INCIDENCE

• Epidemiology


  Study of factors determining disease occurrence and distribution in a population
• Epidemiology vs Clinical Medicine

  • Epidemiology - data is from groups/populations, level of populations in the scientific study of disease
  • Clinical Medicine - single patient, level of individual patients in the study of disease
• Classical Epidemiology


  • Studies distribution and determinants of disease
  • Studies origins of health problems in communities, mostly infectious agents, others include nutrition, environment, behavior, economics, etc.
  • Determines risk factors to be altered to prevent/delay disease, injury, or death
• Clinical Epidemiology


  • Epidemiology applied in clinical medicine
  • Setting is in health care facilities vs community (classical)
  • Studies are done to improve management of patients
• Infectious Disease Epidemiology


  • Historically depended on lab sciences/laboratory (i.e., microbiology, serology) but now also uses statistical analysis
• Chronic Disease Epidemiology


  • Depended on complex sampling and statistical methods but now also uses molecular laboratory markers
• Some diseases are both infectious and chronic at the same time, i.e., TB, AIDS
• Natural History of Disease

  The way a disease progresses in the absence of medical or public health intervention
• Interventions


  • May be preventive or therapeutic
  • Alter the natural history of a disease in a favorable way
• Disease stages


  • Pre-disease stage - before the disease process begin; primary preventive measures intervene to avoid exposure to agent of disease
  • Latent stage - disease process started but without symptoms; secondary prevention measures intervene to prevent/delay progression to symptomatic
  • Symptomatic stage - disease manifestation is evident; tertiary prevention measures slow, arrest, or reverse progression of disease
• Epidemiologic Triangle


  Cause of disease are considered in terms of the HOST, AGENT, and ENVIRONMENT, plus the VECTOR
• Host


  The "who" in the epidemiologic triangle; the degree to which an individual can adapt to stressors in the environment
• Agent


  The "what" in the epidemiologic triangle; what causes the disease (biological agents, chemical, physical agents, social and psychological stressors)
• Environment


  The "where" in the epidemiologic triangle; external factors allowing for disease transmission; influence the probability and circumstances of contact between the host and the agent
• Vector


  Transmitter of disease; has specific relationship with the agent, the environment, and the host
• The BEINGS Model


  • Biologic, behavioral factors
  • Environmental factors
  • Immunologic factors
  • Nutritional factors
  • Genetic factors - the most difficult to change
  • Health services, social factors, spiritual factors
• Incidence


  Number of transitions from well to ill, from uninjured to injured, or from alive to dead in the study population during the study period
• Prevalent case


  Different from prevalence rate
• Period Prevalence


  Point prevalence at the start of the interval + incidence during the interval
• Prevalence can increase due to increase in yearly number of new cases or increase in length of time that symptomatic patients survive (before dying or recovering)
• Point vs Period


  • Point - specific point in time
  • Period - specific time interval; the sum of points during the interval
• Cumulative incidence


  The total number of cases of an epidemic disease reported over time
• Risk


  Proportion of those unaffected at the start of the study period but had a risk event during the study period
• Cohort subsets (levels) of population (from specific to general)


  • Persons who died
  • Persons who fell ill
  • Number of those infected
  • Number of exposed persons
  • Number of susceptible persons
  • Total population
• Case Fatality Ratio


  Proportion of clinically ill who died
• Pathogenicity of organism


  Proportion of those infected who are clinically ill
• Infectiousness of organism


  Proportion of exposed persons who become infected
• Rates


  The number of events that occur in a defined time period, divided by the average number of people at risk for the event during that period
• Prevalence Rate


  The proportion of persons with a defined disease/condition at the time of the study
• Incidence Density


  The number of new events per person-time (e.g. of person-time is person-months, person-years)
• 3 Broad Categories of Rates

  • Crude - applies to entire populations without reference to individual components of the population
  • Specific - population is divided into subgroups based on a common characteristic
  • Standardized - "adjusted rates", are modified crude rates controlling for effects of age or other characteristic in order to allow for valid conclusions to be made
• Direct Standardization


  Most common; removes effects of different age structures
• Indirect Standardization


  Used if ASDRs are not available, and if the population to standardize is small
• Cause-Specific Rates


  Comparing CSRs are risky! Differences in diagnostic standards, in availability of health services, etc. makes conclusions invalid. Always be mindful of the context and these differences whenever possible.
• Maternal and Child Health Rates

  • Livebirth
  • Fetal death - early, immediate, late
  • Infant death - neonatal, postnatal
  • Perinatal death - stillbirths + <7 days old, Perinatal Mortality Ratio, Hebdomadal Death