epidemiology

definition of epidemiology

the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems

• research and action

distribution

frequency and pattern

frequency

number of health events with respect to the population

pattern

time, place, and person 

determinants

causes and other risk factors that influence disease

descriptive vs. analytic epidemiology

• descriptive describes occurrence: who, what, when, where, forms hypothesis

• analytic searches for determinants: tests hypothesis, why??

hallmark of analytic epi

control group

scopes of epidemiology

infectious disease, non-infectious disease, genetic conditions

infectious diseases

notifiable diseases - 60 required by law to be reported

non infectious disease

birth defects, chronic conditions (Alzheimer’s disease, diabetes, cancer, injuries)

clinician (2)

• individual focus

• provide treatment

epidemiologist (4)

• population focus

• identify exposures

• many people or population

• prevent spread

notable figures in epidemiology

hippocrates, john graunt, william farr, john snow

hippocrates

environmental and personal factors (not supernatural)

john graunt

mortality data: quantified patterns

william farr

vital statistics

john snow

london cholera outbreak

john snow 2 methods of investigation

• spot map

• investigating water companies; Southwark & Vauxhall areas have higher cholera rates while Lambeth was upstream

which area should Snow investigate

BOTH

framingham

cardiovascular disease among residents of a town

doll and hill

smoking and lung cancer

uses for epidemiological information (4)

• assess the community

• individual decisions

• complete clinical picture

• determine causes

core epidemiologic functions (6)

• public health surveillance

• field investigation

• analytic studies

• evaluation

• linkages

• policy development

public health surveillance nickname

information for action

what does public health surveillance involve (3)

• ongoing data collection

• analysis

• dissemination

what are sources of public health surveillance (5)

• health department reports

• reports from investigations

• public health program data

• disease registries

• health surveys

morbidity

cases of illness/disease

mortality

deaths

field investigation methods (3)

phone calls, survey, interviews

field investigation nickname

shoe leather epidemiology

4 components of analytic studies

• design

• conduct

• analysis

• interpretation

effectiveness vs. efficacy vs. efficiency

effectivenes: actual conditions

efficacy: ideal conditions

efficiency: minimal time and resources

linkages

multidisciplinary collaboration

case definition

set of standard criteria for classifying whether a person has a particular disease, syndrome, or other health condition 

what is the purpose of a case definition

consistency and comparison

how can case definitions vary (2)

by emerging diseases, and by location

what do case definitions always include

symptoms, signs, lab confirmation NOT always required

symptoms vs. signs

subjective complaints, while signs are objective, physical findings

levels of certainty for cases

suspect, possible, probable, confirmed, not a case

• not a case is most definite

sensitive case definition

broad or loose

sensitive case advantage and disadvantage

• advantage: catches ALL possible cases

• disadvantage: includes a lot of non-cases

strict case definition

• narrow

strict case advantage and disadvantage

• advantage: only true cases

• disadvantage: might exclude some actual cases

incidence rate

new cases in a period

• (number of new cases of disease occurring during a specific time)/(total number of people exposed)

• risk of getting the disease

• high ir means population has greates risk of disease

prevalence rate

• all cases (new + old)

• (number of existing cases)/(total population)

• burden of disease in a population

• high prevalence may mean it’s chronic

experimental vs. observational

• experimental: actually manipulating, clinical trial

• observ: not manipulating, cohort, cross sectional, case control

cohort

group chosen by exposure and not the disease

• control group would not be exposed to disease

• can either be retrospective or prospective

case control

subjects chosen based on disease outcome, ALWAYS retrospective

• control group is without the disease

cross sectional

frozen snapshot of exposures and disease in a given population at a given time, more suited to descriptive epi, indicates prevalence

rothman’s causal pies

better for non-infectious diseases with numerous factors

sufficient cause

whole pie, complete set of causes that together can produce the disease

component cause

slice in a pie, one factor among others

necessary cause

required factor, must be present for disease to occur

stage of susceptibility end

exposure happens

middle of subclinical disease

pathologic changes

end of subclinical disease

onset of symptoms

middle of clinical disease

usual time of diagnosis

infectivity

proportion of exposed who become infected

pathogenicity

proportion of infected to develop apparent symptoms

virulence

proportion of clinically apparent diseases that are severe or fatal

carriers

infected individuals who carry and transmit disease without symptoms 

incubatory

before they show symptoms

convalescent

recovered but still contagious

chronic

harbor and transmit disease long term

chain of infection

reservoir, portal of exit, mode transmission, portal of entry, susceptible host

fomites

indirect vehicles of transmission

vectors

insects, rodents, indirect vehicles of transmission

outbreak

epidemic limited in an area

incidence rate among exposed

a/a+b

incidence rate among unexposed

c/c+d

rate difference (RD)

a/a+b - c/c+d

relative risk (RR)

a/a+b -/- c/c+d