Introduction to Epidemiology and the Epidemiology Modules

Today

What is Epidemiology?

Measures that are important in all of epidemiology

Why I never believe anything I read in the newspaper

The three modules and how they fit into the big picture

What is Epidemiology?

(English)

epi (among)

demos (people)

logy (study)

What is Epidemiology?

The study of the distribution and determinants of health and disease related states in populations,and the application of this study to control health problems. (John M. Last)

What is Epidemiology?

The study of the distribution and determinants of health and disease related states in populations,and the application of this study to control health problems.

“epidemiology’s full value is achieved only when its contributions are placed in the context of public health action, resulting in a healthier populace.” (Koplan et al. 1999)

“the product of [epidemiology] is research and information and not public health action and implementation”. (Atwood et al. 1997)

“Epidemiology . . .

is a Greek word that means to put people to sleep with charts and graphs.”

- Dr. Mark JohnsonIn testimony before the House Judiciary Committee on the spread of HIV

“Epidemiologists . . .

Are like bookies of disease, stalking the globe to determine point-spreads on which groups of people are most likely to get which diseases.

Part detective and part statistician, part anthropologist and part physician, epidemiologists hope to track down the agents of illness by deducing which of the differences between peoples lie at the root of their distinctive disease patterns.”(H. Shodell, Science ’82, September, p. 50)

Functions of Epidemiology

(Schoenback and Rosamond)

Epidemiologic approaches

DESCRIPTIVEHealth and disease in the community

What? Who? When? Where?

What are thehealth problemsof the community?

What are theattributes of these illnesses?

How many peopleare affected?

What are theattributes ofaffected persons?

Over whatperiod of time?

Where do theaffected peoplelive, work orspend leisuretime?

ANALYTICEtiology, prognosis and program evaluation

Why? How?What are the causal agents?

What factorsaffect outcome?

By what mechanism do they operate?

Forms of Epidemiology

•Clinical Epidemiology•Descriptive Epidemiology•Predictive Epidemiology•Etiologic Epidemiology•Genetic Epidemiology•Occupational Epidemiology•Social Epidemiology•Spatial Epidemiology•Surveillanceetc…

US Accredited Schools of Public Health, 2007

Academia The Colorado School of Public Health

As of July 1st!

Government Agencies

•Local health departments

•State health departments

•Office of the Surgeon General of the United States

•FDA

•Centers for Disease Control (CDC)

•World Health Organization (WHO)

Julie Gerberding, MPH, MD

Industry

•Occupational epidemiologists – monitor and investigate effects of worker exposures

•Pharmaceutical companies – mostly clinical trials of drugs, but also follow-up epidemiologic studies

•Contract research organizations (CRO)– design and implement epidemiologic studies for government or other industries

A Brief History of “Modern” Epidemiology

John Snow (1813-1858) Public Health intervention

Late 1800s -> 1950s The “hey day” of infectious diseaseepidemiology and prevention

John Ryle (1899-1950) The Institute of Social Medicine

Doll & Hill (1954) Smoking and Lung Cancer

Archie Cochrane (1909-1988) Evidence-based medicine

Mary-Claire King (1946- ) BRCA1 gene and Breast Cancer

What is Epidemiology?

The study of the distribution and determinants of health and disease related states in populations,and the application of this study to control health problems. (John M. Last)

What are “disease” and “health”?

Dorland's Illustrated Medical Dictionary (28th ed.):

Disease – "any deviation from or interruption of the normal structure or function of any part, organ, or system (or combination thereof) of the body that is manifested by a characteristic set of symptoms and signs . . .".

Health – "a state of optimal physical, mental, and social well-being, and not merely the absence of disease and infirmity."

What is “disease”

Manifestional criteria:

Manifestational criteria refer to symptoms, signs, and other manifestations of the condition. Defining a disease in terms of manifestational criteria relies on the proposition that diseases have a characteristic set of manifestations. This defines disease in terms of labeling symptoms.

Causal criteria:

Causal criteria refer to the etiology of the condition, which, must have been identified in order to be employed. This defines disease in terms of underlying pathological etiology.

The Acquired Immunodeficiency Syndrome (AIDS) was initially defined by the CDC in terms of manifestational criteria as a basis for instituting surveillance.

