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CHAPTER 5

Epidemiologic Principles and Methods

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Copyright © 2021 by Jones & Bartlett Learning, LLC an Ascend Learning Company. www.jblearning.com

Definition of Epidemiology

Epidemiology is defined as “the study of the distribution and determinants of disease frequency in human populations.”

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Step 1: Define the Disease

Death is easy to determine.

A death certificate states cause of death.

A blood test or stool culture is needed to verify a diagnosis of certain diseases.

Some diseases are hard to define.

EMS and SARS

Sometimes a definition changes as more is learned.

AIDS

Other health outcomes include injuries and risk factors.

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Disease Frequency

Count the number of people with a disease and relate that to the population at risk (PAR).

PAR (denominator) may be the total population or exposed population, or one gender or age group.

PAR often comes from a census.

Two ways to measure frequency are:

Incidence, the number of new cases

Prevalence, the number of existing cases

Incidence is used for studying causes of disease.

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Disease Frequency (cont.)

Prevalence depends on incidence and prognosis.

If causes or risk factors increase, incidence and prevalence increase.

If ability to diagnose increases, incidence and prevalence appear to increase.

Prevalence rates are most useful in assessing the societal impact of a disease and planning for healthcare services.

Mortality rates are used to measure frequency for diseases that are often fatal.

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Distribution of Disease

Who:

Sex, age, occupation, race, and economic status

When:

Looks for disease frequency over time: Season, year (long-term trends), elapsed time since an exposure (epidemic curve)

Is crucial in tracking an outbreak of infectious diseases such as hepatitis and legionellosis

Where:

Neighborhood (e.g., clusters), latitude (climate), urban vs. rural, national variations

Looks at comparisons of disease frequency in different countries, states, counties, or other geographical divisions

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Determinants of Disease

Why is distribution as it is?

We can make inferences from distribution.

Epidemiologists usually speak of risk factors not causes.

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Human Population

Epidemiology studies human population, usually using observational rather than experimental methods.

Biomedical approach uses animal models to investigate the causes of disease.

Experiments conducted on animals can yield clear answers as to cause and effect.

For ethical reasons, experiments cannot usually be done on humans.

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Kinds of Epidemiologic Studies

Goal is to determine an association between an exposure and a disease or other health outcome.

Studies may be prospective or retrospective.

Intervention study

Cohort study

Case-control study

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Intervention Study

Epidemiologists do not perform the experiments.

Closest thing to an experiment.

Start with two groups:

Experimental group (gets the intervention or exposure)

Control group

Watch them over time and compare outcomes.

Experimenter chooses who is in which group.

Two groups should be as similar as possible so that intervention is the only difference.

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Intervention Study (cont.)

Randomized, double-blind, placebo control is ideal.

Pharmaceutical companies conduct many clinical trials for new drugs.

Physicians’ Health Study was a prevention study:

Aspirin to prevent heart disease

Beta carotene to prevent cancer

Field trial of polio vaccine in 1954 was randomized and double-blind.

Kingston–Newburgh study of fluoridation to prevent tooth decay was a community trial.

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Cohort Study

Are for situations when doing an intervention study would be unethical or too difficult.

Considered the next most accurate

Choose a large number of healthy people, collect data on their exposures, and track outcomes over time.

The only difference from intervention is that people choose their own exposures.

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Cohort Study: Examples

Framingham Heart Study

Nurses’ Health Study

British study of physicians on smoking and lung cancer

Hammond–Horn study on smoking and lung cancer in the U.S.

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Case-Control Study

Faster and cheaper are the advantages.

This is the least accurate approach.

It is commonly done to follow up on a hypothesis generated by shoeleather epidemiology.

Choose people who already have disease.

Choose a healthy control group of individuals, as similar as possible to cases.

Interview them all and ask for their previous exposures.

Estimate the strength of the association between exposure and disease by calculating an odds ratio.

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Discussion Question 1

What is the difference between incidence and prevalence?

Why is incidence more useful in identifying the cause of a disease?

When is it most useful to use prevalence?

Give examples for each.

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Discussion Question 2

Explain the interaction between incidence, prevalence, and prognosis.

Give examples.

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Discussion Question 3

Why are the who, when, and where questions useful in determining the causes of disease?

Give examples.

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Discussion Question 4

Explain the three major types of epidemiologic studies.

Which is most likely to yield a valid result? Why?

Which is likely to yield an answer in the shortest period of time? Why?

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Discussion Question 5

Visit the National Institutes of Health website on clinical trials, www.clinicaltrials.gov.

Browse by condition.

How many clinical trials are going on right now?

