Introduction to Epidemiology Flashcards
Epidemiology
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
Distribution
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
Determinants
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)
Data used to inform public health efforts
“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
Cholera outbreaks in London in 1800’s - John Snow
- 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
Core epidemiologic functions
- 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
Social determinants of health
health care access and quality; neighborbood and built environment; social and community context; economic stability; education access and quality
Time in epidemiology
- 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
Endemic
- 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
Hyperendemic
- persistent, high levels of disease
Sporadic
- a disease that occurs infrequently and irregularly
Epidemic
- 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
Pandemic
- global epidemic
- an epidemic that has spread over several countries or continents
Epidemics occur when
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
Common source outbreak
- 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
Propagated outbreak
- 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)
Incubation periods may create multiple peaks
- 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
Mixed epidemic
- common-source outbreak followed by propagated (person-to-person) spread
COVID-19
- 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
Serial Interval
- 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
Geographic occurrence of disease
can track spread, inform containment efforts
Incidence
- 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
Incidence rate (IR) can incorporate person-time
- 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
Incidence rate example
- 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
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
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
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
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
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
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.
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
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
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
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
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
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
