Epidemiology Exam 1

epidemiology

the science of public health

the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems

study

quantitative discipline based on principles of statistics and research methods

distribution

distribution of health events within groups in a population, characterizing health events in terms of person, place, and time

descriptive epidemiology

determinants

causes or factors associated with increased risk/probability of disease

analytical epidemiology

health-related states

anything you can think of

populations

groups of people (not individual patients)

control

epi methods steer public health decision-making and aid in developing and evaluating interventions

applied epidemiology

objectives in epidemiology

identify causes of disease (etiology) and risk factors

determine the extent of the disease in the community

examine the natural history of the disease and prognosis

evaluate effectiveness of interventions and treatments

provide input on public policy

identify causes of disease (etiology) and risk factors

descriptive statistics often evaluated first

means, counts, percentages

next, try to determine possible causes of observed differences: measurement error? greater exposure? differences in characteristics of people in our popultion of interest?

determine the extent of the disease in the community

in addition to reporting extent of disease, epidemiology also focuses on changes in threats to health over time

similar changes found as countries become more industrialized (“epidemiologic transition”)

changes in causes of death necessitate changes in type of research done

epidemiologic influence on clinical practice

many aspects of clinical practice are developed from research on populations of people

diagnostic practices

knowledge about association between certain assessment findings (heart murmurs) and specific disorders (mitral regurgitation) came from population-based research

prognosis estiamtes

based on observations of large groups of people with the same disease at the same stage who received the same treatment

treatments

randomized trials on large groups of people provide evidence about the efficacy of different treatment options

scientific method used in epi

observation

hypotheses: what causes (or correlates) with the disease

data collection and analysis

conclusions/recommendations

epi as an interdisciplinary field

math and biostats

demography and geography

history

sociology

behavioral sciences

law

before epidemiology, how did we determine what caused diseases?

no concept of testing hypotheses in a systematic way in groups of people

no structured way of evaluating information

“causes” of bubonic plague

14th century plague that swept through Asia, Europe, and Africa

killed 50 mil; between 1346-1352, 25-33% of European population died

believed “black death” was caused by punishment from God/gods, contact with lepers, walking in the hot sun, miasmas

miasma

Greek for pollution

noxious vapors/gases, caused by decaying matter and foul smell

diseases attributed to miasma: cholera, bubonic plague, and malaria (bad air)

actual cause of plague

bacterium

transmitted through bites of infected fleas and other vectors, person to person via air, contact with contaminated fluid or tissue

key figures in development of epi

John Graunt: summarized pattern of mortality in 17th century London

James Lind: used experimental study to determine cause of scurvy in 18th century

John Snow: showed that cholera was transmitted by fecal contamination of drinking water in 19th century

cholera background

swept into Europe in early 1800s, epidemics in London and Paris

symptoms:

nausea, dizziness

violent vomiting and diarrhea

rice water stools

extreme muscle cramps

insatiable thirst

cardiovascular collapse

death

bacterial infection of the intestine transmitted by ingestion of water or food contaminated with sewage

cholera theories

connection between micro-organisms and disease had not yet bee determined so believed it was due to miasmas

John Snow

English anesthesiologist and father of epidemiology

hypothesized cholera may be transmitted by water or food, not miasma

conducted a landmark series of studies to test hypothesis and create an intervention

create mapping of Broad Street

London Water Supply

Some families carried water from community pumps; others got water from companies that pumped water from the Thames River via pipes

Two different water companies supplied water from the Thames River to houses in the same area

In 1852, the Lambeth Company relocated its sources of water to a less polluted portion of the river

Snow noted that residents served by the Lambeth Company had fewer cases of cholera than residents served by the other company

Significance of Snow’s work

based on observation and reason, he proposed a new hypothesis for how cholera was transmitted

tested hypothesis by collecting data systematically and comparing groups of people

established an association between certain (contaminated) drinking water and getting cholera

argued for an intervention that prevented more cases

after Broad Street outbreak

removed pump handle temporarily to halt the outbreak

public officials refused to accept waterborne theory; clung to miasma

fecal-oral route finally became accepted after the isolation of the cholera bacterium in 1883 (well after Snow’s death)

2015 epi example

residents of Flint, MI were concerned about water quality and skin rashes

Dr. Mona Hanna observed elevated blood lead levels in her patients and decided to conduct an analytic epidemiologic study using data available in electronic medical records

revealed findings publicly and advocated for action in Sept 2015 press conference before her research was scientifically peer-reviewed

issued health advisory

findings met with skepticism - said she was inciting a panic

continued to publish her work and MI ultimately began to take public health action

wealth of information on health status of populations around the world and in the US