Choose a condition and investigate what treatments are being tested for that condition.

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Discussion Question 6

Visit the website of the Nurses’ Health Study, www.channing.harvard.edu/nhs.

Read the most recent annual newsletter.

What are the most recent findings of this cohort study?

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CHAPTER 6

Problems and Limits of Epidemiology

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Problems with Studying Humans

Intervention study problem:

Subjects may not follow prescribed behavior throughout study period.

Cohort study problem:

Sometimes it is hard to isolate which of many factors are responsible for health differences.

Case-control study problems:

Control group may not be truly comparable.

Errors may exist in reporting or recalls.

For all studies, differential drop-outs are worrisome.

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Sources of Error

Random variation

Confounding variables

Bias

Selection bias

Reporting bias or recall bias

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Factors That Lend Validity to Results

Strong association

Dose–response relationship

Known biological explanation

Large study population

Consistent results from several studies

High relative risk or odds ratio

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Hormone Replacement Therapy

Conflicting results exist between two major studies.

Previous positive evidence has all come from observational studies.

Clinical trial is the gold standard.

Results of cohort study were confounded by associated factors that made women taking HRT healthier, even without the therapy.

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Ethical Issues

Nazi experiments on humans

Tuskegee syphilis study

AIDS epidemic

Bone marrow treatment for advanced breast cancer

New rules

Informed consent

Institutional review boards

Importance of clinical trials

Possibility of conflict of interest with medical providers who stand to profit

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Conflicts of Interest in Drug Trials

Drug companies are required to conduct randomized controlled trials on a new drug before it can be approved.

Harmful side effects have frequently become obvious after drugs were approved.

There is evidence that drug companies sometimes suppress negative findings.

All clinical trials must now be registered in advance with a public database.

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Discussion Question 1

What are strengths and weaknesses of each of the major types of epidemiologic study?

Randomized controlled trial

Cohort

Case-control

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Discussion Question 2

Have you heard of the Tuskegee syphilis study?

Why was it unethical?

What influence has it had on the conduct of clinical trials?

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Discussion Question 3

Visit the website of the Hastings Center, www.thehastingscenter.org.

What issues is the Hastings Center concerned with this month?

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CHAPTER 6

Problems and Limits of Epidemiology

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Problems with Studying Humans

Intervention study problem:

Subjects may not follow prescribed behavior throughout study period.

Cohort study problem:

Sometimes it is hard to isolate which of many factors are responsible for health differences.

Case-control study problems:

Control group may not be truly comparable.

Errors may exist in reporting or recalls.

For all studies, differential drop-outs are worrisome.

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Sources of Error

Random variation

Confounding variables

Bias

Selection bias

Reporting bias or recall bias

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Factors That Lend Validity to Results

Strong association

Dose–response relationship

Known biological explanation

Large study population

Consistent results from several studies

High relative risk or odds ratio

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Hormone Replacement Therapy

Conflicting results exist between two major studies.

Previous positive evidence has all come from observational studies.

Clinical trial is the gold standard.

Results of cohort study were confounded by associated factors that made women taking HRT healthier, even without the therapy.

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Ethical Issues

Nazi experiments on humans

Tuskegee syphilis study

AIDS epidemic

Bone marrow treatment for advanced breast cancer

New rules

Informed consent

Institutional review boards

Importance of clinical trials

Possibility of conflict of interest with medical providers who stand to profit

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Conflicts of Interest in Drug Trials

Drug companies are required to conduct randomized controlled trials on a new drug before it can be approved.

Harmful side effects have frequently become obvious after drugs were approved.

There is evidence that drug companies sometimes suppress negative findings.

All clinical trials must now be registered in advance with a public database.

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Discussion Question 1

What are strengths and weaknesses of each of the major types of epidemiologic study?

Randomized controlled trial

Cohort

Case-control

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Discussion Question 2

Have you heard of the Tuskegee syphilis study?

Why was it unethical?

What influence has it had on the conduct of clinical trials?

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Discussion Question 3

Visit the website of the Hastings Center, www.thehastingscenter.org.

What issues is the Hastings Center concerned with this month?

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CHAPTER 8

Role of Data in Public Health

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Uses of Data

Assessment of the health of a community

Raw material for research

Identification of special risk groups

Detection of new health threats

Planning of public health programs and evaluation of their success

Preparation of government budgets

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Collection of Data

Local records are sources of data:

Birth certificates

Death certificates

Other vital statistics

Data is transmitted:

From local governments to states

From states to National Center for Health Statistics (NCHS is part of CDC)

Surveys are sources of data.

