Epidemiology I Flashcards
5-year survival
the percentage of patients still living 5 years after diagnosis (or treatment)
-a proportion but often referred to as a rate
-used in clinical medicine when evaluating
treatment
-requires lengthy follow-up and cannot be used for
treatments less than 5 years old
acquired immunity
immunity due to deliberate actions to prevent disease
active immunity
immunity due to an individual’s active production of their own antibodies
age-adjustment
an approach for dealing with the problems of rates that are not comparable because the age distributions of the populations are different and the event being measured is associated with age
analytical epidemiology
uses comparison groups to quantify the associations between determinants and health problems to test hypotheses about causal relationships
Belmont report
provides guidance and recommendations for the protection of human research subjects by identifying 3 basic ethical principals (1979)
the quintissential reference in the US for the ethical treatment of research subject
created by the national commission for the protection o f human subjects in biomedical and behavioral research
case definition
a blueprint to determine who is truly
case report
individual level observations to describe a particular clinical phenomenon in a single patient
- main objective is to provide a comprehensive and detailed description of the case under investigation
- key hypothesis generating tools
- very limited external validity due to biased selection of cases
case-based surveillance
used to monitor the occurrence of disease by collecting information on individuals diagnosed as having the disease according to an official definition
case-fatality
the proportion of all people who have a disease who die from that disease
-includes no explicit statement of time
-generally used for acute diseases in which death
occurs relatively soon after diagnosis
chain of infection and 6 links
six links representing the steps of transmission of infection
- all must be present and occur in order for an infection to occur
- each represents an opportunity for disease prevention and control
- infectious agent
- reservoir
- portal of exit
- mode of transmission
- portal of entry
- susceptible host
characteristic epi curve of a continuous outbreak
curve is wider with a gradual increase in the number of cases, followed by a plateau and then a gradual decrease in the number of cases
characteristic epi curve of a point-source outbreak
shows a great spike in the number of cases that gradually declines
-becomes normally distributed when graphed
against log(time)characteristic epi curve of a propagated epidemic
the curve appears sporadic with several rapid increases and decreases that does not occur in any apparent pattern
classes of diseases deemed of considerable public health importance by the CDC and their mandatory report schedules
class A: of major public health importance and must be reported immediately
class b: of public health importance due to potential for epidemic spread and must be reported by the end of the next business day
class C: must be reported by the end of the next business day only when there is a suspected outbreak or unusual incident
cluster
a group of cases in a specific time and place that may or may not be greater than the expected rate of cases
cohort effect
trends in incidence or prevalence for persons who were born in a certain year as they are moving through time together
-examined by moving diagonally down and right
along a life table
colonization
the entry and development of an infectious agent that does NOT cause symptoms of disease
communicable period of disease
the duration of time when the level of pathogen is above critical mass and the disease is transmissible
conditions needed for herd immunity to be achieved
- the infectious agent must be restricted to a single host species
- transmission must be direct from one member of the host species to another
- infections must induce solid immunity
- there must be a random mixing of the population
-outbreaks often occur among small communities
of susceptible individuals
considerations for planning and implementing screening programs
- can the disease be detected early?
- what is the sensitivity and specificity of the test?
- what is the predictive value of the test?
- how serious is the problem of false-positive and false-negative results?
- what is the cost of early detection in terms of funds, resources and emotional impact
- are patients harmed by screening?
- Do patients benefit from the early detection of disease?
count as a type of measure
the number of cases of a disease or the number of health related events that occur
critical mass of a pathogen
level of pathogen where it becomes possible for the pathogen to be transmitted
-once reached, the host is considered infected
cut-point of a screening test
definitions that will differentiate between results that will be considered positive or indicative of the presence of disease and those that will be considered negative or indicative of the absence of the disease
-must be set if the test does not produce
dichotomous results
-should be based on biologic information
-should consider the consequences of false
positives and false negatives
-often based on the distribution of results
-there is a tradeoff between sensitivity and
specificity
declaration of Helsinki
the principal foundational document on the ethical principles that govern research involving human subjects which has influenced the development of formal regulations around the world (1979)
descriptive Epidemiology
studies the frequency and patterns of health problems in specific populations by person, place and time
1. who is at most risk for a disease?
