🦠Epidemiology Unit 2 – Measures of Disease Frequency

Key Concepts

• Measures of disease frequency quantify the occurrence and distribution of diseases in a population
• Incidence and prevalence are two fundamental measures used to describe disease frequency
• Incidence measures the rate of new cases of a disease over a specified time period (usually expressed as cases per person-time)
• Prevalence measures the proportion of a population affected by a disease at a specific point in time or during a given period (usually expressed as a percentage)
• Risk ratios and odds ratios compare the likelihood of disease occurrence between exposed and unexposed groups
• Understanding the difference between incidence and prevalence is crucial for interpreting epidemiological data
  • Incidence focuses on new cases, while prevalence includes both new and existing cases
• Measures of disease frequency help identify risk factors, plan public health interventions, and allocate healthcare resources

Types of Measures

• Incidence measures:
  • Cumulative incidence (risk): proportion of a population that develops a disease over a specified time period (e.g., 5-year risk of breast cancer)
  • Incidence rate (incidence density): number of new cases per person-time at risk (e.g., 10 cases per 1,000 person-years)
• Prevalence measures:
  • Point prevalence: proportion of a population with a disease at a specific point in time (e.g., prevalence of diabetes on January 1, 2023)
  • Period prevalence: proportion of a population with a disease during a specified time period (e.g., annual prevalence of influenza)
• Ratio measures:
  • Risk ratio (relative risk): ratio of disease risk in exposed group compared to unexposed group
  • Odds ratio: ratio of odds of disease in exposed group compared to unexposed group
• Attributable risk measures the excess risk of disease in an exposed group compared to an unexposed group

Incidence Rates

• Incidence rates measure the occurrence of new cases of a disease over a specified time period
• Calculated as the number of new cases divided by the total person-time at risk: $\text{Incidence Rate} = \frac{\text{Number of New Cases}}{\text{Total Person-Time at Risk}}$
• Person-time accounts for the varying lengths of time individuals are observed or at risk of developing the disease
• Incidence rates are often expressed as cases per 1,000, 10,000, or 100,000 person-years
• Age-specific incidence rates can be calculated to compare disease occurrence across different age groups
• Standardized incidence rates (e.g., age-standardized rates) allow for comparisons between populations with different age structures
• Incidence rates are useful for studying disease etiology, identifying risk factors, and evaluating the effectiveness of interventions

Prevalence

• Prevalence measures the proportion of a population affected by a disease at a specific point in time or during a given period
• Point prevalence is calculated as the number of existing cases divided by the total population at a specific point in time: $\text{Point Prevalence} = \frac{\text{Number of Existing Cases}}{\text{Total Population}}$
• Period prevalence is calculated as the number of existing cases divided by the average population during a specified time period: $\text{Period Prevalence} = \frac{\text{Number of Existing Cases}}{\text{Average Population}}$
• Prevalence is often expressed as a percentage or proportion
• Prevalence is influenced by both the incidence and duration of a disease
  • Chronic diseases (e.g., diabetes) tend to have higher prevalence than acute diseases (e.g., influenza)
• Prevalence data is useful for estimating disease burden, planning healthcare services, and allocating resources

Risk Ratios and Odds Ratios

• Risk ratios (relative risks) compare the risk of disease in an exposed group to the risk in an unexposed group: $\text{Risk Ratio} = \frac{\text{Risk in Exposed Group}}{\text{Risk in Unexposed Group}}$
• Odds ratios compare the odds of disease in an exposed group to the odds in an unexposed group: $\text{Odds Ratio} = \frac{\text{Odds of Disease in Exposed Group}}{\text{Odds of Disease in Unexposed Group}}$
• A risk ratio or odds ratio greater than 1 indicates an increased risk of disease in the exposed group, while a value less than 1 indicates a decreased risk
• Risk ratios are more intuitive and easier to interpret than odds ratios, especially for common outcomes
• Odds ratios approximate risk ratios when the disease is rare (<10% prevalence) in both exposed and unexposed groups
• Confounding factors should be considered when interpreting risk ratios and odds ratios, as they may influence the observed associations

Interpreting Disease Frequency Data

• Incidence rates provide information about the speed at which new cases of a disease occur in a population
  • Higher incidence rates suggest a greater risk of developing the disease
• Prevalence data reflects the overall burden of a disease in a population at a given time
  • Higher prevalence indicates a greater proportion of the population is affected by the disease
• Changes in incidence rates over time can indicate changes in disease risk factors or the effectiveness of interventions
• Differences in prevalence between populations can be due to variations in incidence, disease duration, or population characteristics
• Risk ratios and odds ratios quantify the strength of association between an exposure and a disease
  • Values farther from 1 (in either direction) indicate a stronger association
• Confidence intervals and p-values should be considered when interpreting the statistical significance of risk ratios and odds ratios

Practical Applications

• Monitoring disease incidence and prevalence helps public health officials identify outbreaks and track disease trends (e.g., COVID-19 surveillance)
• Comparing incidence rates across different populations or time periods can help identify risk factors and health disparities (e.g., higher incidence of lung cancer in smokers)
• Prevalence data is used to plan healthcare services and allocate resources based on the burden of disease in a population (e.g., estimating the need for diabetes care)
• Risk ratios and odds ratios are used to identify potential causal relationships between exposures and diseases (e.g., assessing the risk of cardiovascular disease in obese individuals)
• Measures of disease frequency inform the development and evaluation of public health interventions and policies (e.g., assessing the impact of vaccination programs on disease incidence)

Common Pitfalls and Limitations

• Incomplete or inaccurate data can lead to biased estimates of incidence and prevalence
  • Underreporting of cases, misclassification of disease status, or loss to follow-up can affect the accuracy of measures
• Differences in case definitions, diagnostic criteria, or surveillance methods can make comparisons between studies or populations challenging
• Incidence rates may not capture the full impact of diseases with long latency periods or recurrent episodes
• Prevalence can be influenced by factors unrelated to disease risk, such as changes in population size or composition
• Risk ratios and odds ratios can be affected by confounding factors, which may distort the true association between an exposure and a disease
  • Adjusting for potential confounders is important to obtain unbiased estimates
• Measures of disease frequency alone do not provide information about the severity, duration, or impact of diseases on individuals and populations