Epidemiology: Key Concepts

• Study notes for Epidemiology

Definition and Objectives

• Epidemiology studies disease occurrence patterns in different populations to identify causes, spread, and prevention strategies

Primary, Secondary, and Tertiary Prevention

• Primary: Prevents disease onset (e.g., vaccinations)
• Secondary: Early detection to halt disease progression (e.g., screening)
• Tertiary: Reduces impact of existing illness (e.g., rehabilitation)

Epidemiologic Triad of Disease

• Includes an external agent, a susceptible host, and an environment facilitating their interaction

Direct vs Indirect Disease Transmission

• Direct: Disease spread person-to-person via physical contact or droplets (e.g., influenza)
• Indirect: Transmission through intermediate objects or vectors (e.g., malaria via mosquitoes)

Preclinical, Subclinical, Clinical Persistent and Latent Disease

• Preclinical: Asymptomatic, destined to become clinical
• Subclinical: Asymptomatic, not destined to become clinical; diagnosis via serology or culture
• Clinical Persistent: Long-term or chronic symptoms
• Latent: Inactive/dormant infectious agent that can reactivate

Endemic, Epidemic, and Pandemic Diseases

• Endemic: Constant presence at baseline level in a specific area (e.g., chickenpox)
• Epidemic: Sudden increase in cases above expected levels (e.g., norovirus outbreak)
• Pandemic: Epidemic spread across countries or continents (e.g., COVID-19)

Types of Epidemic Curves

• Point Source: Sharp rise/fall, exposure limited to a single point
• Continuous Source: Gradual rise/fall, ongoing exposure
• Propagated Outbreak: Cases in waves, person-to-person transmission
• Intermittent Source: Multiple waves, disease source involvement at irregular intervals

Food-Specific Attack Rate

• Calculated as (Number of ill people who ate the food / Total number of people who ate the food) x 100

Surveillance

• Systematically collects and interprets health data for public health planning and evaluation

Passive vs Active Surveillance

• Passive: Relies on healthcare providers reporting cases; cost-effective, may underreport
• Active: Health authorities actively search for cases; more accurate, used in outbreaks

Incidence Rate and Cumulative Incidence

• Incidence Rate: New cases occurring in a population during a specific period.
• Formula: (Number of new cases during the time period / Population at risk during the time period) x 100,000
• Cumulative Incidence: Probability of an event occurring within a specified period
• Formula: (Number of new cases during the time period / Number of individuals at risk at the beginning of the period) x 100

Person-Time

• Used in studies following individuals over time with varying contribution lengths
• Used to calculated rates for longer periods
• Used cases studies where people leave or are added at different times

Attack Rate

• Incidence rate for outbreak investigations
• Proportion of at-risk population developing disease during an outbreak

Prevalence

• Proportion of individuals with a disease at a specific time
• Formula: (Number of existing cases at a particular time / Total population at the same time) x 100

Point vs Period Prevalence

• Point Prevalence: Disease proportion at a specific point in time
• Period Prevalence: Proportion during a specified time, including existing and new cases

Prevalence, Incidence, and Duration

• Formula: Prevalence = Incidence x Duration of Disease
• Requires stable disease incidence and population size

Absolute Number

• Raw count of events (e.g., deaths, disease cases)
• Does not account for population size or risk

Mortality Rate

• All-Cause Mortality Rate: total deaths from any cause
• Formula: (Total deaths in a year / Mid-year population) × 100,000
• Cause-Specific Mortality Rate: deaths from a specific cause
• Formula: (Deaths from a specific cause in a year / Mid-year population) × 100,000
• Sex-Specific Mortality Rate: mortality rates by sex
• Formula: (Total deaths in a specific sex group / Total population of that sex) × 100,000
• Age-Specific Mortality Rate: death rate for specific age groups
• Formula: (Deaths in a specific age group / Total population in that age group) × 100,000
• Mid-year population is used for a more accurate estimation of average population size throughout the year