Most data are collected by governmental & non governmental agencies on a routine basis or by special surveys

Can obtain info on deaths & a wide variety of conditions, including acute illnesses & injuries, chronic conditions, & pregnancy outcomes

Information also available on characteristics that influence a person’s risk of illness

Nutritional habits; immunizations; use of cigarettes, alcohol, & drugs

Also have data on impact of conditions on utilization of health service

US government agencies involved in public health data collection

US department of health and human services (HHS)

centers for disease control and prevention (CDC): monitoring and surveillance and health promotion

national institutes for health (NIH): funds and conducts research on treatment and prevention

US department of agriculture: national dietary guidelines and WIC and SNAP programs

US data sources

US census

vital statistics

health surveys

disease registries

notifiable disease systems

sentinel surveillance

administrative systems

census data

every 10 years, the government conducts a census of the entire US population

household characterisitcs, age, race, education, occupation, insurance coverage, income, disability

essential for calculating rates in populations so we can compare disease burden and trends

vital statistics

information collected at time of birth and death

mortality records are oldest data systems used for disease surveillance

collected via vital registration system in US

process: states register death certificates and transmit files to national center for health statistics for processing; in return, states receive files with international classification of disease codes

death certificates list immediate cause of death, sequence of events that led to death, other contributing causes and manner of death - occasionally incomplete

accurate cause-of-death ascertainment has broad implications for understanding disease burden in US

% of excess death not assigned to COVID varies by state and county

counties less likely to assign excess deaths in the West and South, areas with low SES or non-Hispanic black residents

health surveys

may be used to collect info about self-reported behaviors and health practices

national health and nutrition examination survey (NHANES): gathers data on health and diet of US population, includes interviews and health tests

national health interview survey: gathers data on major health problems and healthcare utilization

national survey of family growth: gathers data on marriage, divorce, family planning, infertitlity; both men and women 15-49 in 50 states

behavioral risk factor surveillance system (BRFSS): telephone survey on health risk behaviors related to chronic disease, injuries, & death; incudes use of screening & preventive services, smoking, alcohol use, physical activities, F/V consumption, seatbelt use, & weight control, being updated to comply with new administration’s DEI orders

national hospital care survey: collects data on patient care in hospital-based settings (inpatient, emergency, outpatient) to describe patterns of health care delivery and use in the US

national immunization survey: collects info on immunization coverage in US

national survey of drug use and health: obtains information on mental health, use of various substances, ended in 2025

pregnancy risk assessment and monitoring system: data on maternal attitudes and experiences before, during, and shortly after pregnancy, 81% of US births, on hold to comply with new administation’s DEI orders

youth risk behavior surveillance system (YRBSS): includes 1- a national school-based survey conducted by CDC & state, territorial, & tribal education groups, & 2-local surveys conducted by state, territorial, & local education groups + health agencies

public health surveillance

systematic, ongoing collection, management, analysis, and interpretation of data

followed by dissemination of data and applying surveillance findings to public health programs

notifiable disease systems

council of state and territorial epidemiologists (CSTE) recommends annual modifications to list

120 illness and disease types, primarily infectious diseases, few noncommunicable diseases

chronic disease registries

Collection of chronic disease info often mandated under state law

Most common state surveillance system: cancer registry

Hospital registries: info on patients diagnosed/treated at specific facility

Often have funds to conduct follow-up

Population-based registries: info on all people residing in a specific geographic area

Often do not have funds to conduct follow-up

Other registries: traumatic brain injury, stroke, birth defects

Surveillance, epidemiology, and end results program (SEER): collects data on prevention, diagnosis, and treatment of cancer in 18 population-based registries across US

sentinel surveillance

“sentinel event” could be a symptom, constellation of symptoms, diseases, or deaths

focused on occupational-related health conditions for chronic diseases (silicosis, lead poisoning)

now COVID and other respiratory illnesses

administrative systems data

hospital discharge info, medicaid and medicare claims

limited by incomplete records, unreliable coding/measurement error, and the fact these data only capture events that occur in a hospital setting

world health organization (WHO)

World Health Statistics Annual: Compiles international morbidity & mortality data on 194 WHO member states

Cancer Incidence on Five Continents: WHO International Agency for Research on Cancer (IARC) collects data on cancer incidence & mortality from many countries around the world

united nation public health data collection

demographic yearbook

collect data on 230 countries and areas of world on population size, distribution and growth, births, deaths, marriages, divorces

global players in data collection

WHO

united nations

non-governmental and multi-lateral organizations

data considerations

specific population covered by the data collection system

calendar period covered by data collection system and frequency with which data are updated

the most current available data typically lag a year or two behind the present

every data collection system has some incomplete and inaccurate material

two types of epidemiology

descriptive and analytic/scientific

descriptive epi

describe disease patterns

1. monitor public’s health

2. evaluate success of intervention programs

3. generate hypotheses about causes of disease

identify and count cases of disease in populations and conduct simple studies (case report, case series, cross-sectional study, ecologic study)

what are the main conditions/diseases in a population? who is getting them? how does it vary across time and place?

