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What changes may occur in an infectious agent that may trigger an epidemic?
increased virulence, novel setting, changes in host susceptibility, changes in host exposure
Epidemiology
study of the distribution and determinants of disease in the human population
Health
complete physical, mental and social well-being and not merely the absence of disease or infirmity
Epidemic
outside of normal rates but restricted to one location
What are the four levels of study of epidemiology?
submolecular, tissue or organ level, level of individual patients (what we look w/), level of population
What is classical epidemiology?
population oriented. studies the community origins of health problems
what is clinical epidemiology?
uses research designs to study pts in healthcare settings to improve
What are the different stages of disease?
-Predisease stage: before it begins (vaccination, prophylaxis)
-Latent stage: disease process started but asymptomatic. contagious
-symptomatic stage: disease manifestations are evident
What does it mean to be a vector?
transmits disease by having a specific relationship to the agent, the environment, and the host
What is the BEINGS model?
-Major categories of risk factors for disease
-B: biologic and behavioral factors
-E: environmental factors
-I: Immunological factors
-N: nutritional factors
-G: genetic factors
-S: services, social, spiritual
Risk may be influenced by...
gender, age, weight, bone density, lifestyle
what is the theory behind herd immunity?
-No herd immunity and everyone is susceptible, the number of cases doubles every disease generation
-50% herd immunity, number of cases is small and remains constant
->50% herd immunity, infection will die out eventually
What does the degree of immunity necessary to eliminate a virus from a population depend on?
type of virus, time of year, density and social patterns of the population.
what occurred with the diphtheria vaccine?
-Vaccine produced immunity decreased over time as people had been getting re-exposed to the disease repeatedly, giving themselves boosters
iceberg phenomenon
disease appears to be a larger problem than it actually is with most cases being seen as bad, even though there are a lot more invisible cases or carriers that are asymptomatic
What is frequency?
frequency of a disease, injury, or death can be measured in different ways, and it can be related to different denominators (who the population is)
What is incidence?
the frequency of new occurrences of disease, injury, or death in the study population during the time period of the study
What is prevalence?
number of persons in a defined population who have a specified disease or condition at a point in time, usually the time the survey is done
What is period prevalence?
number of persons who had the disease at any time during the specified time interval. sum of the point prevalence at the beginning of the interval plus the incidence during the interval
the number of new AIDS cases are going down, but their lifespan is increasing, therefore...
incidence is going down, but prevalence is going up
What is risk?
the proportion of persons who are unaffected at the beginning of a study period, but who undergo the risk event during the study period.
During risk calculations, the denominator is the...
total # of people in the cohort
What are the different subsets of population to assess risk going from largest subset to smallest subset?
-people who are susceptible
-people who are exposed
-people who are infected
-people who are ill
-people who die
What is the case fatality ratio?
proportion of clinically ill persons who die. the higher the case fatality ratio, the more virulent the infection
What is the pathogenicity of the organism?
proportion of infected persons who are clinically ill
What is the infectiousness of the organism?
proportion of exposed persons who become infected
What is the risk rate?
frequency of events that occur in a defined time period, divided by the average number of people at risk during the period being studied
What is the denominator of risk rate?
midperiod population as it can be considered a good estimate of the avg number of people at risk for the outcome
What is the crude death rate?
number of death divided by the midperiod population multiplied by a constant multiplier
When is rate a good approximation of risk?
-the event in the numerator occurs only once per individual
-the proportion of the population affected by the event is small
-time interval is relatively short (<1 year)
When will rate be noticeably larger than the risk?
if the time interval is long or the percentage of people who die is large
What is incidence rate?
number of incident cases over a defined study period, divided by the population at risk at the midpoint of that study period
What is prevalence rate?
indicates the proportion of persons with a defined disease or condition at the time they are studied
What is incidence density?
frequency of new events per person-time
When is incidence density useful?
when the event of interest can occur in a person more than once during the study period
What are crude rates?
Rates that apply to an entire population without reference to any characteristics of the individuals in it
What are specific rates?
when a population is divided into more homogeneous subgroups based on a particular characteristic of interest
When would you want to use crude rates over specific rates?
-frequency of death or disease for certain subgroups is unknown
-size of subgroups is unknown
-number of persons at risk is too small
What is age specific death rate?
number of deaths to people in a particular age group divided by midperiod population
What are standardized/adjusted rates?
crude rates that have been modified to control for the effects of age or other characteristics and allow for valid comparisons of rates. usually applied to death rates.
What is direct standardization?
most commonly used method to remove the biasing effect of the differing age structure of different populations.
What is cause specific death rates?
number of deaths due to a particular cause divided by midperiod population
What is the difference between early, intermediate, and late fetal death?
-early: miscarriage w/in 20 weeks of gestation
-intermediate: delivered 20-28 weeks of gestation
-Late: after 28 weeks, stillbirth
What is an infant death?
live born infant death before their first birthday
what is a neonatal death?
death of an infant before 28th day of life
what is a postneonatal death?
death of infant after the 28th day of life but before the first birthday
What is the crude birth rate?
number of live births divided by midperiod population
What is the infant mortality rate?
-number of deaths to infants <1 yo divided by number of live births.
-overall index of the health status of a nation
What is the maternal mortality rate?
the number of pregnancy related deaths divided by the number of live births
What is the difference between active and passive surveillance?
-Passive is most common where docs and labs report cases of reportable disease
-Active requires periodic telephone calls to the reporting individuals. More costly
What is seasonal variation?
when determining the usual number of cases or rates of disease, the expected seasonal variation is incorporated into the calculations
What is the epidemiologic year?
for a given disease runs from the month of lowest incidence in one year to the same month in the next year, therefore the high incidence months will be in the middle of the graph.