2. what constitutes a health-related state or event that has become problematic?
where is a health problem most common?
when is a health problem most commong?
diagnosis
a definitive decision regarding the disease status of an individual
-typically occurs during the stage of clinical disease
direct age-adjustment
a standard population is used in order to eliminate the effect of any differences in age between two or more populations being compared
-the standard population used is arbitrary
-rates are only comparable when adjusted to the
same standard population
direct transmission
2 types
results from the direct and immediate transfer of a pathogen from a host/reservoir to a susceptible host
person-to-person: by means of direct human contact
perinatal: from mother to baby in utero, during deliver or post-partum
disability-adjused live year (DALY)
the number of years of life lost due to premature death and years lived with a disability of specified severity and duration
-1 DALY represents 1 lost year of healthy life
disease carrier
an individual who harbors an infectious agent but is not infected and shows no evidence of clinical illness
ecologic studies
a study of group characteristics (average values) of a population taking no account of variability between individuals
ecological fallacy
when group characteristics are ascribed to members of a group which they do not possess as individuals
eliminated disease
there is no endogenous (long chains of human-to human) transmission within a geographic area
endemic
the ongoing, constant presence of a disease in a community, within a geographic area or among a group of individuals
Environmental factors that affect infection
climate ecology land use nutrition sanitation vector control access to care
enzooinic
endemic within an animal population
Epi curve of a propagated outbreak is dependent on:
- characteristics of the agent and host population
- Pathogenicity
- susceptibility
- means of transmission
- contact rate
- population density
- Interventions (if there are any)
epidemic
occurrence of cases of an illness, specific health-related behavior, or other health event that is clearly in excess of normal expectancy in a community or region
epidemic (epi) curve
frequency distribution of the number of cases that develop over a period of time
-represents the distribution of incubation periods
epidemiologic surveillance
the ongoing systematic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practice closely integrated with the timely dissemination of these data to those who need to know
epidemiologic triad
a time-dependent interaction of an infectious agent, a host and an environment that promotes disease
infectious agent: pathogen that causes disease
host: organism that harbors the disease
environment: surroundings and conditions external to the host and agent that causes or allows for disease transmission
equation for proportion of individuals who must be immune to achieve herd immunity
1-(1/R0)
Eradicated disease
the disease is permanently gone and no longer circulating in nature
- Smallpox - Rinderpest
ethical norms in public health
- ensure that investigators and practitioners are held accountable for their actions to the scientific community and public as a whole
- help build trust and support among the general public
- promote moral and social values
ethical principals of the Belmont report
- respect for persons: individuals are to be treated as autonomous agents and persons with diminished autonomy are entitled to protection
- beneficence: there is an obligation to do no harm, maximize possible benefits and minimize potential harms
- justice: there must be fairness in the distribution of the benefits and burdens of research to ensure that vulnerable populations are protected
ethics
the norms for conduct that distinguish between acceptable and unacceptable behavior
expected level of disease
the baseline level of observed occurrences of a particular disease within a population
fomite
inanimate objects which harbor microbes
generalized steps to investigate an outbreak
- define the outbreak and validate it’s existance
- examine the distribution of cases
- look for combinations (interactions of variables)
develop hypotheses - test hypotheses
- prepare a written report of the investigation and the findings
- communicate the findings
gold standard for testing
the best available external source of truth regarding the disease status of each individual in the population
-sometimes just watchful waiting
herd immunity
the resistance of a group of people to an attack by a disease to which a large proportion of the group is immune
-accomplished when the number of susceptible individuals is minimized compared to the number of immune individuals
-only part of a population needs to be immunized to
achieve highly effective protection
-will not operate if there is a reservoir for the disease
outside the host
host factors that affect infection
immune status gender age socioeconomic status genetics occupation behavior
iceberg concept of disease
much of the disease present in a population is subclinical disease, but only clincal disease is observed and measured
- we do not know the true prevalence of disease
- we do not know the true extent of the subclinical
prevalence
- subclinical cases may still transmit disease
- fatality rate is calculated from clinical disease,
leading to overestimate
incubation period of disease
interval from exposure to an infectious agent to the time of onset of clinical illness
-a precise incubation period does not exist for a
given infection, but is based on the range
characteristic for the disease
-may reflect the time needed for replication to critical
mass
-can be influenced by the dose of agent received,
mode of transmission or portal of entry
indirect age-adjustment
the mortality rates of a known population are used to determine the expected number of deaths in a population of interest
-used when numbers for each age-specific stratum
are unavailable
-uses standard mortality ratio (SMR)
indirect transmission
3 types
results when the pathogen or infectious agent is transferred or carried from a host/reservoir to a susceptible host by an intermediate item, ,organism, means or process
airborne: transmission via aerosols
vehicle borne: transmission via a common vehicle or fomite
vector-borne: transmission via a mechanical or biological vector
-part of pathogen’s life or development occurs within
the vector
infection
the entry and development or multiplication of an infectious agent within the body
infectious disease epidemiology
the epidemiological study of infections in populations
interobserver variation in test results
variation in the reading of test results by different observers due to differences in application, belief, knowledge or criteria for obtaining or reading the results
intraobserver variation in test results
variation in the reading of test results by the same observer due to the subjectivity of the test and/or reading of the results
intrasubject variation in test results
