Epidemiological Measures Overview

Epidemiological Measures

• Epidemiology is the study of the distribution and determinants of health-related states or events in a specified population, and the application of this study to the control of these health problems.
• Epidemiology focuses on frequency and pattern of health events in a population.
• Frequency considers the number of health events (e.g., meningitis cases, diabetes cases) and the relationship of that number to the population size (rate).
• Pattern examines the occurrence of health-related events by person, place, and time. Time patterns can be annual, seasonal, daily, etc. Place patterns include geographic variation, urban/rural differences, and work/school locations.
• Personal characteristics include demographic factors linked to illness/injury risk (age, sex, marital status, socioeconomic status) as well as behaviors and environmental exposures.

Determinants

• Epidemiology is used to identify determinants (causes and other factors influencing disease/health events).
• Analytic epidemiology/epidemiologic studies explain the "why" and "how" of health events.

The 5Ws of Epidemiology

• What: Health issue of concern
• Who: Person affected
• Where: Location of the issue
• When: Time of the event
• Why/how: Causes, risk factors, transmission

Time Distribution

• Disease patterns are described by the time of occurrence (week, month, year, hour of onset).
• Time patterns include seasonality, periodic increase/decrease, and consistent trends.

Three Kinds of Time Trends/Fluctuations

• Short-term fluctuations: Epidemics are the best known short-term fluctuations.
• Periodic fluctuations (seasonal trends): Diseases like measles and varicella have seasonal peaks. Upper respiratory infections peak in winter, and bacterial gastrointestinal infections are common in summer.
• Long-term (secular) trends: Diseases like coronary heart disease, lung cancer, and diabetes have shown consistent increases in developed countries over the past 50 years

Types of Epidemics

• Common-source epidemics: Single exposure (point-source) – rapid, simultaneous exposure; continuous/multiple exposure – prolonged exposure from a common source
• Propagated epidemics: Person-to-person; arthropod vector; animal reservoir

Common Source - Single Exposure Epidemics

• Exposure to disease agent is brief and simultaneous.
• Epidemic is explosive; cases cluster within a short time frame.
• All exposed individuals typically develop the disease within the same incubation period.

Herd Immunity

• Herd immunity is the resistance of a group of people to a disease when a large proportion are immune.
• Presence of many immune individuals reduces the likelihood of susceptible individuals encountering the disease.

Periodic Fluctuations (Cyclic Trend)

• Some diseases occur in cycles (days, week, months or years). Examples include measles (2-3 years), rubella (6-9 years). Naturally occurring variations in herd immunity affect cycles.

Long-term or Secular Trends

• Secular trends show consistent changes in disease occurrence over years or decades (e.g., increasing or decreasing).

Place Distribution (Geographical Comparisons)

• Geographic patterns are an important source for understanding disease causes.
• These patterns can be examined in relation to factors like socio-economic factors, dietary differences, and cultural/behavioral variations (international, national, rural-urban, and local).

Rural-Urban Variations

• Chronic bronchitis, accidents, lung cancers, cardiovascular diseases, and mental illnesses are more common in urban areas than rural areas.
• Some diseases (skin/zoonotic diseases) are more frequent in rural areas than urban areas.

Person Distribution

• Disease occurrence varies by age, sex, occupation, marital status, habits, social class, and other host factors.
• Measles is a childhood disease; cancers are more common in middle age; atherosclerosis tends to occur in older individuals.
• Some diseases (diabetes, hyperthyroidism, obesity) are more common in women; lung cancer and coronary heart disease are more frequent in men.

Epidemiological Application Examples

• Activities like comparing food histories of people with and without Staphylococcus food poisoning, mapping birth defects near hazardous waste sites, graphing syphilis cases per year, or tabulating chickenpox cases in a specific region.

Endemic

• Endemic means a disease's constant presence in a geographic area or population (without external introduction).
• Endemic may relate to the typical frequency of a disease in a region or population.
• Hyperendemic refers to constant presence of a high incidence/prevalence affecting all age groups.

Sporadic

• Sporadic disease occurs irregularly, haphazardly and less frequently. Examples include polio, tetanus, herpes-zoster, meningococcal meningitis.
• A sporadic disease may evolve into an epidemic if conditions become favourable for its spread.
• Many zoonotic diseases manifest as sporadic infections in humans.

Epidemic

• An epidemic is an unusual occurrence of a disease, or specific health event, in a community or region clearly exceeding expectations.
• An "outbreak" is a small, localized epidemic.
• Epidemics can include diseases like measles, chickenpox, cholera.
• Some modern infectious diseases, like specific cancers, may take several years to manifest across a population.

Pandemic

• A pandemic is a widespread epidemic that occurs across a wide geographic area or globally.

Control, Elimination, Eradication

• Control: Reducing disease incidence, prevalence, morbidity, or mortality to an acceptable level via deliberate efforts (requires continued intervention)
• Elimination: Interrupting transmission to zero of a specific pathogen within defined geographical areas (minimal risk of reintroduction)
• Eradication: Permanent reduction of a pathogen to zero in nature or in laboratories, without any risk of reintroduction.
• Extinction: Eradication of the pathogen that cease to exist (either naturally or deliberately).

Surveillance

• Surveillance is the ongoing systematic collection, analysis, and interpretation of health data.
• It's essential for planning, implementing, and evaluating public health policies aimed at preventing and controlling illness and injury.
• Surveillance is vital for detecting changes in the environment/health status of populations and for assessing the effects of interventions (useful evidence).

Passive Surveillance

• Passive surveillance uses available data and mandates disease reporting.
• Reporting responsibility often rests with health care providers or district health officers.
• Completeness/quality of data depends on their efforts and resources. Underreporting is common.

Active Surveillance

• Active surveillance involves staff actively seeking health information from providers or populations.
• It gives accurate, timely information but is costly.

Monitoring

• Monitoring involves routine measurements to detect environmental or population health status changes.
• This can involve measuring health program effectiveness, air quality, water quality, intrauterine growth, nutritional status, and health facility functions.