The operational definition grouped diverse manifestations – Kaposi's sarcoma outside its usual subpopulation, PCP and other opportunistic infections in people with no known basis for immunodeficiency.

This was based on similar epidemiologic observations (similar population affected, similar geographical distribution) and a shared type immunity deficit (elevated ratio of T-suppressor to T-helper lymphocytes).

Manifestational Criteria

Causal Criteria

Human immunodeficiency virus (HIV, previously called human lymphotrophic virus type III) was discovered and demonstrated to be the causal agent for AIDS.

AIDS could then be defined by causal criteria.

Challenges with Disease Classifications

1.A single causal agent may have multiple clinical effects.

2. Multiple etiologic pathways may lead to apparently identical manifestations, so that a manifestationally-defined disease entity may include subgroups with differing etiologies.

3. Multi-causation necessitates a degree of arbitrariness in assigning a causative versus a contributing factor to a disease.

4. Not all persons with the causal agent develop the disease.

The Natural History of Disease

UnderlyingGeneticSusceptibility

onset ofdisease

diagnosisof disease

Environmental & Behavioral Factors

PhysiologicAbnormalities Clinical disease

cause specificmortality

XSub-clinical disease

Measures of Disease Occurrence

• To study disease, need measures of its occurrence

• Some measures of disease occurrence– Counts– Prevalence – Incidence– Mortality

Epidemiologic approaches

DESCRIPTIVEHealth and disease in the community

What? Who? When? Where?What are thehealth problemsof the community?

What are theattributes of these illnesses?

How many peopleare affected?

What are theattributes ofaffected persons?

Over whatperiod of time?

Where do theaffected peoplelive, work orspend leisuretime?

Each of the measures can be calculated for different combinations of “What? Who? When? and Where?”

Each of the W’s needs to be defined carefully to get comparable measures across state, nation, world

Prevalence

• The prevalence of a disease is the proportion of individuals in a population with disease (cases):

• Prevalence is a proportion – range of 0 to 1

• Removes the effect of total population size – makes estimates from different populations or over time more comparable.

Number cases in population at specified timeNumber of persons in population at that specified time

Prevalence

• Often expressed as a percent (%) – Prevalence *100

• Also often expressed as the prevalence per 1,000 or 10,000 or 100,000.

• Prevalence * 1,000 = prevalence per 1,000.

1991 1995

Obesity Trends* Among U.S. Adults

BRFSS, 1991-2006

2002

No Data <10% 10%–14% 15%–19% 20%–24% ≥25%

(*BMI ≥30, or ~ 30 lbs overweight for 5’ 4” woman)

2006

Prevalence – Salmonella Cases infected with the outbreak strain of Salmonella Saintpaul, as of July 15, 2008 9pm EDT

Incidence

• Incidence is a measure of risk of developing disease

• Often multiplied by 100,000 (or 1000 or 100) and reported as “Incidence per 100,000”

Number of NEW cases in population DURING specified timeNumber of persons AT RISK of disease in population during that specified time

If population size is 3.81 million, then652

100,0003,810,000

.00017 100,000

17.1

I

Incidence – Salmonella Incidence of cases of infection with the outbreak strain as of July 15, 2008 9pm EDT

Prevalence and Incidence – Salmonella

Cases infected with the outbreak strain of Salmonella Saintpaul, as of July 15, 2008 9pm EDT

Incidence and Prevalence

• Incidence and prevalence measure different aspects of disease occurrence

Prevalence IncidenceNumerator:

Denominator

Measures:

Most useful:

All cases, no matter how long diseased

Only NEW cases

All persons in population

Only persons at risk of disease

Presence of disease

Risk of disease

Resource allocation

Risk, etiology

Mortality Rate – Incidence of death

• Numerator– Number of deaths

• Denominator– Number of individuals in population

• Time interval– 1-year: Annual Mortality Rate– Typical to use annual rate

• Specifier– age, sex, race, etc.

