EPI Flashcards
Epidemiology purpose
Quantify disease/death in pop
Quantify causes of disease in pop
Identify causal links to disease and death
Test claims of interventions efficiency
Break causal links
Classical Epidemiological Practice Steps
- Identify a health problem
- Describe its distribution (descriptive epi)
- Hypothesize potential causes
- Determine causal associations (analytical epi)
- Intervene
- Assess for results
- Repeat
Descriptive Epi purpose
describing distribution of disease in pop/ health conditions and determinants
Variables used in descriptive epi
time, person, place, measures of morbidity (and why we are using these). use to analyze outcomes AND determinants
We have to see if associations are…
causal
An observed association can be due to 4 things:
- cause
- chance
- confounding
- bias
Intervention can help with all 3 levels of prevention
- Primary: Prevent initial development
- Screen and catch early to reduce severity
- Reduce clinically apparent disease’s ultimate impact through treatment or rehabilitation
Goals of Descriptive Epi
Understand burden of disease in population
Understand causal links to disease, death, and disability
Generate hypotheses for further study
Track trends in pop. health
Morbidity
Disease and disability
Mortality
Death
Counts
of cases in a pop
Fractions
Proportion of population that is a case
Rate
Cases per time (fraction per time)
In descriptive epi, analyse outcomes and determinants by these 3 things
time, person, place
“Place”
urban/rural, community, political
“Person”
education status, wealth status, race, gender/sex, disability
Incidence
of new cases in a defined pop
Prevalence
Current cases present in a defined pop.
What does a population at risk exclude
Those already w/ the disease
Those who are biologically unable to contract disease
The 3 measures of incidence
Count, rate, proportion
Incidence Count
Number of new cases in a population (say in 2018)
Incidence Proportion/Fraction
of new cases / # of people in population at risk in a given time
What is incidence proportion a measure for?
RISK
Cumulatative incidence is a measure of ______ ______ and measures
Incidence proportion: measures the number of new cases of a disease occurring during a specific time period
“20% of men develop diabetes before 60th birthday)
Incidence Rate
of people who develop case / total person time at risk
When to use incidence rate?
When comparing different people/groups in a study for a different amount of time. Assume rate of contraction remains constant
How to calculate Incidence rate?
total # of new cases in a period / population at risk x time
Difference between incidence proportion and rate
Incidence proportion is over population at risk, incidence rate is person time. Proportion can show if there is an alarmingly large amount of cases given a pop. size, unlike a rate
Attack rate
Same as incidence proportion, but usually refers to outbreaks or risks related to particular exposure
Prevalence proportion
of people with disease / # of people in at risk pop
Point prevalaence
prevalence at particular point in time (convention)
Period Prevalence
count/proportion of who had disease at any time within a period
Relationship between incidence and prevalence
Prevalence depends on incidence and duration of the disease
_____ is an example of high incidence, low prevalence
_____ is an example of low incidence, high prevalence
common cold
chronic heart disease
Illness duration depends on
Speed of cure/death
Prevalence is a good measure of ______ but not of _________
burden of disease, but not of risk
Prevalence = Incidence x Duration assuming
steady state, closed pop, somewhat low prevalence
When is seeing increasing prevalence good
For incurable chronic diseases, means life spans are increasing
Problems with Numerator in Morbidity Data
Over/under inclusive case definition (must be precise)
Errors/bias in data collection
Changing case detection (better screening procedures)
Problems with Denominator in Morbidity Data
Only those AT RISK in denominator
Having strict/precise definitions for sub populations
What does mortality depend on
lethality of disease and prevalence
Mortality Count
of deaths in a defined pop
Annual mortality rate
of deaths in a pop in a year / population size on avg/at year’s midpoint
Cause specific mortality rate
of deaths from particular cause/# of people in pop. in time period
Age-specific mortality
of deaths in age group/pop of that age group
Years of potential life lost
Total yrs of life lost before standard age, measures premature mortality
Excess mortality
of rate of deaths observe in pop - expected # of rate of deaths in pop historically
Proportionate mortality
of deaths from particular cause in a pop. / total # of deaths in a pop
Case fatality
of people who die from disease / number of people who have acquired disease (best for acute time period diseases)
Case fatality rate doesnt measure population impact, it measures _______
lethality
5 year survival rate
After 5 years, this % of infected survive (denominator should include only people who were diagnosed at least 5 years ago)
Problems w/ mortality data
Assigning cause of death may be difficult (immediate, underlying, or contributing causes)
Case definition of cause of death can change
If multiple causes of death, hard to attribute
Life expectancy
Crude but actual measure of population level mortality. Very sensitive to early mortality
Prognosis and natural history stages
- Biological onset
- Pathological evidence (if sought
- Signs and Symptoms emerge
- Seek Case
- Diagnosis
- Treatment
- Outcome (death, cure, disability, control)
Where is the border between non clinical and clinical stages of natural history
signs and symptoms
Why might aids diagnoses fall after routine testing is implementing?
