Lecture 2: Descriptive Epi and Measures of Disease Freq Flashcards
2 aspects of DISTRIBUTION of a disease
Frequencies and Patterns
Frequency of disease occurrence considers
Counts in relation to size of a population
3 aspects of PATTERNS of disease occurrences
Person, Place, Time
Distribution of disease= __ Epidemiology
Descriptive Epi (who, where, when)
When to use Descriptive Epidemiology
When occurrence of disease is more frequent than usual or greater compared to other areas.
3 types of surveillance systems
Passive, active, syndromic
System waits for disease/conditions to come to them
Passive Surveillance System
Search through communities for new cases of disease/conditions
Active Surveillance System
Focus is looking for signs/symptoms that could lead to disease/conditions
Syndromic Surveillance System
subtype of Syndromic Surveillance; looks for signs/symps not just in humans, but also in animals, plants, environment.
Biosurveillance
Induction/Incubation period
Time between exposure and onset (disease process starts) of disease
Latency period
Time between onset and detection of disease
Timeline for Disease
- Susceptability
- 5 (Exposure)
- Subclinical changes (pathologic changes occur)
- 5 (Onset of symptoms)
- Clinical Stage (usual time for diagnoses)
- Recovery, Disability, or Death stage
Element that must be defined before ‘Who’
Case Definition
Define disease/condition for public health surveillance
Case Definitions
Department that makes case definitions (infectious and non)
Council of State and Territorial Epidemiologists (CSTE)
Department that collects specific types of disease from subnational (state, local, etc) health departments in order to monitor, control, prevent disease
National Notifiable Diseases Surveillance System (NNDSS)
Confirmed vs Probable case defs
Probable-likely to have condition but key factor for accurate diagnosis has not been met
Confirmed-key factor for diagnosis has been met
Newsletter based from NNDSS that reports notifiable diseases at the national level
Morbidity and Mortality Weekly Report (MMWR)
Disease occurrence is higher than normal.
Epidemic
Localized higher than normal increase in disease occurrence (smaller scale compared to epidemic)
Outbreak (aka cluster)
Higher than normal disease occurrence is a norm in a given area.
Endemic
Pre-pandemic alert that epidemic should be taken under cautionary measures
Emergency of International Concern
Epidemic spread world-wide
Pandemic
Graphical study monitoring outbreak/epidemic cases over time and is used to form hypotheses of occurrences; incorps Descriptive Epi (3 W’s)
Epidemic Curve
3 visual factors an epidemic curve presents
- Magnitude (number of cases, vertical axis)
- Timing (occurrences over time; horizontal axis)
- Pattern (if disease is continuously spreading or not; shape of curve)
Index or sentinel case/case zero
First documented person with onset of disease
2 aspects of PATTERNS in an epicurve
- Point source-common, single source that passes the disease continuously or intermittently; not propagated (ex. broadstream pump)
- Propagated- person-person spread
Hypotheses from epicurves include understanding
- transmission route
- probably exposure period (graph, begins at point 0)
- incubation period
No sentinel case; continuous over time, not propagated or repeated
common/point source graph with no exposure
Yes sentinel case, continuous over time, not propagated or repeated
common/point source graph with index case
There is repetition of disease occurrence
Common/point source graph with intermittent outbreak
Yes sentinel case, there is propagation of infected infecting others (secondarily) and others infect others (spread)
person-to-person outbreak
Maximum incubation period
period between time of exposure and last incident case
Average incubation period
Period between exposure and highest peak of a continuous epicurve
Minimum incubation period
Period between exposure and first detected incident cases
3 numerical measures of disease frequencies
Proportions, Ratios, and Rates
Proportion
part over whole; simple percentage; numerator is related to the denominator (ex. same individs in population)
Ratio
Division of two unrelated numbers (ex. different individs from a population of interest)
Rate
Proportion with time incorporated in the denominator
3 key factors when comparing disease freqs BETWEEN groups. If factors 2 and 3 are not the same between groups, what do we do?
- number of people who are infected (count)
- size of source pop (those who have disease) OR those at risk (those who do not have disease)
- Time-length that the population was followed
We need standardize values (have a common denom)
New cases of a disease
Incidence
Existing plus new cases
Prevalence
______ and ______ are proportions; they factor in the ‘at risk’ OR ‘base’ populations into the denominator
Incidence, prevalence
How does prevalence decrease?
infected persons have gotten cured or have died
How does prevalence increase?
infected persons are being treated, prevent from dying sooner
How do you prevent incidence?
Prohibit actively (vaccinations) or are immune
Equation for incidence
aka risk/attack rate
incidence=new cases/AT RISK population
(REMEMBER: in denom, subtract those who already have disease or are immune!)
When is the equation for incidence useful?
When numerator and denom are unchanging (non-dynamic)
When the incidence/risk/attack rate for a NON-DYNAMIC POPULATION is summed over multiple time periods
Cumulative incidence
What to do when we are evaluating a population
in which it would be near-impossible to determine
who IS and who ISN’T at risk during known time?
Will have to estimate the population.
