and the location of work sites or schools.
    • Personal characteristics: Include demographic factors (age, sex, marital status, socioeconomic status), behaviors, and environmental exposures.
• Determinants: These are the causes and other factors that influence the occurrence of disease and health-related events. A determinant is any factor, characteristic, or definable entity that brings about a change in a health condition. It is assumed that illness is not random but occurs when a specific accumulation of risk factors exists in an individual.
• Health-related States or Events: Originally focused on communicable disease epidemics, the field now encompasses endemic communicable diseases, non-communicable infectious diseases, chronic diseases, injuries, birth defects, maternal-child health, occupational health, and environmental health. It also includes behaviors related to well-being, such as exercise habits, seat belt use, and genetic markers of disease risk.
• Specified Populations: While clinicians focus on the individual patient, epidemiologists focus on the community or the collective health of a population. For example, in a diarrheal disease outbreak, the clinician treats the individual, while the epidemiologist identifies the source, the number of exposed persons, the potential for community spread, and interventions to prevent recurrence.
• Application: Epidemiology involves applying knowledge to community-based practice. It is considered both a science and an art, requiring scientific methods, experience, judgment, and an understanding of local conditions to "diagnose" the health of a community.

Common Uses of Epidemiological Information

• Assessing Community Health: Public health officials use descriptive epidemiology as a factual framework for decision-making. Important questions include:
  • What are the actual and potential health problems in the community?
  • Where are they occurring and which populations are at increased risk?
  • Which problems are declining, and which are increasing?
  • How do these patterns relate to the distribution of available public health services?
• Making Individual Decisions: Epidemiological risk assessments influence daily choices such as quitting smoking (based on the $1950s$ findings on lung cancer), engaging in exercise/diet (based on $1970s$ heart disease studies), or using condoms (based on mid-$1980s$ findings on HIV risk).
• Completing the Clinical Picture: Epidemiologists contribute to the understanding of the natural history of disease. Examples include:
  • Identifying eosinophilia-myalgia syndrome in late $1989$ after reports of unexplained eosinophilia and myalgias.
  • Characterizing SARS (Severe Acute Respiratory Syndrome) caused by a new coronavirus in China in late $2002$.
  • Identifying conditions associated with cigarette smoking, ranging from pulmonary disease to throat and lung cancer.
• Searching for Causes: Searching for causal factors to enable public health action. Historical examples include John Snow removing the Broad St. pump handle in $1854$ to stop cholera, the withdrawal of a rotavirus vaccine in $1999$ due to intussusception risk, and the identification of risk factors for Legionnaires' disease in $1976$ before the bacillus was isolated in a laboratory.

Core Epidemiologic Functions

• Public Health Surveillance: The ongoing, systematic collection, analysis, interpretation, and dissemination of health data ("information for action"). It portrays patterns of disease occurrence so prevention measures can be applied efficiently.
  • Sources: Morbidity and mortality reports from health-care providers, laboratories, and infection control practitioners. Other sources include case investigations, immunization coverage data, disease registries, and health surveys.
  • Competencies: Designing data collection instruments, data management, descriptive methods, graphing, and scientific writing.
• Field Investigation: Actions resulting from surveillance reports, ranging from phone calls to verify cases to large-scale investigations to characterize epidemics. It often involves identifying unrecognized cases (e.g., sexual partners in STD investigations) or identifying a vehicle of infection (e.g., ground beef or fruit juice for Escherichia coli $O157:H7$).
• Analytic Studies: Used when more rigorous methods are needed to evaluate the credibility of hypotheses generated by surveillance or descriptive epidemiology.
  • Design: Determining research strategy, writing protocols, calculating sample sizes, and choosing comparison groups.
  • Conduct: Securing clearances, adhering to ethics, interviewing subjects, and data management.
  • Analysis: Calculating rates, creating two-by-two tables, computing risk ratios, odds ratios, tests of significance (chi-square), confidence intervals, and advanced modeling.
  • Interpretation: Putting findings into perspective and making recommendations.
• Evaluation: Determining the relevance, effectiveness, efficiency, and impact of public health activities.
  • Effectiveness: Ability of a program to produce expected results in the field.
  • Efficacy: Ability to produce results under ideal conditions.
  • Efficiency: Ability to produce results with minimum expenditure of time and resources.
• Linkages: Field epidemiology is a "team sport," requiring multidisciplinary collaboration between epidemiologists, laboratorians, sanitarians, nurses, and computer specialists. It involves formal (Memoranda of Understanding) and informal networking.
• Policy Development: Epidemiologists provide input, testimony, and recommendations regarding control strategies, reportable disease regulations, and health-care policy.