NCHS set up a computer system to link vital records of infants because infant mortality is an important public health issue.

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The Census

Is mandated by the U.S. Constitution

Serves as the denominator for most public health data:

Age, sex, race, ethnicity

Is conducted every 10 years

Determines the political composition of the U.S. Congress

The citizenship status question excluded from 2020 census

New household categories included for 2020 census

American Community Survey is done on an ongoing basis in between censuses:

Education, housing, health insurance

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Surveys

National Health Interview Survey (NHIS)

National Health and Nutrition Examination Survey (NHANES)

National Health Care Survey

Behavioral Risk Factor Surveillance Survey (BRFSS)

State-by-state, with results transmitted to NCHS

Surveys done by other government agencies

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How Much Data Is Necessary?

There can never be too much data.

Data is critically important in developing surveillance systems.

Data collected is used by federal, state, and local agencies in all areas of public health.

Public health problems are defined according to available data.

Without data, experts can’t recognize when an epidemic is starting.

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Accuracy and Availability of Data

Data collection is imperfect.

Census is most accurate.

Still, there are overcounts and undercounts.

Information technology increases accuracy and availability.

Public health informatics has vastly improved the accessibility of public health information for public health workers and the general public.

The CDC and most other federal and state public health agencies make information available over the Internet.

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Confidentiality of Data

Governments have safeguards to protect information on individuals.

Use of data may involve removal of information identifying individuals.

Use of data requires permission granted by an institutional review board or data protection committee.

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Discussion Question 1

Vital statistics are the most accurate and complete data collected on the American population, and yet there are still many errors in the information submitted.

What are some possible sources of error in records collected on births, deaths, spontaneous fetal deaths, and induced abortions?

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Discussion Question 2

Why did the U.S. Census Bureau propose to use statistical sampling in conducting the year 2000 census?

Can you think of some reasons why some people might not be counted?

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Discussion Question 3

Of what value to public health is information gathered by the NHIS?

NHANES?

BRFSS?

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Discussion Question 4

Visit the website of the U.S. Census Bureau, www.census.gov.

What is the population of your city or town?

Is it increasing or decreasing?

How does the age distribution of your city or town compare with that of the U.S. overall?

How does the ethnic composition compare?

How does the education level of the population compare with that of the U.S. overall?

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Discussion Question 5

Visit the NCHS website, www.cdc.gov/nchs.

What surveys does NCHS conduct in addition to those described in this chapter?

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CHAPTER 10

The Resurgence of Infectious Diseases

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HIV/AIDS

Was first recognized in the U.S. in 1981

Is now a worldwide killer

Is caused by a retrovirus

Attacks the immune system

Screening test recognizes antibodies.

We can measure viruses in the blood.

Now many drugs are available but no cure.

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HIV/AIDS Transmission

Sexual contact:

Homosexual

Most common in U.S.

Heterosexual

Most common around the world

Sharing needles:

Intravenous drug use

Medical use of unsterile needles

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HIV/AIDS Transmission (cont.)

Mother to infant

Prenatal or during birth

Breastfeeding

Blood transfusions

No longer in the U.S.

Circumcision helps protect men against contracting HIV from women, but it doesn’t protect women from contracting it from men.

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HIV/AIDS Treatments

Highly active antiretroviral therapy (HAART), a drug combination, led to dramatic improvements in survival rates.

Reduces viral load to undetectable levels in blood and body fluids

Encourages infected people to be tested and treated, preventing transmission

Fusion inhibitors

Integrase inhibitors

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Where Did HIV Originate?

Probably originated in Africa

Transmitted cross-species from monkeys or apes

Spread in human populations due to disruption of traditional lifestyles

Spread to Western countries due to changing patterns of sexual behavior and international travel

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Other Emerging Viruses

Ebola

Monkeypox

Zika

Other hemorrhagic fevers

West Nile virus

SARS

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Factors That Lead to Emergence of New Infectious Diseases

Human activities that cause ecological damage and close contact with wildlife

Modern agricultural practices

International travel

International distribution of food and exotic animals

Breakdown of social restraints on sexual behavior and intravenous drug use

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Influenza

Influenza is an RNA virus like HIV.

Virus is constantly mutating.

Vaccine must be changed frequently.

New lethal strains appear periodically.

Epidemic of 1918–1919 killed 20 million to 40 million worldwide.

Bird flu and H1N1 are concerns.

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New Bacterial Threats

Legionnaires’ disease

Lyme disease

Group A streptococci

E. coli O157:H7 in food

Antibiotic resistance

From improper medical use

Use in agriculture

MRSA

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Tuberculosis

TB is the leading cause of infectious-disease death worldwide.

