Introduction to Epidemiology Flashcards
1
Q
Epidemiology
A
The study of the distribution and the determinants of health related events in a population, and the application of this information to the control of health problems
2
Q
Distribution
A
focused on the frequency and patterns of health events in a population
* frequency: number of events; rate or risk of a disease (relationship of number of events to size of population)
* patterns: person (demographics); place; time
3
Q
Determinants
A
determinants: causes and other factors that influence disease
* why/how
* demographic characteristics, genetics, immunologic patterns, behaviors,
environment, etc.
* exposures and outcomes (in clinical terms, analogous to treatment and effect)
4
Q
Data used to inform public health efforts
A
“the basic science of public health”
* describe health of the population – examples?
* explain disease etiology/cause
* predict disease occurrence
* control the spread of disease
* assess efficacy of public health efforts
* inform decisions at the individual level
* complete the clinical picture - e.g., SARS, COVID-19
5
Q
Cholera outbreaks in London in 1800’s - John Snow
A
- applied systematic scientific approach to identify source of outbreak
- used ”spot map” to identify sources of contaminated water
- established a systemic sequence of steps to investigate disease
6
Q
Core epidemiologic functions
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- public health surveillance: systematic collection of morbidity and mortality reports; identify new diseases, changes in patterns of known diseases - e.g., HIV in Scott County, IN
- field investigations: e.g., environmental investigations, food borne illnesses
- analytic studies: strongly linked to biostatistics
- programmatic evaluation: e.g., vaccination efforts
- policy development
7
Q
Social determinants of health
A
health care access and quality; neighborbood and built environment; social and community context; economic stability; education access and quality
8
Q
Time in epidemiology
A
- change in occurrence of disease over time
- displayed graphically as rate of disease or number of cases vs. time: rapid changes in disease; seasonal trends; long term trends; epidemic period - time course of a disease outbreak, epidemic curve
9
Q
Endemic
A
- baseline level of disease typically found in a community for a
disease that is habitually present in that community * the expected level of disease over time
10
Q
Hyperendemic
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- persistent, high levels of disease
11
Q
Sporadic
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- a disease that occurs infrequently and irregularly
12
Q
Epidemic
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- increase (potentially sudden) in number of cases of disease above what is expected in that population: relative to the usual frequency of the disease; in infectious disease can be a single case of a long absent
communicable disease; can be first invasion of a communicable disease
13
Q
Pandemic
A
- global epidemic
- an epidemic that has spread over several countries or continents
14
Q
Epidemics occur when
A
an agent and susceptible host are present in adequate numbers and the agent can be spread to susceptible hosts
* may result from: an increase in amount/virulence of the agent, recent introduction of the agent into a new setting, change in transmission so more people are exposed, change in host susceptibility, change in host exposure
15
Q
Common source outbreak
A
- exposure originates from same source: point source - all exposed at one time (e.g., nuclear disaster, food borne illnesses) within one incubation period; cases occur suddenly; stops unless secondary spread; curves with steep upslope, gradual downslope
- continuous common source: exposures occurs over time from a common source
- intermittent common source: exposure reemerges over time
16
Q
Propagated outbreak
A
- results from transmission from one person to another
- typical of community wide outbreaks: typically direct, person-to-person transmission; can also be vehicleborne (e.g., HIV transmission through shared syringes) or vectorborne (e.g., malaria transmission via mosquitos)
- incubation period: the amount of time between initial contact with the agent and the onset of disease
- generation period: the amount of time between peaks in a spread (an estimate of
incubation period)
17
Q
Incubation periods may create multiple peaks
A
- secondary cases appear one incubation period after peak of first wave due to secondary spread
- usually wanes after a few “generations” because number of susceptible people falls below a critical level, or due to intervention measures
18
Q
Mixed epidemic
A
- common-source outbreak followed by propagated (person-to-person) spread
19
Q
COVID-19
A
- incubation period (the amount of time between initial contact with the agent and the onset of disease)
- most estimates 4-5 days
- median 4 days (IQR 2-7) (China)
- median 5.1 days (95% CI 4.5-5.8) (global, however mainly from mainland China) * median 5.2 days (95% CI 4.1-7.0) (Wuhan, China)
- provide basis for quarantine recommendations
- median 5.1 days (95% CI 4.5-5.8 –> 98% of patients who develop symptoms will do so
in 11.5 days - median 5.2 days (95% CI 4.1-7.0) (Wuhan, China –> 95% percentile is 12.5 days
20
Q
Serial Interval
A
- time between successive cases – interval between clinical onset of disease
- COVID-19
- mean 7.5 days (95% CI 5.3-19) (Wuhan, China)
- mean 3.96 days (95% CI 3.53–4.39 days) (China); 2.6% of case reports indicated presymptomatic transmission
- mean 4.0 days (95% credible interval [CrI]: 3.1, 4.9) (Hong Kong)
- serial interval < incubation period can indicate that disease may be transmitted prior to onset of symptoms
21
Q
Geographic occurrence of disease
A
can track spread, inform containment efforts
22
Q
Incidence
A
- number of new cases of a disease that occur in a group over a defined time period
- describes rate of development of a disease in a group over a certain period
- incidence = (number of new cases over a time period/total population at risk during the same time period): some call this incidence proportion or incidence rate interchangeably
23
Q
Incidence rate (IR) can incorporate person-time
A
- typically for longer term follow-up
- IR = (number of new cases over a time period/time each person was observed totaled for all persons)
- describes how quickly a disease occurs in a population
- assumes probably of disease is constant throughout the time period
- report the results a cases per person years
24
Q
Incidence rate example
A
- Adult opioid naive patients who received an opioid prescription
between January 2012 and December 2017 - 1,328,287 opioid prescriptions were identified for 341,722 patients.