Descriptive vs. Analytic Epidemiology

• Descriptive Epidemiology: Catalogs the circumstances surrounding a health event by person, place, and time. Its primary considerations are frequency and pattern.
  • The 5 W's: What (health issue), Who (person), Where (place), When (time), Why/How (causes/risk factors/transmission).
  • Variables: Includes age, education, socioeconomic status, race, gender, and behaviors (drug abuse, eating patterns).
• Analytic Epidemiology: Uses data from descriptive studies to test hypotheses about causation. It searches for the "Why" and "How" of disease occurrence.

Epidemiologic Study Designs

• Experimental Studies: The investigator determines the exposure through a controlled process (e.g., clinical trials). Participants are randomly assigned to receive an intervention (vaccine) or a placebo, and outcomes are tracked.
• Observational Studies: The epidemiologist observes exposure and disease status without intervention. The three main types include:
  • Cohort Study: Participants are categorized by exposure status and followed to see if they develop disease. The unexposed group serves as the comparison.
    • Prospective (Follow-up): Participants are enrolled before the outcome occurs (e.g., Framingham study of over $5,000$ residents).
    • Retrospective: Both exposure and outcomes have already occurred (e.g., $2004$ Pennsylvania cyclosporiasis outbreak linked to snow peas).
  • Case-Control Study: Investigators enroll a group with the disease (case-patients) and a group without the disease (controls) and compare previous exposures. The hepatitis A outbreak traced to green onions is an example.
  • Cross-Sectional Study: A sample of a population is enrolled and exposures and outcomes are measured simultaneously to assess prevalence at a specific point in time.

Concepts of Disease Occurrence and Causation

• Epidemiologic Triad: The traditional model for infectious disease consisting of three components:
  1. Agent: A microorganism (virus, bacterium, parasite) that must be present for disease, though its presence alone may not be sufficient. Factors include pathogenicity and dose.
  2. Host: The human who can get the disease. Susceptibility is influenced by genetics, nutrition, immunologic status, and behaviors.
  3. Environment: Extrinsic factors affecting the agent or opportunity for exposure, including physical (geology/climate), biological (insects), and socioeconomic (sanitation/crowding) factors.

Levels of Disease Occurrence

• Sporadic: Occurs only occasionally or in scattered incidents (e.g., typhoid fever in the U.S., with approximately $700$ cases per year).
• Endemic: Constant presence and stable frequency of a disease in a specific geographic location (e.g., malaria in parts of Africa).
• Hyperendemic: Present at a high and constant frequency that equally affects all groups in a population (e.g., hypertension or osteoarthritis).
• Holoendemic: High prevalence in early childhood, infecting most children, so children show evidence of disease more often than adults (e.g., malaria in certain communities).
• Epidemic: Larger-than-expected number of cases occurring over a short time in a geographic region (e.g., Influenza).
• Pandemic: An outbreak affecting a large percentage of the global population over a vast geographic area.
  • Black Death (Black Plague): $1347$ to $1351$, caused by Yersinia pestis via rat fleas, killing $75$ to $200$ million people in Europe and Asia.
  • $1918$ Influenza Pandemic: Over $500$ million infected, $50$ million deaths.
  • AIDS (HIV): Destroys CD4 helper T cells, making patients susceptible to infections. Risk factors include lack of education, unsafe blood transfusions, and IV drug use.
  • COVID-19: Caused by SARS-CoV-2. Originating in China in $2019$, it was declared a pandemic by the World Health Organization (WHO) in March $2020$.

Global Health Organizations and Research Challenges

• World Health Organization (WHO): A specialized agency of the United Nations that provides leadership, creates policy, assesses trends, and monitors statistics for diseases like malaria (e.g., insecticide-treated nets, drug funding, vaccine research).
• Research Error: Epidemiological studies are prone to bias because they occur in natural settings. Major problems include:
  • Selection Bias: Errors in selecting study populations.
  • Information Bias: Poor quality of health data.
  • Confounding: Factors that lead to errors in data interpretation.

Questions & Discussion

Question: What are the different factors that affect the size of human population? Answer: Population size is influenced by factors such as birth rates, death rates, and migration (immigration and emigration).

Question: Enumerate the basic components of population change. Answer: The basic components are fertility (births), mortality (deaths), and migration.

Question: Is population growth essential in the study of public health? Why? Answer: Yes, population growth is essential because it dictates the demand for public health services, influences the spread of diseases (through density and urbanization), and helps in planning for health infrastructure and resource allocation.

Question: Which of the following is used by public health officials to set health goals for the nation? Answer: A. epidemiologic data. Rationale: Epidemiologic data provides the baseline and factual framework necessary to set measurable health goals, such as those set for the nation in $2000$ and $2010$.