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Week 4: The Basics of Epidemiology

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Ashley Conway

on 2 October 2016

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Transcript of Week 4: The Basics of Epidemiology

Week 4
Epidemiology: The Study of Disease, Injury, and Death
"Aide Medicale" - an educational health channel to aid students of medicine and related subjects: http://www.youtube.com/user/AideMedicale

Atkinson, W. et al. (2012) Epidemiology and Prevention of Vaccine-Preventable Diseases, 12th Edition (the Pink Book). US Department of Health and Human Services, Centers for Disease Control and Prevention.

Kanchanaraksa, Sukon. (2008) Epidemiologic Investigation - presentation. Johns Hopkins Bloomberg School of Public Health.

McKenzie, J., Pinger, R., and Kotecki, J. (2012) An Introduction to Community Health, 7th ed. Sudbury, MA: Jones & Bartlett Learning.

Miquel Porta (2008). A Dictionary of Epidemiology. Oxford University Press.

Pickard Nelson, Amy. E is for Epi: Epidemiology Basics for Non-Epidemiologists - presentation. UNC Center for Public Health Preparedness (CPHP).

US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention. (1992) Principles of Epidemiology, Second Edition.
Learning outcomes:
Define epidemiology
Describe the difference between descriptive and analytic epidemiology
Recognize when and why incidence, prevalence, relative risk, and attack rates are used
What is epidemiology?
Epidemiology is the science of distribution and determinants of health, disease, or injury in human populations and the application of this study to the control of health problems.
The 2 "D"s
The 3 "P"s
- Porta
Objectives of epidemiology:
From the Greek:
Epi =upon, among
Demos =people
Ology =science, study of
Epidemiology is the scientific method of disease investigation and typically involves the disciplines of biostatistics, medicine, and public health.
- Kanchanaraksa
- Aide Medicale
- Aide Medicale
Descriptive Epidemiology
Analytic Epidemiology
• Study risk associated with exposures
• Identify and control epidemics
• Monitor population rates of disease
and exposure
I KEEP six honest serving-men
(They taught me all I knew);
Their names are What and Why and When
And How and Where and Who.

- Rudyard Kipling
"The Elephant's Child"
Descriptive vs. Analytic Epidemiology
Provides a systematic method for characterizing a health problem:
• Ensures understanding of the basic dimensions of a health problem
• Helps identify populations at higher risk for the health problem
• Provides information used for allocation of resources
• Enables development of testable hypotheses
Descriptive Epidemiology
The importance of rates...
A rate is the number of events (e.g. a case of disease) in a given population over a given time period or a certain point in time.

Rates allow for comparison of outbreaks at different times or in different places
Cases: people afflicted (those who are sick)
Natality (birth), morbidity (sickness), mortality or fatality (death) rates
Population at Risk: those susceptible to particular disease or condition
Determining the number of cases of a disease is dependent on having a case definition:
• Standard diagnostic criteria that must be fulfilled to identify a person as a case of a particular disease
• Ensures that all persons who are counted as cases actually have the same disease
• Typically includes clinical criteria and restrictions on person, place, and time
But first... establish a case definition!
Who, Where, & When
- Pickard
- Pickard
Determining prevalence is useful for:

Assessing the burden of disease* within a population
The study of chronic diseases (>3 months)
Not useful for determining what causes disease
Incidence = Number of new cases over a specified period of time
Number of persons at risk of disease
over a specific period of time

Number of new flu cases from 2/3 to 2/9 (10)
Number of nursing home residents (200)
0.05 x 100 =
The incidence of confirmed flu in the nursing home
in the one week period from 2/3 - 2/9 was 5%
Determining incidence is useful for examining how common it is for people to get a certain disease:

High incidence represents diseases with high occurrence; low incidence represents diseases with low occurrence
Can be used to help determine the causes of disease
Can be used to determine the likelihood of developing disease
Important in the study of acute diseases (<3 months)
Attack rate
Number of people infected
Number of people exposed
X 100
An incidence rate calculated for a specific disease for a specific time period.
Used in epidemics, particularly food borne outbreaks
Expressed as a percentage
Attack Rates (or Ratio)
Case definitions: Time, place and person
The use of case definitions is very important in epidemiology in order to standardize criteria for identification of cases. All case definitions must include the three classical dimensions of epidemiological variables: time, place and person.
Clinically compatible case
: a clinical syndrome generally compatible with the disease, as described in the clinical description.
Confirmed case
: a case that is classified as confirmed for reporting purposes.
Epidemiologically linked case
: a case in which the patient has had contact with one or more persons who either have/had the disease or have been exposed to a disease.
Laboratory-confirmed case
: a case that is confirmed by one or more of the laboratory methods.
Probable case:
a case that is classified as probable for reporting purposes.
Supportive or presumptive laboratory results:
specified laboratory results that are consistent with the diagnosis, yet do not meet the criteria for laboratory confirmation.
Suspected case:
a case that is classified as suspected for reporting purposes.
Source: World Health Organization
8 students who ate the turkey sandwich at a 2/6 campus event have suspected salmonella. Interviews with all of the people who were at the event determined that 15 students ate the turkey sandwich.
What is the attack rate?
Attack rate
8 people who ate the turkey sandwich with suspected Salmonella
15 people ate the turkey sandwich at the event on 2/6
The attack rate is 53.3%
This is the attack rate for people
who ate a SPECIFIC food, so the
attack rate is a "food-specific attack rate".
Specific attack rates are calculated to identify persons in the population who are at a higher
risk of becoming ill than others. Examples of other commonly used specific attack rates are attack rates by age group, residence, sex or occupation.
But what about the people at the 2/6 event who
eat the turkey sandwich?
Interviews determined that 36 people at the event did not eat the turkey sandwich. Two of the 36 people who didn't eat the turkey sandwich developed illness consistent with the case definition for suspected Salmonella.