- the incidence of death was 3.52 per 1,000 person-years
25
Prevalence
* proportion of people who have a disease at a specified time, or over a specified period of time: number of existing (TOTAL) cases in a population at some designated time; can be point prevalence (at a point of time) or period prevalence (over an interval of time)
* Prevalence = all new and pre-existing cases during/at a time period/population during or at that period
* can be expressed as a number, a percentage, or a number of
cases per unit size of the population
* provides an idea of the extent of a health problem
* prevalence is proportional to incidence rate and disease duration: if duration is short, then prevalence is similar to incidence
26
Attack rate
alternate form of incidence rate in outbreak settings
* used for diseases observed in a population for short times
* often due to specific exposure (e.g., foodborne illnesses)
* not a true rate because time dimension may be uncertain
* measure of risk
* AR (overall AR) = number of new cases/total population
27
Secondary Attack Rate
* rate of disease in a group among those exposed to an initial case
* can use to document transmission in community vs community in a defined/closed population
* an index of spread in a defined group
* a measure of contagiousness
* can be useful in evaluating control measures
* unlike AR, denominator is restricted to susceptible contacts
* SAR = number of new cases/number of exposed susceptible individuals
28
Secondary attack rate for COVID-19 SAR estimates vary
* CDC: 12 travel-related COVID-19 cases in late Feb 2020, identified 445 close contacts (19 household members), 14 days of active symptom monitoring
* “symptomatic SAR”à54 symptomatic, of these 2 tested positive (household members)
* (2/445) x 100 = 0.45%
* “symptomatic SAR” for family contactsà(2/19) x 100 = 10.5%
* meta-analysis of 54 studies reporting secondary household transmission
* SAR 16.6% (95% CI 14.0%-19.3%)
* higher in symptomatic index cases, transmission to adult contacts,
transmission to spouses
29
Basic Reproductive Number: R0
* average number of secondary cases produced by one infected
individual introduced into a population of susceptible individuals
* estimates the epidemic potential
* <1: may indicate disease dying out
* >1: may indicate likelihood of spread
* depends on many variables including location and population density
30
Mortality (or morbidity) rate
* frequency of death in a defined population during a specified period
mortality rate = (deaths occurring during a time period/size of the population among which deaths occurred) x 10n
* denominator can vary
* e.g., vital stats, may use size of population in middle of time period
* may report as a number per 1,000 or 100,000
* variations: crude mortality rate (all causes), cause-specific, age- specific, infant mortality rate (< 1 year of age/number of live births), maternal mortality rates, race specific, age adjusted, etc.
31
Case Fatality Rate
* proportion of people with a given condition who die from that condition
CFR = (number of cause specific deaths among incident cases/number of incident cases) x 10n
* this is a proportion, not a true rate
32
Death to case ratio
* number of deaths attributed to a disease during a specified time period/number of new cases identified in that time period
* ratio (not a proportion) because some deaths in the numerator may not be in people who developed the disease in the same period
33
Proportional Mortality
* proportion of deaths in a specified population over a period of time attributable to different causes
proportional mortality = (number of deaths of a particular cause/number of deaths from all causes)
* can calculate a proportional mortality ratio (PMR) to compare proportional mortality in a given population to the broader population
* PMR > 1.0 indicates a particular cause accounts for a greater proportion of deaths in that population
34
Risk Ratio (Relative Risk)
* measures of association in cohort study (review Principles of Drug
Information and Literature Evaluation)
* risk of an outcome (e.g., disease) among one group with the risk (or exposure), among another group
* RR = risk of disease in group of interest/risk of disease in comparator
* incidence of disease in exposed/incidence of disease in unexposed
35
Rate ratio
* compares incidence rates between 2 groups (again, in cohort studies)
* may include person-time (e.g., person years of follow up)
* Rate Ratio = Rate for group A/Rate for group B
RR = risk in exposed/risk in unexposed
* RR > 1 indicates increase risk in exposed group
* RR < 1 indicates decrease risk in exposed group
36
Odds Ratio (OR)
* used for case control studies (review Principles of Drug Information and Literature Evaluation)
* estimates relative risk
* don’t use RR in case control since we don’t know the true denominator
(ie, total population)
* OR = odds of an exposed person being a case/odds of an unexposed person being a case
* statistical significance indicated by 95% CI