variation within individual subjects due to conditions under which the test was performed
latent disease
an infection with no active multiplication of the agent
-viral nucleic acid sequence is incorporated into cell
DNA as a provirus
-only the genetic component is present, not the
viable organism
latent period of disease
from the point of infection until critical mass is reached
lead time
the time interval between an earlier diagnosis made possible by improved screening tests and the later, usual time of diagnosis
-increases the survival time by pushing back the
time of diagnosis without changing the natural
history of the disease
natural immunity
immunity due to naturally occurring biological processes
negative predictive value (NPV)
the ability of a screening test to produce a negative result among non-diseased people
-probability of no disease given a negative result
-proportion of patients who do not have the disease
among all those who test negative
net sensitivity in sequential screening tests
sensitivity of the first test x sensitivity of the second test
nuremberg code
set the standard by which to judge the human experimentation conducted by the Nazis by identifying 10 points for the ethical conduct of human experimentation (1947)
-created the framework for establishing
documented guidance regarding the ethical
treatment of research subjects
outbreak
an epidemic that is limited in scope and confined to a localized area or population
pandemic
an epidemic that affects the population of an extensive region, country or continent
passive immunity
immunity due to the transfer of antibodies from one individual to another
pathogen factors that affect infection
resistance to therapy pathogenicity infectivity infective dose virulence environmental stability novelty
person-years
a unit used for counting observation time when different subjects were observed for different lengths of time
-assumes the risk is the same for all person years
observed (usually not the case)
-a rater per person years can be compared to a vital
statistic yearly rate based on the period’s midpoint
population estimate
positive predictive value (PPV)
the ability of a screening test to produce positive results among those with the disease
-the probability of disease given a positive result
-proportion of patients who have the disease
among all those who test positive
preclinical disease
an infection which is not yet clinically apparent but is destined to progress to clinical disease
predictive value of a test
the probability of a true test result among people who test positive or negative
-tells the patient how likely it is that their test result
is correct
prognosis
the probable course and outcome of a disease
-there are many points in the natural history of
disease that can be involved in the measure
-most common is survival time
-problematic because the patient experience is
subjective
proportion as a type of measure
the fraction of the population that is affected by a disease or health problem
rate as a type of measure
describes the speed at which a disease or health problem occurs in a population
ratio as a type of measure
a comparison of any two groups
relationship between prevalence and predictive value of a test
an increase in disease prevalence will increase PPV and decrease NPV
if a high-risk subset of the population can be identified and screenied, screening is likely to be more productive
-high risk populations tend to be more motivated to
participate
-high risk individuals tend to be more likely to take
recommended action if they test positive
results of any test must be interpreted in the context of the prevalence of the disease within the population being tested
reservoir
the habitat in which an infectious agent normally lives, grows and multiplies
screening
a process used to detect disease among populations of people without signs and symptoms of disease
-secondary pretention measure
-intended to identify disease during more treatable
stages
-occurs in stage of pre-clinical disease
sequential screening
uses a less expensive/less invasive test on the entire population and then a more expensive/more invasive test with greater sensitivity and specificity on those who tested positive on the first test
simultaneous screening
two or more screening tests used at the same time
-often in a clinical setting when a battery of tests
are performed
-a positive result on one or more of the testis is
considered a net positive
-negative results on all tests is considered a net
negative
standardized mortality ratio equation
observed number of deaths / expected number of deaths
subclinical disease
an infection which is not yet clinically apparent and not destined to progress to clinical disease
surveillance
the routine collection, analysis and dissemination of all data that may be relevant for the prevention and control of a public health problem
surveillance case definition
a set of uniform criteria used to define a disease for public health intended to aid public health officials in recording and reporting cases
symptomatic period of disease
duration of time when the host experiences signs and symptoms
test sensitivity
a test’s ability to identify correctly those who have the disease
-proportion of diseased people who were correctly
identified as positive
-probability that someone with the disease will test
positive
test specificity
a test’s ability to identify correctly those who do not have the disease
-proportion of diseased people who were correctly
identified as negative
-probability that someone without the disease will
test negative
test validity
a test’s ability to distinguish between who has the disease and who does not
-provides evidence of the inherent accuracy of a
screening test
-evaluated by 2 measures: sensitivity
specificity
-to calculate validity measures, we must know who
“really” has the disease and who does not from a
source other than the test being evaluated
time-bias in prognosis
survival time among those screened for disease and diagnosed early as a result and have artificially lengthened survival time as compared to those not screened and diagnosed early
transmission
the spread of microbes
-most microbes have a main route of transmission
types of common source outbreaks (3)
point source: caused by exposure to a common source over a limited time period
-characterized by a sudden and rapid increase in the
number of cases
-cases are limited to only those who share the
common exposure
intermittent: caused by intermittent exposure to a common source at varying levels
continuous: caused by continuous exposure to a common source at relatively low levels
types of epidemics/outbreaks (3)
common source: caused by exposure to a specific common source
propagated: caused by the gradual spread of an infection from person to person
mixed: mix of common source and propagated
validity consequences of sequential screening
- a loss in net sensitivity
2. a gain in net specificity
validity consequences of simultaneous screening
- a gain in net sensitivity
2. a loss in net specificity
