Epidemiology Flashcards
What is epidemiology?
Epidemiology is the science that studies the patterns and causes of health characteristics and their impact/ burden on defined populations.
Population perspective.
Basic science of public health that answers the questions:
- What causes the disease?
- How does the disease spread?
- What prevents the disease?
- What works in controlling the disease?
What do we use epidemiology for?
- Scientific basis to prevent disease and injury, promote health.
- Determine relative importance to establish priorities for research and action.
- Identify sections of the population at greatest risk to target interventions.
- Evaluate effectiveness of program to improve health of population.
- Study the natural history of the disease.
- Surveillance of disease and injuries.
- Investigate disease outbreaks.
What is the natural history of disease?
Natural history of disease looks at the entire process of development of the disease; it tells us what we can expect to happen. It is fundamental for studying and controlling the disease.
What are key characteristics of population?
What are key events and processes of the population?
Key characteristics: size/density, age, sex, place, ethnicity, education, economic resources
Key events and processes: birth, marriage, migration, aging and death
What is the difference between medical statistics and epidemiology statistics?
Medical statistics = individual perspective
focus on health, risk factors, exposures, causal mechanisms in individuals. approach to health problems: diagnosis (history, physical exam, lab tests), treatment
Epidemiology statistics = population perspective
focus on: mass disease, exposures, causal mechanisms in ppl as a group.
approach to health problems: community diagnosis (surveillance, descriptive data, surveys, analytical studies), interventions via the health care system, policies.
The two perspectives complement each other: every health condition results from a combination of individual level factors and population level factors. Different emphasis, both important.
What was the first epidemiological study?
The first epidemiological study was by John Snow in 1854: observing the different cholera outbreak frequency in different areas of the city of London
Stage 1: Descriptive Study
- Consider the factors that could differ between geographic areas
- Designed an epidemiological study to test the hypothesis that contaminated water was the causal factor
Stage 2: Analytical Study
- Proved that cholera was spread by contaminated water, conducted natural experiments
What is an infectious disease? What is infectious disease epidemiology?
Illness due to a specific agent or its toxic products that develops through transmission.
(from an infected person, animal or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector or inanimate environment).
Infectious disease epidemiology deals with two or more populations, the risk factor is that the infection can be transmitted and the cause is known. (See chart)
What is infectious disease epidemiology used for?
Infectious epidemiology is used for:
- Identification of CAUSES new and emerging (ex SARS)
- SURVEILLANCE of infectious disease (ex Ebola)
- Identification of the SOURCE of outbreaks
- Studies of ROUTES OF TRANSMISSION and NATURAL HISTORY of infections
- Identification of new INTERVENTIONS
What are the features of disease epidemiology?
Features of disease epidemiology include:
- a case may also be a risk factor
- people may be immune
- a case may be a source w/o being recognized as a case (asymptomatic)
- there is sometimes a need for urgency
- preventive measures usually have good scientific basis
-all diseases are caused by micro organisms
- disease can be transmitted from one infected person to another, directly or indirectly
- disease can be transmitted from one person to another by unnatural routes (example in water)
What is the epidemiological triad?
The epidemiological triad shows the dynamics of disease transmission.
Human disease results from the interaction between the AGENT, the HOST and the ENVIRONMENT. A VECTOR may be involved in transmission.
What is the SIR model?
The SIR model aims to predict the number of individuals who are susceptible, infected or recovered at any given time.
Susceptible - Infected - Removed
Each individual considered to be in one compartment at a given time, but can move from one to another:
Susceptibles have no immunity from disease
Infected have disease and can spread
Removed have recovered and are immune to further infection
What are some modes of transmission of infectious disease?
- Person to person: respiratory, urogenital, skin. Examples: HIV, measles
- Vector: animals, insects. Examples: rabies, yellow fever
- Common Vehicle: food, water. Examples: salmonellosis.
- Mechanical Vectors (personal effects, i.e. doorknobs, toothbrushes.
What are the possible outcomes of exposure to an infectious agent? (Infectious disease)
Possible outcomes of exposure to an infectious agent include:
- Nothing
- Carrier (individual who has the organism, but is not infected, can pass along)
- Recovered
Host Susceptibility depends on:
- genetic background
-nutritional status
- vaccination
- prior exposure
What is crucial to understand in epidemiology?
It is crucial to recognize the architecture of a disease: clinical and sub clinical disease.
What is the incubation period?
The incubation period is the interval between the time of contact and/or entry of the agent and the onset of the illness. Dynamic of disease.
What is a latent period (for infectious disease)?
Latent period is the time interval from infection to development of infectious disease.
