Epidemiology: Disease Prevalence and Incidence

• Epidemiology involves estimating the frequency and distribution of diseases in populations
• Epidemiology involves comparing the effects of suspected risk factors on disease frequency

Measures of Disease Frequency

• Measures of disease frequency are used to describe how common an illness is in a population within a specific time frame
• Key elements include cases, population size, and the time period of observation

Case Definition

• A case is a person in a population with a particular disease or experiencing an event of interest
• A case can be an infection, disease (morbidity), disability, death (mortality), recovery, presence of antibodies, or utilization of health services
• A case can be an event or outcome of interest

Prevalence and Incidence

• Prevalence and incidence are two measurements of disease frequency

Incidence

• Incidence shows the occurence of new cases
• Incident cases are new cases

Prevalence

• Prevalence shows the frequency of existing cases
• Prevalent cases are existing cases
• Point prevalence is the proportion of people in a defined population (N) with the outcome of interest (d) at a specific point in time
• HIV prevalence in adults in Africa in 2005 was 7.2%
• Point prevalence measures existing or prevalent cases of a disease
• Point prevalence is only measurable in cross-sectional studies
• The formula for point prevalence is: Point prevalence = d/N, where d is the number of cases in a defined population at one point in time and N is the total number of persons in the defined population at the same point in time

Incidence Details

• Incidence refers to the frequency of new disease cases in a defined population within a specified time
• Risk, odds, and rates are the three ways of measuring incidence
• Risk, odds, and rates are numerically similar if the disease is rare
• As an exmaple, 5% of students of Queen's Hall had at least one episode of malaria in the last academic year

Risk or Cumulative Incidence

• Risk refers to the proportion of disease-free persons who initially develop a disease during a specified time interval
• Risk is often called cumulative incidence
• Risk represents the probability of getting the disease in a given time period
• The formula for risk is: Risk (r) = d/N, where d is the number of new cases of disease in a given time period, and N is the total number of disease-free persons at the beginning of that time period
• Risk of disease in exposed (r1) = a/(a+b), where a is the number of exposed individuals with the disease and b is the number of exposed individuals without the disease
• Risk of disease in unexposed (r0) = c/(c+d), where c is the number of unexposed individuals with the disease and d is the number of unexposed individuals without the disease

Odds Calculation

Odds show the ratio of people who get the disease to people who do not get the disease

• Odds are rarely used to measure disease frequency
• The odds of disease to non-disease is the ratio of people who get the disease (d) to people who do not get the disease (N-d)
• Formula: Odds = d / (N-d), where 'd' represents the number of new cases of disease in a given time period and 'N' represents the total number of persons who did not become a case during that time period
• Odds of disease in exposed = a/b, where a is the number of exposed individuals with the disease and b is the number of exposed individuals without the disease
• Odds of disease in unexposed = c/d, where c is the number of unexposed individuals with the disease and d is the number of unexposed individuals without the disease

Rates or Incidence Rates

• Rates are a measure of the frequency of new cases of a disease
• Rates relate the number of new cases to the person-time at risk (Y)
• The rate (R) is estimated by dividing the total number of events observed among all individuals, (d), by the sum of the individual observation times (Y)
• Formula: Rate (R) = d/Y, where 'd' represents the number of persons who have become cases in a given time period and 'Y' represents the total person-time at risk during that period

Estimating Population-Time at Risk

• There are 3 main methods
• Method 1: Add up the time that each person is at risk
  • This is the most common method
• Method 2: Add up the population at risk during each time segment
• Method 3: Multiply the average size of the population at risk by the length of the time interval
  • This is usually used for large populations
  • Multiply the average population at the mid-point of the calendar period of interest (mid-period population) by the number of years of the study

Uses of Incidence & Prevalence

• Incidence
  • Incidence is generally used for acutely acquired diseases
  • Incidence can be used to investigate causal relationships
  • Incidence needs a duration
• Prevalence
  • Prevalence is used for more permanent health conditions
  • Prevalence is used for assessing the public health impact of a specific disease within a community, estimating required services, and estimating cost and resources
  • Prevalence may or may not need a duration