One third of the world population is infected.

There was a resurgence in the U.S. in the early 1990s.

People with HIV are at much higher risk.

TB is transmitted by aerosol.

Fatality rate is 50% for untreated TB.

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Tuberculosis (cont.)

Antibiotics are effective but must be taken for several months.

Improper use of antibiotics leads to resistance, including multidrug resistance.

Then, mortality rate can exceed 50%.

Directly observed therapy works.

It is the best approach for preventing antibiotic resistance.

In 2007, the CDC revised its requirements for overseas medical screening of applicants for immigration to the U.S.

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Prions

Creutzfeldt–Jakob Disease (CJD)

Sporadic CJD is most common.

CJD usually appears in later life.

Bovine spongiform encephalopathy (BSE) or “mad cow disease” has a history in Britain.

New variant of CJD (vCJD) in the UK in younger people is thought to be caused by eating infected beef.

Regulations have been tightened on animal feed.

Gerstmann–Straussler–Scheinker syndrome

Fatal familial insomnia

Kuru

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Public Health Response to Emerging Infections

Global surveillance

Improved public health capacity

Veterinary surveillance

Reduction of inappropriate use of antibiotics

Institute of Medicine recommendations:

New vaccines

New antimicrobial drugs

Measures against vector-borne diseases

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Threat of Bioterrorism

Approach to bioterrorism is the same as that for natural disease outbreaks.

Bioterrorism will probably first be recognized by surveillance.

First signs of an attack are likely to be seen by physicians and hospital emergency room staff.

It is best defended against by the same methods as natural outbreaks.

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Discussion Question 1

Consider the spread of the following emerging infectious diseases, using the chain of infection: E. coli O157:H7, Lyme disease, Ebola virus, hantavirus, influenza, tuberculosis, vCJD.

For each disease, explain how the chain of infection can be interrupted.

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Discussion Question 2

What are the reasons for the rapid emergence of AIDS as a worldwide epidemic?

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Discussion Question 3

What are three public health measures that could reduce the probability that bacteria will develop resistance to antibiotics?

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Discussion Question 4

Visit the website of the National Institute of Allergy and Infectious Diseases, www.niaid.nih.gov.

Under “News & Events,” review the news releases over the past year.

On what diseases has the Institute released news about progress in research?

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Discussion Question 5

Visit the website of the CDC’s journal, Emerging Infectious Diseases, www.cdc.gov/ncidod/eid.

What emerging diseases are discussed in the current issue?

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CHAPTER 7

Statistics: Making Sense of Uncertainty

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Statistics

Is a set of concepts and methods used to analyze data in order to extract information

Is the science used to interpret the numbers that describe the health of the population

Makes possible the translation of data into information about causes and effects, health risks, and cures of diseases

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The Uncertainty of Science

Most science is of a probable nature.

In many cases, there is not enough data to give us a degree of certainty, or the existing data is too ambiguous to allow a valid conclusion.

Science is ongoing; studies may contradict each other.

The science of statistics can quantify the degree of uncertainty.

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Probability

The probable is what usually happens.

Probabilities are used to describe the variety and frequency of past outcomes under similar conditions as a way of predicting what should happen in the future.

A p-value:

When p ≤ 0.05, it usually means that a result is statistically significant.

When p = 0.05, there is still a 5% chance that the result is wrong.

Key concepts are:

Confidence interval

Law of Small Probabilities

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Power of a Study

There is the probability of finding an effect if there is, in fact, an effect.

Large numbers confer power.

Studies with low power are likely to produce false-negative results.

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Statistics of Screening Tests

Examples of screening:

Mammography for breast cancer

HIV tests

Newborn screening

Sensitivity versus specificity

False positives versus false negatives

Lead-time bias

Overdiagnosis bias

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Rates

Rates relate raw numbers to the size of the population being considered.

Birth rates

Mortality rates

Other rates commonly used as indicators of community health are:

Infant mortality rate

Maternal mortality rate

Crude rates

Adjusted rates

Age adjusted

Group specific rates

Gender specific

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Other Calculated Statistics

Life expectancy

Years of potential life lost (YPLL)

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Risk Assessment and Risk Perception

Risk assessment:

Identifies events and exposures that may be harmful to humans

Estimates the probabilities of their occurrence

Estimates the extent of harm they may cause

For well-known risks, we can calculate from historical data.

For poorly understood risks, we must make many assumptions.

Risk perception:

Involves psychological factors

Is the result of the apparent irrationality of the public in response to risks that experts estimate to be small

Is classified on two scales: dread and knowability

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Cost–Benefit Analysis

Weighs the estimated cost of implementing a policy against the estimated benefit, usually in monetary terms

Costs are easier to calculate than benefits.