The attack rate of people who didn't eat the turkey sandwich at the 2/6 event is: 0.056 or 5.6%

These two attack rates (those who did and didn't eat the ill-fated turkey sandwich) are compared and expressed as a ratio. It is useful to understand what the probability is that the event (in this example, suspected cases of Salmonella) will occur in the exposed group (turkey sandwich eaters) compared to the non-exposed group (people who didn't eat the turkey sandwich).

In a 4 X 4 table (often used in descriptive epidemiology) it looks like this
The relative risk (RR) of eating the turkey sandwich is 53.3/5.6 or 9.5
People attending the 2/6 campus event who ate the turkey sandwich were 9.5 times as likely to become ill as people who didn't eat the turkey sandwich.
Prevalence =
Incidence, Prevalence, and Attack Rates
McKenzie et al
Some more important rates in epidemiology
McKenzie et al
McKenzie et al
This week you will be viewing a documentary by Hans Rosling (60:00). Rosling isn't a statistician, but he is a global health expert who loves statistics and uses them in a way that can make sometimes confusing concepts fun and interesting. I love the way he gives distribution and determinants meaning and makes them come alive.

As you watch the video:
pay particular attention to the public health application of statistics
listen for epidemiology terms and concepts presented in this lecture and Chapter 3 of the text
keep in mind your task in the Week 4 assignment - you might want to read the assignment instructions before viewing the video
The Joy of Stats
Now you know...
Epidemiology studies groups - not individuals.
And another type of attack rate....
Secondary Attack rate
A measure of the frequency of new cases of a disease among the contacts of known cases. In other words, the probability that infection occurs among susceptible persons within a reasonable incubation period following known contact with an infectious person or an infectious source.
What's the difference between....?
Symptoms: What the ill person experiences e.g. fatigue or nausea
Signs: Indicators of a problem e.g. heart rate or oxygen saturation
Incubation periods matter!
An incubation period is the time from the exposure to an infectious agent (or another kind of harmful agent such as a chemical or radiation) until signs and symptoms of the disease appear.
attack rate
Number of new cases among contacts of cases during a specific period of time

Total number of contacts during the specific period of time
Incubation period - the period between
infection and clinical onset of the disease
Latent period - the time from
infection to infectiousness
In some diseases - as shown in this diagram - the latent period is shorter than the incubation period. This means a person can transmit infection without showing any signs of the disease! Such infection is called subclinical infection.

Knowing the incubation period of a disease is necessary in order to know what time period should be used for the secondary attack rate.

So understanding the incubation period and period of communicability for measles,
knowing the clinical course of Jim's illness (when he first started feeling ill, got a fever, and developed a rash) is important for conducting surveillance and controlling measles.
Determining the secondary attack rate is useful for measuring contagiousness and for designing control measures.
attack rate
Relative Risk
Number of people who got measles from being in contact with Jim

Total number of people who were in contact with Jim while he was contagious
Incubation period: The period between infection and clinical onset of the disease
Latent period: The time from infection to infectiousness

The incubation period of measles, from exposure to
prodrome* averages 10–12 days. From exposure to rash onset averages 14 days (range, 7–18 days).
The prodrome lasts 2–4 days (range 1–7 days). It is characterized by fever, which increases in stepwise fashion, often peaking as high as 103°–105°F. This is followed by the onset of cough, coryza (runny nose), or conjunctivitis.

Measles is highly communicable, with greater than 90%
secondary attack rates among susceptible persons. Measles may be transmitted from 4 days before to 4 days after rash onset. Maximum communicability occurs from onset of prodrome through the first 3–4 days of rash.