If a vaccine is a failure, what can be used to detect this?
-A lack of change of disease states
-An increase in disease rates after an initial decrease
-An increase in disease rates in a recently vaccinated group
What is an epidemic?
unusual occurrence of a disease
What is endemic?
when a disease in a population occurs regularly and at a constant level
What is the attack rate?
number of new cases of a disease divided by the number of persons exposed in a particular outbreak
What are the steps for investigating epidemics?
1. establish a diagnosis
2. establish epidemiologic case definition
3. establish the time, place, and person
What is a case definition?
a list of specific criteria used to decide whether or not a person has the disease of concern
What is an epidemic time curve?
time on the x-axis and the number of new cases on the y-axis. Units of time on the x-axis are considerably smaller than the probable incubation period
What is a propagated outbreak?
if theres person-to-person spread, the epidemic curve has a prolonged irregular pattern
what is an index case?
the case that introduced the organism into the population
What is the source of infection?
the person or vehicle that brought the infection into the affected community in the first place
What is the pattern of spread?
the pattern by which infection can be spread from the source to the individuals infected
What is a propagated pattern?
occurs when the infection propagates itself by spreading directly from person to person over an extended period
What is a mixed pattern?
persons acquire a disease through a common source and spread it to family members or other by personal contact
What are four common types of intervention used to control an outbreak?
-Sanitation: modification of environment
-Prophylaxis: putting a barrier w/in susceptible hosts
-Diagnosis and treatment
-Control of disease vectors
What are the three fundamental types of causation?
sufficient cause, necessary cause, risk factor
What is sufficient cause?
if the factor is present, the disease will always occur. precedes a disease.
What is necessary cause?
the factor must be present for the disease to occur but the factor may be present without the disease occurring. precedes the disease
What is risk factor?
if the factor is present, the probability that the effect will occur is increased
What is directly causal association?
the factor exerts its effect in the absence of intermediary factors (intervening variables)
What is indirectly causal association?
the factor exerts its effect via intermediary factors
what is noncausal association?
the relationship between two variables is statistically significant, but no causeal relationships exists because the temporal relationship is incorrect
what are the three steps for determining causation?
-investigation of the statistical association
-investigation of the temporal relationship
-elimination of all known alternative
What factors increase the likelihood that a statistical association is causal?
-strength of the association
-consistency of the association
-specificity of the association
-biologic plausibility
-presence of a dose-response relationship
What is assembly bias?
deals with the initial assemblage of the groups into. can either be selection or allocation bias
What is selection bias?
When subjects are allowed to select the study group they want to be in. Happens in terminal diseases
What is allocation bias?
when investigators choose a nonrandom method of assigning subjects to study groups or if staff involved do not follow the random method chosen.
What are associated problems of validity?
-ethically random clinical trials must allow study subjects to participate or not voluntary, introducing self-selection
-Therefore, results have internal validity but lack external validity
What is detection bias?
the result of the failure to detect a case of disease, a possible causal factor, an outcome of interest
What is measurement bias?
occurs while collecting baseline data or follow up data (shoes w/ height and weight w/ clothes)
What is recall bias?
subjects who have experienced an adverse event are more likely to recall previous risk factors than subjects who have never experienced the event.
What is random/nondifferential error?
produces findings that are too high and too low in approximately equal amounts, decreasing the probability of finding a real association by reducing the statistical power of a study
What is confounding?
the confusion of two supposedly causal variables-part or all of the purported effect of one variable is actually due to the other
What is synergism?
the interaction of two or more presumably causal variables, so that the combined effect is clearly greater than the sum of the individual effects
What is effect modification/interaction?
the direction or strength of association between two variables differs depending on the value of a third variable
What is research design?
the rules that govern the process of collecting and arranging the data for analysis
What is hypothesis generation?
process of developing a list of possible candidates for the "causes" of the disease and obtaining initial evidence that supports one or more of the candidates
What is a cross-sectional study?
survey of a population at a single point in time. quick and easy but looks for disease and risk factors simultaneously and more chronic diseases
What is a cross-sectional ecologic study?
relate the frequency with which some characteristic and some outcome of interest occur in the same geographic location
What is a longitudinal ecologic study?
ongoing surveillance or frequent cross-sectional studies to measure trends in disease rates over many years in a defined population
What are cohort studies?
a clearly identified group of people to be studied to determine whether they develop the disease of interest or if the risk factors at the beginning of the study predict the diseases that occur
What are prospective cohort studies?
-assembles the study groups in the present time
-collects baseline data on them
-continues to collect data for a period that can last many years
What are retrospective cohort studies?
-goes back into history to define a risk group and follows the group members up to the present to see the outcomes
What are case control studies?
selects the case group and the control group on the basis of the outcome and compares the groups in terms of their frequency of past exposure to possible risk factors. uses odds ratio instead (relative risk of the outcome)
What is the major difference between the two types of randomized controlled trials?
clinical trials are usually used to test therapeutic interventions in ill persons while field trials are usually done to test preventive interventions in well persons
What is publication bias?
selection bias that makes the overall interpretation of the literature difficult regarding the results of clinical trials
What is the absolute risk difference?
the risk in the exposed group minus the risk in the unexposed group. denominator is entire population
What is the absolute rate difference?
the rate in the exposed group minus the rate in the unexposed group. denominator is midpoint population.
When the level of risk in the exposed group is the same as the level of risk in the unexposed group, the risk difference (attributable risk) is....
zero. no significance.