Mortality Rate vs Case-Fatality

• Mortality Rate: Overall deaths in a population
• Case-Fatality Rate: Severity of disease, percentage of deaths among those with the disease.
• Formula: (Number of deaths from a disease / Number of people diagnosed with the disease) × 100

Proportionate Mortality

• Percentage of total deaths caused by a specific disease
• Formula: (Deaths from a specific disease / Total deaths from all causes) × 100

Direct vs Indirect Adjustment

• Direct: Uses age-specific mortality rates and a standard population to adjust for age differences
• Indirect: Uses standard population's rates to estimate expected deaths when age-specific rates are unavailable

Adjustment Procedures

• Direct Standardization: Applies age-specific rates to a standard population
• Indirect Standardization: Compares deaths vs base population
• Age Adjustment: Adjusts for differences in age distribution
• Risk Adjustment: Accounts for additional factors when comparing health outcomes

Validity

• Diagnostic Accuracy: Measures diagnostic accurately with disease presence or absence

Sensitivity, Specificity, PPV, NPV

• Sensitivity: Ability to correctly identify those with the disease (true positive rate)
• Specificity: Ability to correctly identify those without the disease (true negative rate)
• Predictive Value Positive (PPV): Probability those with a positive test have the disease
• Predictive Value Negative (NPV): Probability those with a negative test do not have the disease
  • Sensitivity = a / (a + c)
  • Specificity = d / (b + d)
  • PPV = a/(a + b)
  • NPV = d / (c + d)
• PPV increases in higher prevalence of disease.
• NPV increases as the prevalence decreases.
• Sensitivity and specificity are inherent test properties not influenced by prevalence.

Randomization

• Assigns participants to groups using random mechanisms, reducing selection bias

Randomized Controlled Trials (RCTs)

• RCTs typically involve at least two groups: intervention (treatment group), placebo or standard treatment (control group)
• Balances known/unknown variables for accurate study of intervention effects

Parallel Designs

• Fixed Parallel Design: Random assignment to treatment groups, each receiving one treatment
• Cluster Parallel Design: Randomization by group, each receives one treatment
• Crossover Parallel Design: Each participant receives multiple treatments, serving as their own control

Stratified Randomization

• Dividing participants into strata based on characteristics like age and disease severity
• Ensures balanced treatment groups relative to key characteristics

Efficacy

• Intervention performance under ideal, controlled conditions
• Used in studies in monitored clinical trials

Effectiveness

• Intervention performance in real-world conditions

Number Needed to Treat (NNT)

• Number of patients needed to be treated to prevent one additional bad outcome.
• NNT = 1 / (Control event rate - Treatment event rate)

External vs Internal Validity

• External Validity: Generalizability of study results to other situations/people
• Internal Validity: Ability to establish a causal relationship, minimizing biases

Cohort Study

• Observational study following groups to assess exposure-outcome associations
• A cohort study starts with the exposure, risk factors are tracked to determine if the tested population will develop issues later
• The population in a cohort study is not randomized unlike randomized controlled trials (RCTs)

Measures of Association

• Relative Risk (RR): Risk in exposed group vs unexposed group. RR = [Risk of disease in exposed group] / [Risk of disease in unexposed group]
• Risk Difference (RD): Absolute risk difference between groups. RD = [Risk of disease in exposed group] - [Risk of disease in unexposed group]
• RR is a measure of risk, more than 1 suggests a chance of increased disease, and less than 1 a suggests a protective effect

Cohort Study Advantages and Disadvantages

• Good for rare exposures and examining multiple outcomes
• Good for understanding the timing or events
• Can be expensive and time-consuming

Case Control Studies

• Compares people who have the disease (cases) with similar people who don't have the disease (controls)
• A case-control study starts with the disease (outcome), after its determined individuals with the disease and those without is studied

Odds Ratio

• Odds Ratio: Odds of exposure with disease to without (formula)
• Helps determine the odds, with high levels suggesting an increased risk of illness

Case Control Study Advantages and Disadvantages

• Cost and resource efficient, for quick data collection
• Vulnerable to recall bias, with difficulty determining a time relationship