What is endemic?
Endemic is the habitual presence of a disease in a given area.
What is epidemic? Pandemic?
Epidemic is the occurrence of an illness within a community or region, in excess of normal expectancy.
Pandemic is worldwide.
What is herd immunity?
Herd immunity is resistance of a group to an attack by a disease to which a large proportion of members of the group are immune.
The underlying principle is that the presence of enough immune people in a community interrupts the transmission of an infectious agent.
Conditions for herd immunity to exist include:
- the disease agent must be restricted to a single host species within which transmission occurs
- The transmission must be relatively direct from one to another
- 94% of the population must be immune before the chain of the transmission is interrupted
What is virulence?
Virulence is the severity of the disease produced by the organism.
What are the three critical variables to investigate a disease outbreak?
- When did the exposure take place?
- When did the disease begin? (onset)
- What was the incubation period for the disease?
If we know any two of these, we can calculate the third.
What is the attack rate?
Attack rate is those who are ill over those who were exposed.
It is useful for comparing the risk of disease in groups with different exposures.
Could also be specific to a given exposure, for example can calculate the attack rate for those exposed to radiation.
For example, number of ppl who ate a certain food and became ill over the number of people who ate that food.
x 100 to express as a percentage.
Secondary attack rate is the attack rate in susceptible people who were exposed to a primary case. (could think of family members)
What is R0? How do you calculate it? What is the magnitude influenced by?
R0 is the basic reproductive number: the average number of secondary cases caused by an infectious individual in a totally susceptible population.
R0 = D x C x B (probability)
Interventions can address each part.
If R0 is less than 1, the disease dies out. Implications are that infection control is unnecessary.
If R0 = 1; the disease is endemic.
If R0 is more than 1; it could become an epidemic. Implications: control measures are necessary to prevent/delay an epidemic.
If average number is high, it is easy to transmit.
If average number is low, it is difficult to transmit.
R0 usually goes down when we start testing asymptomatic individuals.
The magnitude of R0 is influenced by birth rate, population density and behavioral factors.
What were some challenges in COVID-19, thinking about infections and R0 formula?
What counts as a case?
(Those who test positive, suspected cases, asymptomatic?)
Time intervals are important; there were delays in test requests, reporting symptoms, receiving results, getting results to health authorities.
What is the force of infection?
The force of infection / rate of infection is the risk of being infected.
What are the aims of immunization / vaccine programs?
The aim of vaccines is to either:
1. Protect those at highest risk (selective strategy)
Selective vaccination, ex: HPV. New frontier, new possibilities with cancer.
- To eradicate, eliminate or control disease (mass immunization strategy)
Mass strategy very hard. Ex: Europe has no shared health goals/ indicators. Health is sensitive, culturally based.
Vaccines are the only treatment given to healthy people.
Eradication: Infection has been removed worldwide (ex: smallpox)
Elimination: Disease has disappeared from one area, but remains elsewhere (ex: polio, measles).
Control: Disease no longer constitutes a significant public health problem. (ex: neo-natal tetanus).
What is the ideal vaccine?
The ideal vaccine is:
- immunogenic
- long lasting immunity
- safe
- stable in field conditions
- combined
- single dose
- affordable and accessible to all
How do you evaluate a vaccine?
VE = Vaccine Evaluation
Pre-licensing:
Randomised, Blinded, Controlled Clinical Trials
Testing vaccine efficacy: Protective effects under Idealised Conditions
Post-licensing:
Observational Studies
Testing vaccine effectiveness: Protective effects under Ordinary conditions of a public health program
What is vaccine efficacy vs vaccine effectiveness?
Vaccine efficacy is the protective effect under idealised conditions (in trial phase).
Vaccine effectiveness is the protection effects in the general population.
99% efficacy in “ideal” population will have 50-70% effectiveness in general population.
What is the basic calculation for VE?
VE is Vaccine Evaluation. It tells you the % reduction in attack rate of disease in a vaccinated (ARV) compared to unvaccinated (ARU) individuals
Effectiveness is always lower than efficacy.
What is surveillance?
What is the objective of public health surveillance?
Surveillance is the ongoing, systematic collection, analysis and interpretation of data. It may be carried out to monitor changes in disease frequency or in the levels of risk factors.
The objective of public health surveillance is to provide the scientific and factual database essential for informed decision making and appropriate public health action.
What are types of surveillance?
Active surveillance: system employing staff members to regularly contact health care providers of the population to seek info about health conditions.
Passive surveillance: a system by which a health jurisdiction receives reports submitted from hospitals, clinics, public health units or other sources. Issues could arise with timing, depends on the source.