After people are aware of their status, they may engage in safer sex
Where were hiv outbreaks in dc
- center city
- southeast
Population density contributes to _______ incidence. Lack of transportation contirbutes to ________ incidence.
higher, lower
Incidence is the # of cases, not necessarily the number of diagnoses
just remember this
For chronic diseases, we actually want prevalence to ______ in the short term
increase, indicates longer life spans
What is a sample statistic a best measure of?
population parameter
The standard error and CI calcualted from the sample are our best approximations of how much our population parameter would…….
vary if we were to draw a different sample in the same manner from the same population
Unlucky sample are
always possible
The larger the sample
the less random variation we expect, closer to pop. value we expect our sample estimates to be
random (non-systematic error):
discordance between calculated sample value and true population value
If we were to take hundreds of samples,
any given sample may randomly diverge, but the average value estimated would match the population value
Non-random error
everything else besides random sampling error that causes an inaccurate estimate of population value –> LEADS TO BIAS
coverage error
over or underinclusive samples
systematic sampling error
some americans less likely to be selected for sample (like homeless)
non-response error
not everyone is going to respondmeasurem
measurement errors
people lie, forget, feel pressure
Characteristics of Simple random sampling
Everyone in pop has equal % of being selected
Some list of all eligible units
Some way of randomly selecting people
There is _____ bias from other sources than random sampling error
always
How to interpret a 95% confidence interval
We are 95% confidence that the interval calculated for our sample contains the true population value, assuming no source of error other than random sampling error.
95% confidence interval 3 key aspects
- Estimation of true pop. value (an interval)
- A degree of confidence
- Assuming only random sampling error
Screening is the basis of ______ prevention
secondary
Screening is
the systematic application of diagnostic test to identify cases and intervene early
Screening vs testing
Testing is more specified, looking to identify individuals w/condition and initiate intervention
Between which stages of the natural history of disease doees screening target
Pathological evidence stages and signs/symptoms stage
Sensitivity
Proportion of people with disease who are positive
Specificity
Proportion of people who are disease free who test negative
PPV
If you are positive, what is the probability you actually have the disease.
NPV
If you are negative, what is the probability you do not have the disease
Reliability
Test results can be different at times due to
different conditions
who is conducting the test
To improve reliability of screening
Have very clear screening/testing protocols
-Consistent use of calibrated instrutments
-Training for test admin
Multiple tests/people determining the results
Screening without subsequent intervention is
pointless and unethical
Validity tell us
Does a test measure what it is supposed to
Two dimensions of validity
Accuracy in measuring
Does the thing being measured actually mean anything
Two basic forms/results of tests
- yes/no result
- continuous result requiring a cutoff
PPV is affected by
the specificity of the test (if you don’t have the disease, what proportion test negative)
Prevalence
Because when you have a lot of false positives, this decreases PPV a lot
How to increase PPV
administer sequential screening tests
1. cheaper, lower sens/spec test
2. most costly, expensive, valid test
or seek higher prevalence populations –> higher risk increases PPV
Impacts of increased sequential tests on sensitivity and specificity
You only progress if you are positive, so we can eliminate more false positives by doing sequential testing, but we may also generate more false negatives
Sensitivity: reduced, the likelihood of if you have the disease and test positive decreases
Specificity: increased, the likelihood of if you don’t have the disease and you test negative increases
When do we care more about PPV?
In clinical practice, we want high PPV to avoid overtreating
When do we care more about NPV?
For highly infecitous and dangerous diseases, we want to make sure people DON’t have the disease. Would be willing to accept some false positives to avoid false negatives (so effectively quanranting more people than needed)e
What is surveillance
Systematic collection, analysis, and interpretation of health data to help with planning, implementation, and evaluation of public health practice + dissemination
Pointless if not give to public health officials
Two major purpose of surveillance
Case surveillance
Statistical surveillance
Case surveillance
Focuses on individuals or groups to identify those w/disease and taken action
Statistical surveillance
Focuses on pop to identify differentials and trends informing public health policy making –> includes allocation of resources
Two major methods of surveillance
- active
- passive
Active surveillance
extensive efforts by surveillance system to identify cases that have sought care but may not have been reported through normal means
Passive surveillance
Relies on providers to voluntarily submit public health to authorities
Uses for surveillance
estimate magnitude of problem, descriptive epi, evaluate disease control measures, facilitate planning, identify individual and local interventions to control diseae
Individual level surveillance
Case finding, patient tracking
Local level surveillance
removal of contaminated sources, pollutants
Population level
Identify issues and form basis for devleoping/implementing targeted programs, public edu, allocation of resources
Syndromic Surveillance
Focuses on symptoms, not confirmed diagnoses, builds on existing data systems
4 principles of surveillance
representativeness
timeliness
completeness
sensitivity and specificity
What are the major data sources used for surveillance?