Mortality rates

Case Fatality “Rate”

• Cumulative incidence of death among those who develop an illness

• Technically not a rate, but a proportion

• Time period should be stated

• Example– 15/300 persons died in 30 days after surgery

– CFR = 0.05 x 1,000 = 50 per 1,000

– Risk of dying within 30 days of surgery is 50 per 1,000 surgeries, or 5%

Importance of defining numerator and denominator

• For each of the measures, carefully defining both the numerator and denominator crucial for interpretation

• In order for measures to be comparable across studies, need consistent definition and reporting strategies for numerator

• Also need consistent approaches for counting (or estimating) the persons or person-time for the denominator

Prevalence Numerator – case definition

From Gordis Text

Prevalence Numerator – case definition

Gordis: Fig 4-17

Result ofnew definition

1st Quarter of 1993:Expansion of

surveillance casedefinition

AIDS cases, United States 1984-2000

Incidence Denominator – Who is at risk?

Importance of looking at multiple measures

The “demi” in Epidemiology

Understanding population dynamics is crucial to epidemiology.

Demography = the study of population dynamics including fertility, mortality and migration

denominator…denominator…denominator…denominator…denominator…denominator…

Comparing rates between two different populations

What is your guess regarding the incidence of HIV/AIDS in Kenya compared to Denmark?

Comparing rates between two different populations

Crude Incidence of HIV/AIDS, 2005

0

2

4

6

Kenya Denmark

Rate / 1,000

Kenya data from UNAIDS/WHO, Denmark data from UNICEF

Comparing rates between two different populations

0

25

50

75

100

Age (years)

% ofcases

HIV/AIDS by Age

(from CDC, U.S. 2001-04)

Potential problems in interpretation of rates over time

• Numerator– diagnostic abilities– diagnostic criteria– reporting completeness– payment schemes (affecting items above)– others . . .

• Denominator– census methods for classification– age misclassification– removal of persons at risk– cohort effects (changes in exposure/prognosis or other

confounding variables over time)– others . . .

Breast Cancer Incidence in Colorado in 2000

Age (years)

20 - 29 30 - 39 40 - 49 50 - 59 60 - 69 70 - 79 80 - 89

Incid

en

ce

of B

reast C

ancer

(per

1,0

00)

0

10

20

30

40

50

Age / Period / Cohort Effects

Breast Cancer Incidence in Colorado by year of Survey

Age (years)

20 - 29 30 - 39 40 - 49 50 - 59 60 - 69 70 - 79 80 - 89

Incid

en

ce

of B

rea

st C

an

ce

r (p

er

1,0

00

)

0

10

20

30

40

50

1910-191920-291930-391940-50 1950-59

Age / Period / Cohort Effects

The Three Epidemiology Modules

• Imperfect testing

• Pass it on

• Computer modeling of disease outbreaks

Imperfect TestingScientific questions that require or are made

easier to answer by binary tests

• Diagnosis of disease in a clinical setting

• Screening individuals to identify those who might need further testing to determine disease status

• Surveillance for - Cancer and other diseases - Infectious disease outbreaks - Bioterror agents

• Identifying sequence characteristics across the genome

Imperfect Testing• Focus on medical testing, but applicable to all sorts of testing

scenarios

• Follows story of a woman who gets a positive mammogram and wants to know the implications – does she have breast cancer?

• She goes on to get tested for some of the known breast cancer genes – and learns what that means for her daughter

• Emphasis on understanding the implications of the results from imperfect tests – and the personal decisions that are often necessary.

• Uses simple and conditional probabilities and concepts of relative risk (ratio of risk for those with and without genetic risk factor)

Pass it On/Modeling of OutbreakSome questions we might want to ask about infectious

disease spread and epidemics

• How many individuals (or what proportion) will become infected?

• Will a given disease become epidemic? How far will it spread?

• How long will the disease or pathogen persist in the population?

• Would vaccination prevent an epidemic? If so, what type of vaccination program is most efficient?

• What other measures could be taken to prevent an epidemic?

Pass it On

• Focus is on transmission of infectious disease

- explore routes of transmission

- probability of transmission based on a certain route or type of contact

• Relates probabilities to expected numbers of exposed and infected individuals

• Relate probabilities to evolutionary success of the disease-causing organism

Computer Modeling of Disease Outbreaks

• Focus is on modeling characteristics of an outbreak

• Explores parameters expected to influence incidence and prevalence of an infectious disease

• Uses a simple compartmental model

• Explores types of interventions and their impact on the parameters that influence incidence, prevalence, scope of outbreak

The Three Epidemiology Modules

• Imperfect testing

• Pass it on

• Computer modeling of disease outbreaks