What monetary value can we put on a life saved?

Analysis is often controversial.

Cost-effectiveness analysis compares the efficiency of different methods of attaining the same objective.

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Discussion Question 1

Why is statistics so important for public health?

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Discussion Question 2

Try organizing a group of students in a coin-tossing experiment.

How often does the same side come up five times in a row?

Ten times in a row?

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Discussion Question 3

Think of a medical test that you or someone you know has undergone.

Was it more important for the test to be sensitive or for it to be specific? Why?

What would be the implications if the result was a false positive?

What if there had been a false-negative result?

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Discussion Question 4

Consider the list of activities and technologies presented in Table 7-4.

Rank them in order of your willingness to accept the risk.

How does your ranking compare with those of the other groups shown in the table?

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Discussion Question 5

Visit Rice Virtual Lab in Statistics by statistician David Lane, onlinestatbook.com/rvls.html.

Choose one of Lane’s case studies that is relevant to health and describe the experimental design.

What are the conclusions of the experiment?

Are the results significant?

Is a p-value given?

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CHAPTER 9

The “Conquest” of Infectious Diseases

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Infectious Diseases: Major Killers in the Past

Bubonic plague

“Black Death”

Tuberculosis

Smallpox

Cholera

Typhoid

Typhus

Yellow Fever

Diphtheria

Measles

Influenza

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Infectious Diseases Were “Conquered” by the 1960s

Immunization

Antibiotics

Public health measures

Purification of water

Proper disposal of sewage

Pasteurization of milk

Improved nutrition and personal hygiene

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Infectious Agents

Bacteria

Tuberculosis, cholera, typhoid, tetanus, diphtheria, dysentery, syphilis, streptococci, staphylococci

Viruses

Smallpox, poliomyelitis, hepatitis, measles, rabies, AIDS, yellow fever

Parasites

Malaria, cryptosporidiosis, giardiasis, roundworms, tapeworms, hookworms, pinworms

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Means of Transmission

Directly from one person to another

Aerosol

Touching contaminated object and putting hands to mouth, nose, or eyes

Contaminated water or food

Fecal-oral route

Vectors

Sexual contact

Carriers

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Chain of Infection

The transmission pattern is composed of links:

Pathogen (infectious agent)

Reservoir

Means of transmission

Susceptible host

Public health measures to control the spread of disease are aimed at interrupting the chain of infection at whichever links are most vulnerable.

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Interrupting Chain of Infection

Kill pathogen with antibiotics.

Eliminate the reservoir.

Prevent transmission:

Hand washing

Quarantine

Condoms

Increase resistance of host by immunization.

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Public Health Measures

Epidemiologic surveillance

Contact tracing

Immunization and treatment of identified patients to prevent further spread

Quarantine if necessary

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Examples

SARS

Controlled by classic public health measures

Rabies

Surveillance of wildlife

Immunization of dogs

Post-exposure prophylaxis

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Eradication

Eradication is possible if there is no nonhuman reservoir and if a vaccine exists.

Smallpox was eradicated in 1977.

Polio was eradicated from the Western Hemisphere.

Now it is only in a few countries.

There is religious opposition in some countries.

Measles is the next target.

Now it is no longer endemic in the U.S.

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Fear of Vaccines

Rumors spread of vaccines causing autism and SIDS.

Side effects do exist for some vaccines.

Some parents refuse to accept risks.

Herd immunity is lost if many people do not get vaccinated.

Pharmaceutical companies are reluctant to develop vaccines.

Low profits

Risk of lawsuits

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Discussion Question 1

Choose several infectious diseases and analyze how they are spread, using the chain of infection.

Examples are AIDS, rabies, cryptosporidiosis, cholera, malaria, smallpox, syphilis.

For each disease, how can the chain of infection be interrupted?

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Discussion Question 2

Why is AIDS not on the list of diseases targeted for eradication?

Why is rabies not targeted?

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Discussion Question 3

Visit the website of the Centers for Disease Control and Prevention, www.cdc.gov.

Where in the U.S. is rabies endemic in animals? Have there been any cases of human rabies in the last year or two?

Where in the U.S. have there been measles outbreaks in the last year or two? What was the source of the outbreak?

Where in the world have there been polio outbreaks in the last year?

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Discussion Question 4

Visit the CDC website on vaccines and immunizations, www.cdc.gov/vaccines.

What evidence can you find about the safety of childhood vaccines?

Do you believe the benefits outweigh the risks of vaccination?

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