identify problems, trends, high risk groups; planning and where to spend resources; generate hypothesis for analytic epi

cannot identify causes of disease!!

analytic/scientific epi

search for disease causes and preventions

1. evaluate hypothesis about causes of disease

2. evaluate success of intervention programs

compare groups and systematically determine: is there an association? (clinical trial, experimental study, case-control study, cohort study)

descriptive epi measures

person

place

time

person

important to examine because personal characteristics may affect illness

permanent: age, sex, race/ethnicity

biological: immune status

acquired: marital status

activities: occupation, leisure activities, use of medications/tobacco/drugs

conditions: SES

age

likely the single most important “person” attribute

almost every health-related event varies with age

can be categorized by year or year groupings

factors that vary with age

susceptibility

opportunity for exposure (hygiene hypothesis)

incubation period of disease

physiologic response

sex

refers to a person’s biological status (male/female)

indicators of biological sex: sex chromosomes, gonads, internal reproductive organs, external genitalia

gender

refers to attitudes, feelings, and behaviors that a given culture associates with a person’s biological sex

gender identity

refers to “one’s sense of oneself as male, female, or transgender”

descriptive epi: person/sex

differences in disease rates may occur because of genetic, hormonal, anatomic, or other inherent differences between the sexes

inherent differences affect susceptibility or physiologic responses

differences may also be due to difference in opportunity or levels of exposure

descriptive epi: person/gender ex.

trachoma - infectious cause of blindiness

4x as common in women vs. men

not by biological difference but attributed to sociocultural factors

no consensus on race vs. ethnicity

one view: ethnicity is cultural heritage while race is a societally-imposed identity based on phenotype (ex. skin color, hair texture, etc.)

descriptive epi: person/race and ethnicity

differences in racial, ethnic, or other group variables may reflect differences in susceptibility or exposure, or differences in other factors that influence risk of disease, such as SES and access to health care

descriptive epi: person/SES

many variables: occupation, family income, educational achievement, living conditions, social capital

epidemiologists commonly use occupation, income, and educational achievement (easiest to measure) but recognize not precise measures of SES

adverse health conditions increase with decreasing SES (ex. infant mortality, life expectancy, infectious disease) —> may reflect harmful exposure, low resistance, less access to care

few adverse health conditions occur more frequently among persons of higher SES

melanoma

breast cancer (first child later in life, fewer children, menopausal hormone therapy, drink alcohol)

tennis elbow

marital status

singe or non-married (ex. never married, divorced, widowed)

married

living with a partner

in general, people in partnerships tend to have lower rates of morbidity and mortality

partnership may operate as a protective or selective factor

protective: provide environment conducive to health

selective: people who have/seek partnership are healthier than those who don’t

descriptive epi: place

place helps us determine where diseases are occurring and if frequency varies by location

any geographic location relevant to disease occurrence (place of residence/diagnosis, birth place, place of employment, school district, recent travel destinations)

even if place data cannot reveal why risk may be increased, it can help generate hypotheses to test with additional students (ex. crowding, homes built in wooded areas and Lyme)

time

occurrence of disease change over time

some are predictable and some aren’t

ex. flu, RSV, pertussis (predictable), salmonella (unpredictable)

for diseases that occur seasonally, health officials can

anticipate their occurrence

implement control and prevention measures (ex. flu vaccination campaign)

for diseases that occur sporadically investigators can

conduct studies to identify causes and modes of spread

develop appropriately targeted action to control or prevent further occurrence

regardless of predictability, displaying patterns of disease occurrence by TIME is critical for:

monitoring disease occurrence in the community

assessing whether public health interventions made a difference

years, months, days, specific day or time

hypothesis

tentative explanation for an observation, phenomenon, or scientific problem than be tested by further investigation

process of generating hypotheses is creative, and combines observations with biomedical and sociodemographic information

as epis, we generate hypotheses by:

Comparing groups with different disease rates to try & identify characteristics that may account for those disparate rates

