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PHRM 849 > Introduction to Epidemiology > Flashcards

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

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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

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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)

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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

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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
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6
Q

Core epidemiologic functions

A
  • 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
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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

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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
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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
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10
Q

Hyperendemic

A
  • persistent, high levels of disease
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11
Q

Sporadic

A
  • a disease that occurs infrequently and irregularly
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12
Q

Epidemic

A
  • 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
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13
Q

Pandemic

A
  • global epidemic
  • an epidemic that has spread over several countries or continents
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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

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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
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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
Q

Prevalence

A
  • 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
Q

Attack rate

A

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
Q

Secondary Attack Rate

A
  • 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
Q

Secondary attack rate for COVID-19 SAR estimates vary

A
  • 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
Q

Basic Reproductive Number: R0

A
  • 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
Q

Mortality (or morbidity) rate

A
  • 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
Q

Case Fatality Rate

A
  • 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
Q

Death to case ratio

A
  • 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
Q

Proportional Mortality

A
  • 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
Q

Risk Ratio (Relative Risk)

A
  • 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
Q

Rate ratio

A
  • 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
Q

Odds Ratio (OR)

A
  • 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

PHRM 849 (10 decks)

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