Routine health information system: a passive system in which regular reports about diseases and programs.
Health information and management system: a passive system by which routine reports about financial, logistic, and other processes involved in admin of PH and clinical systems are collected.
Categorical surveillance: system that focuses on one or more diseases or behaviors of interest to an intervention program. (active or passive)
Integrated surveillance: combination of active and passive systems, using a single infrastructure that gathers info about multiple diseases or behaviors of interest to several intervention programs.
Collection and response of those who collect (Government, Public Health Institutions, WHO) is that it is disconnected and hard to compare data.
What are the main measures of disease frequency?
The main measures of disease frequency include:
- Incidence Rate
- Attack Rate
- Prevalence Rate
- Mortality Rate (indicator of health)
Prevalence = probability of having a disease (to assess burden)
Prevalence increases with new cases/incidents.
It decreases with cure or death.
Incidence is the probability of developing the disease (to assess risk)
What is proportion? Rate? Ratio?
Proportion = Prevalence: % or fraction of a population with an illness or other characteristic
Rate = Incidence: How fast the disease is occurring in population with a time specification.
Ratio: Dividing one quantity by the other.
What is mortality rate vs incidence rates?
Mortality data is generally more available. Fatality reflects many factors, so mortality rates may not be a good surrogate of incidence rates.
Death certificate cause of death are not always accurate or useful.
Mortality does not equal incidence. (COVID example)
What is Exposure? What is Disease?
Exposure (E)
Risk factor, potential health determinant; the independent variable (example: smoking)
Disease (D)
Outcome after exposure to the risk factor, development of disease (death or disability included), the dependent variable (example: lung cancer)
What is incidence rate?
Incidence rate measures the number of new cases of a disease that occur in a specified time period in a population at risk.
It is dynamic.
Some factors that influence incidence rate include:
- New risk factor
- changing habits
- changes in virulence of the causative organisms
- changes from intervention program
- selective migration
-population pattern (ex: aging? increased degenerative disease)
- reporting
- screening
- new diagnostic tools
We need to understand when changes are due to changes in the disease or other factors (like out migration of health people, improved diagnostic facilities, etc).
What is the relationship between incidence and prevalence?
Prevalence = incidence x duration of disease
With higher incidence, there is higher prevalence (short duration of illness and recovery, prevalence may or may not change)
Longer disease duration leads to higher prevalence.
What is population attributable risk (PAR)?
Population attributable risk is the excess risk of disease in total population attributable to exposure. It is the reduction in risk which would be achieved if population entirely unexposed. It helps us to determine which exposures are relevant to public health community.
Example: fast driving , risk of dying for any cause, take out the 80 slow drivers, 44% of deaths presumably due to fast driving.
What is relative risk? (RR)
Relative risk (RR) is a measure of effect. It is the ratio of the risk of disease in people exposed compared to the risk in people unexposed.
The probability of having a disease with risk factor respect to the probability of having a disease without risk factor, it is used in cohort studies.
What is odds ratio? (OR)
The Odds ratio (OR) is the ratio of odds of a condition in the exposed compared to the odds of the condition in the unexposed. It is usually applied in prevelance studies.
It is the ratio between exposure ODDs and non exposure ODDs; it is used in case control studies and in corss-sectional studies.
What is the difference between a screening and a diagnostic test?
Screenings are a measure of secondary prevention, it detects early asymptomatic phase whereby early treatment can be given and disease can be cured or its progression can be delayed.
Screening is the search for unrecognized disease by means of applied tests, exams, or other procedures in apparently healthy individuals.
Screening programs work better when the time lag between the disease’s onset and the disease’s final critical point is long.
Quality of screening and diagnostic tests is critical.
What is prognosis?
Prognosis is the prediction about the likely outcome/course of the disease. (natural history)
Prognosis has to do with prediction and risk factors.
We express prognosis in terms of death from disease or survivors.
What is diagnosis?
Diagnosis is the identification and recognition of a possible disease.
It has to do with association and screening. Very often the time to diagnosis is the baseline time. (Latency - time to detection)
What happens to the prevalence and incidence rates when you remove part of the population from the denominator?
Looking at prevalence and incidence rates, everyone in the denominator needs to also be able to join the numerator. If you remove part of the population from the denominator, the prevalence rates will go up.
The example in the book is about women with hysterectomy and uterine cancer rates. Women with hysterectomies cannot get uterine cancer, so they need to be removed from the calculation, and therefore incidence rates go up.
What is a standarized mortality ratio?
Standarized mortality ratio is used when number of deaths for each age-specific stratum are not available. It is used to study mortality in an occupationally exposed population.