Identifying common characteristics that link affected groups

Describing the presence/variation over time for an associated risk factor

Making analogies with other conditions/diseases

descriptive epi: hypothesis generation

specific enough to be tested using scientific method

defines two or more groups to be compared (ex. exposed vs. unexposed)

gives direction of relationship (ex. exposure increases risk or exposure decreases risk of disease)

gives a time frame for the expected outcome

Sandy Ford

1981: Sandy Ford, US CDC drug technician, major role in beginning of HIV/AIDS epidemic

noticed increase in requests for pentamidine isethionate - a drug available in US only through CDC for treatment of Pneumocystis pneumonia (PCP) - immunocomprised and organ transplant patients who were immunosuppressed

after communication with treating physicians, she learned that all of these patients were otherwise adults men who had sex with men

unusual and communicated to supervisor

origins of disease

descriptive epi can be used to study origins of disease

we are interested in learning: who are the people at greatest risk for developing the condition and when and where the condition occurs

natural history of disease

1. onset of disease

2. pathological evidence

3. signs and symptoms begin

4. person seeks medical care

5. diagnosis

6. treatment

7. outcome

non-clinical disease

not apparent or obvious signs or symptoms

pre-clinical disease

disease at an early stage that will ultimately present with signs/symptoms

subclinical disease

low severity disease that may not ultimately present with signs/symptoms

chronic disease

persistent disease in which symptoms wax and wane

latent disease

organism causing disease has stopped replicating but lays dormant in body

clinical disease

high severity disease that produces signs and symptoms

signs: objective manifestations of disease

symptoms: subjective manifestations of disease

epidemiologic triad and onset of disease

disease occurs as the result of an interaction between a host, an agent (causes the disease), and the environment

sometimes a vector facilitates transmission of a given disease

host must be susceptible to disease in some way

interaction permits transmission of a pathogenic organism or an alteration of body function

genetic components can be activated due to interaction so difficult to identify these as symptoms and often takes time to develop

bacteria

Staphylococcus aureus

• Streptococcus pyogenes

• Escherichia coli

• Helicobacter pylori

viruses

Herpes simplex

• Hepatitis A, B, C

• Varicella zoster

• SARS CoV-2

fungi

Candida albicans

• Aspergillus

• Pneumocystic jirovecii

protozoa

Giardia lamblia

• Trypanosoma brucei

• Plasmodium

dietary insufficiency

Vitamin C

• Vitamin D

• Folic acid

characteristics of the environment could influence the presence of the agent and nature of the interaction

temperature

humidity

water supply

pollution

population density

disease transmission

directly or indirectly

direct transmission

direct contact

indirect transmission

by exposure to a common vehicle

water

air

vectors

formites (objects such as furniture, utensils, etc. on which organisms can survive)

carrier

someone who can transmit an infectious organism or trait that causes disease but shows no signs or symptoms of disease

ex. Mary Mallon (Typhoid Mary) is believed to have been responsible for 10 outbreaks of typhoid in NYC in 1900s

common characteristics of outbreaks

Sudden, sharp increase in the number of cases following an exposure

Increase in cases is limited to those who experienced the same exposure

Example: There was a sharp increase in leukemia among people living in Hiroshima following the atomic bomb

disease outbreaks

extent to which disease affects a population is associated with susceptibility and mount of immunity in the population

susceptible

those at risk for developing condition

immune

those who cannot develop condition

may have had condition and may have developed antibodies against condition

may have been immunized against condition

herd immunity

large % of people in a community become immunized against a condition

low likelihood of diseased person coming into contact with a susceptible person

chain of infection broken and less likely to be an outbreak

for herd immunity to be effective

1. Immunity needs to be complete (If only partial immunity, a person could become re-infected & spread would not be contained)

2. Condition has to be spread from person-

to-person (If organism can survive outside of a human host & be transmitted by another means (e.g., fomite), herd immunity may not develop)

immunization and herd immunity

% of people who need to be immunized for herd

immunity to occur varies based on condition

Measles is highly contagious; ~94% of population needs to be

immunized to restrict spread of infection

Poliomyelitis is ~80%

Immunization programs are developed to get as many

people immunized as possible

incubation and disease spread

once an organism has infected a host, it begins to replicate

a person may not experience symptoms: incubation period

may express measurable biomarkers of disease

symptoms begin once number of organisms reaches a threshold

an infected person may be contagious during the incubation period

quarantine

separates and restricts movement of people exposed to a contagious disease to see if they become sick

incubation and chronic disease

Chronic diseases also have an “incubation period” (the time it takes for symptoms

to appear)

Person may be exposed to a carcinogen, but it will take time for

cancer to develop

Person may be exposed to coal dust, but it will take time before lung disease develops

Person may be exposed to radiation, but it may take time before effects of are observed

epidemic curve

number of cases of a disease following an exposure can be plotted over time

each vertical bar represent number of cases

connect tops of each bar in a graph