Infectious disease case reports
Vital Records
Registries
Sentinel Surveillance
Population-based surveys
Medical and administrative records
Endenmic
Customary presence of disease in pop
Epidemic
Occurrence of disease in excess of what is normally expected in pop
Pandemic
Epidemic over a very large geographic range
Attack rate
Used synonymously w/cumulative incidence proportion of people infected over a short period of time
Case-fatality rate
Among those who acquire disease, % who die
Can case-fatality rate vary?
Yes. by pop., risk factors, treatment, and overcrowding
Incubation period
time from infection to onset of clinical symptoms
latent period
time from infection to infectiousness
Two modes of transmission
direct and indirect
Direct transmission
physical, oral, sex bodies
Case definition Levels
Suspect, Probably, confirmed
A vector moves in between
host, agent, and environment (agent is the cause of the disease)
Infectious disease transmission requires _____ and ________
pathogens and susceptible hosts
R0 does NOT measure
speed of spread
Generation time
Average amount of time from primary infection to secondary infection
Serial interval
Time from primary symptoms to secondary
What determines monitoring/quarantining time
incubation period
Asymptomatic transmissoin
transmissions before clinical symptoms develop in the preclinical stage
(icubation > latent)
Immunity
resistance to infection
Natural immunity
infection will produce permanent or temp resistance against some pathogen
Artificial
vaccination and immune system products to produce immunity
Sterilizing
Protection against effective infection
Therapetuic
person can be infected against but not seriously
R0: the basic reproductin number
of secondary infections an infectious person would cause if everyone else in the population was non immune
Does each disease have a unique R0
No. R0 is depedent on both disase characteristics and interactions of populations
Can low R0 and gen time spread faster than high R0 and gen time
YES
Herd immunity calc.
1-(1/R0)
Vaccines can be use for 2 purposes
- protect vulnerable
- prevent disease in a pop.
Purpose of epidemic control measures
Not always to prevent or stop, but to slow it down and srpead it out, buy time for meds and basic science to respond
Are epidemic control measures purely scientific?
No. Also weigh economic costs and social disruptions
Framwork of epidemic
Susceptible –> Infectious (some die) –> Recovered (immune)
Which three people matter to pathogen
susceptible, infectious, recoevered/immune
susceptible:
able to acquire disease
infectious
able to spread disease
recovered/immune
unable to acquire or spread disease
Where do new cases come from
Transmission-capable interactions between susceptible and infectious people
3 ways we can reduce epidemic disease
Prevent infectiousness
Minimize/prevent transmission capable interaction between susceptible and infectious
Make susceptible people immune
2 purposes of reducing infecitousness
- decrease # of people who can infect others
- reduce likelihood an infected individual will infect contacts
How to reduce infectiousness
- medical treatment
- death
What to target when preventing transmission capable contact between infectious and susceptible?
target mode of transmission
prevent contact or make contact safer
Social distancing
Closing of schools, businesses, or
places of public gathering.
Physicial methods to stop infection
masking
handwashing
decontamination of fomites/vehicles
personal protective equipment
What do travel restrictions do
reduce pop. mixing
prevent cases from entering new pops
Isolation
separation of patients known or suspected to be infected
Qaurantine
separation of patients known or believed to be exposed who are not yet sick
When is isolation more valuable
if disease can be spread casually
When is quarantine more valuable
disease can be spread before person shows symptoms (asymptomatic tansmission)
Isolation requires _____, Quarantine requires ______
testing, contact tracing
Goal of harm reduction
make risky behavior less dangerous
make interactions less transmissoin-capable
Goal and tool of making susceptble people immune
- reduce # of susceptbles infectious can interact with
- primary tool is vaccination
Outbreak
largely synonymous with epidemic, but often implied directly linked and localized cases
Case definition
working description used to define who is and is not a case
includes clinical/lab criteria and restrictions by time, place, and perosn
Case definition should start _____ and end ______
broad, narrow
Confirm outbreak and find cases
what is sounds like
Describe the Outbreak using
TIme, place, person
Develop and Test Hypotheses for outbreak
Use info online list and descriptive epi to hypothesize about
exposures
mode of transmission
factors leading to exposure or contamination
Line list
while investiating cases, make table showing
1. identifying info
2. demographics
3. risk factors
4. Source of reporting
Perform environmental or lab investigaitons
sampling
lab tesiting
identify pathogens
Implement control measures
outbreak control!
Communicate Preventive Info and findings
Provide opp for public educaiton
6 steps of outbreak investigation
- Develop case definition
2.Confirm outbreak and look for cases - Describe an outbreak by time person place
- Develop and test hypotheses
- Perform environmental or lab investigations
- Implement control measures
- communicate preventive info and findings