Relative Measures of Association & Impact

• Concerned with relative measures
• Ratio of measures of disease frequency for two groups of subjects
• Probability of developing the disease in exposed individuals relative to those unexposed
• Estimate of the size of an association between exposure and disease
• Indicates how much more likely people in an exposed group are to develop the disease than those in an unexposed group
• 3 relative measures
  • Risk ratio
  • Rate ratio
  • Odds ratio

Risk Ratio

• Risk ratio = Risk in the exposed group divided by Risk in the unexposed group
• Risk ratio = a/(a+b) divided by c/(c+d)
• Where a is the exposed with disease, b is the exposed without disease, c is the unexposed with disease and d is the unexposed without disease

Rate Ratio

• Rate ratio = Incidence rate in the exposed group divided by Incidence rate in the unexposed group

Odds Ratio

• Odds ratio=Odds of disease in the exposed group divided by Odds of disease in the unexposed group
• Odds ratio = (a/b) / (c/d)
• Odds ratio = ad/bc, where a is the exposed with disease, b is the exposed without disease, c is the unexposed with disease and d is the unexposed without disease
• Risk ratio ≈ odds ratio ≈ rate ratio for a rare disease

Absolute Measures of PH Relevance

• Absolute measures of PH relevance are also called Absolute measures
• Measures the difference between measures of disease frequency in the two groups
• Includes absolute effect of exposure in those exposed compared with those unexposed
• Absolute measures indicate exactly what impact a particular disease/condition will have on a population
• Important implications for any public health prevention measures
• There are 4 main absolute measures of public health relevance
  • Attributable (absolute) risk -Attributable risk per cent
• Population attributable (absolute) risk
• Population attributable fraction

Attributable Risk

• Attributable risk is the excess risk caused by exposure in the exposed group
• Assumes that the relationship between the exposure and disease is causal
• The attributable risk is the excess risk (or rate) in the exposed due to exposure
• Attributable risk = risk in the exposed - risk in the unexposed

Attributable Risk Per Cent

• Attributable risk can be expressed as the proportion of disease in the exposed group attributable to the exposure (the proportion of additional cases)
• Attributable risk per cent is the attributable risk expressed as a percentage
• Formula = (Incidence in exposed group - Incidence in unexposed group) / Incidence in exposed group

Population Attributable Risk (PAR)

• Impact of an exposure on the whole population
• Equals excess rate in the population due to the exposure
• PAR = Incidence in the whole population - Incidence in the unexposed population
• Only applicable when the exposure is causal action

Population Attributable Fraction (PAF)

• Indicates the amount of disease in the population that could be prevented if the exposure to the risk factor were eliminated and the entire population was unexposed
• i.e. the proportion of cases that could be avoided if the exposure were removed from the population entirely
• PAF = Population attributable risk divided by Risk of disease in the population

Types of Heath Data

• Health statistics assess a population's health status by collecting, analyzing, and interpreting data about events
  • Deaths (mortality data)
  • Sickness (morbidity data)
• Use of health services
• Such data is important for health service planning, implementation, and evaluation

Sources of Health Data

• Vital statistics involve Records of vital events that are obtained by registration
• Morbidity statistics involve occurrence and severity of sickness in a community
• Health service statistics are obtained from the operations of health services
  • Resource data and Institutional records
• Data from other sectors
  • education (eg literacy rates)
  • public works (housing, water supply, sanitation)
  • agriculture (food production and distribution)
  • economic planning (poverty, economic indicators)

Examples of Sources

• Census
  • Local and national
  • Data includes total count, age, sex dist
• Epidemiological surveys, Questionnaires, Physical examination or Special investigations
  • Sickness survey,Nutritional survey and Serological or TB surveys
• Data includes Sickness, absence from work, Anthropometric measurements and Prevalence of HIV infection
• Medical installations of Outpatient or Special Clinics
  • Heath centre, ANC, STI and Diabetes clinics
  • Data includes Clinical records and Attendance records and health Profile

More Examples

• Inpatient Services
  • Gen & specialist hospitals
  • Data included is Clinical, lab, autopsy data
• Data collected for other purposes from Routine medical exams
  • Sch entrants, employment Data
  • Data included is Nutritional statuses, immunization status and health profile
• Sickness absence records from Schools and Industry are measured
  • Data included is used for Early warning system for epidemics