*Prodrome - A symptom or group of symptoms that appears shortly before an acute attack of illness. The term comes from a Greek word that means "running ahead of."
Clinical Features of Measles:
Communicability of Measles:
- CDC, Epidemiology and Prevention of Vaccine-Preventable Diseases (a.k.a. the"Pink Book")

Jim has the measles. What do you need to know about measles to calculate the secondary attack rate?
Investigations carry out
studies when disease or death occurs in unexpected or unacceptable numbers.
So, what does it mean?
An RR < 1
An RR = 1
An RR > 1
Exposure associated
with decreased risk
of outcome
No association
between exposure
and outcome
Exposure associated
with increased
risk of outcome
- Pickard
Jennifer Gardy and Tom Scott. The A - Z of epidemiology. (2:31)
CityTownInfo. Epidemiology Career Overview. (1:56)
Number of people infected
Number of people exposed
Attack rate =
X 100

Should parents be allowed to "opt out" of vaccinating their children against VPDs (vaccine preventable diseases)?

Reasons for granting child vaccination exemptions:
Prevalence is about "proportion"
Prevalences are usually expressed as three types:

Point prevalence: relates to prevalence with respect to a specific point in time –
Did you have an asthma attack on Monday?
Period prevalence: related to prevalence over a defined period of time –

Did you have an asthma attack in January?
Lifetime prevalence:

Have you ever had an asthma attack?
- Moulton, Georgina
New Jersey 7/2012:
Total Population, Estimate - 8,864,590
Persons Living with HIV/AIDS -37,511
Calculate the Prevalence of HIV/AIDS in NJ
= 0.00423155
X 100,000 =
423 per 100,000
- NJ Department of Health and CDC
# of cases
# of people in population
X sample size
In an outbreak of gastroenteritis among attendees of a corporate picnic, 90 persons ate potato salad,
30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad.
= 0.303 x 100 =
The Happy Trails nursing has 200 residents. From February 3 to February 9 there were 10 new cases of influenza. What is the incident rate?
Patwari, R. (2013) The Relationship Between Incidence and Prevalence. (3:39)
* Burden of disease - the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators
This is a visual depiction of the relationship between public health and the study of disease. I think it is a good way to remember the concept.
Epidemiology is the science of
of health, disease, or injury in human populations and the application of this study to the control of health problems.
Welcome to Week 4. Last week the prevention and control of communicable diseases was covered (epi triangle, chain of infection, etc.). This week we will take a closer look at epidemiology and the tools used by epidemiologists and others in the field of public health. Epidemiology is used not only by epidemiolgists, but any one working in the public health field who needs to understand the who, what, when, where, and why of a disease; this includes doctors, nurses, researchers, health educators, administrators, social workers, community organizers, program managers, grant writers, and many more!

If you have taken biology, statistics or epidemiology this week will be a review of the basics. Be sure to read the assigned chapters on epi in the text - I think the authors do a good job on the topic. This presentation will highlight the important epi concepts and terminology that you should know for future public health courses and for your practice in any health field.

This Prezi isn't narrated, so as you move from frame to frame be sure to read the text notes and view the embedded videos.

Let's get started!
For this course you will not be asked to do any calculations, but you should be able to define the terms in this presentation and identify when it is appropriate to use a particular type of rate and why. Even if you aren't a "numbers person" I hope that the logic and usefulness of epidemiology will impress you and make you excited about studying and using epi.

Might be useful to know if you are a contestant on
. You never know...
The back story of this video is that a bunch of public health nerds (using "nerds" in a good way here!) got together at a bar and started riffing about epidemiology and then put it in a video. If you enjoy it and haven't declared your major yet - you really need to think seriously about public health. You are one of us!
Who decides what a case definition is?

Source: National Notifiable Diseases Surveillance System (NNDSS), CDC
In the United States, requirements for reporting diseases are mandated by state laws or regulations, and the list of reportable diseases in each state differs. In October 1990, in collaboration with the Council of State and Territorial Epidemiologists (CSTE), CDC published a report entitled Case Definitions for Public Health Surveillance (MMWR 1990;39[No. RR-13]) (the 1990 Report), which, provided uniform criteria for reporting cases.

The 1997 updates were published in a report entitled Case Definitions for Infectious Conditions Under Public Health Surveillance (MMWR 1997;46[No. RR-10]) (the 1997 Report). Case definitions for some infectious conditions designated as not nationally notifiable are also included in this report. Non-nationally notifiable diseases may have been nationally notifiable at some point or may become notifiable at some point. The definitions for non-nationally notifiable diseases are included to facilitate interpretation of data for these diseases, and the diseases that are not notifiable at a national level may be reportable at the state level. Download the 1997 Report.

The 1990 and 1997 case definitions were developed in collaboration with epidemiologists at CDC and the CSTE. They were approved by a full vote of the CSTE membership and also endorsed for use by the Association of State and Territorial Public Health Laboratory Directors which is now the Association of Public Health Laboratories (APHL).
Clinical Criteria

A person with one or more of the following:

acute onset of fever (measured or reported)
maculopapular rash
complication of pregnancy
fetal loss in a mother with compatible illness and/or epidemiologic risk factors; OR
in utero findings of microcephaly and/or intracranial calcifications with maternal risk factors
Guillain-Barré syndrome not known to be associated with another diagnosed etiology.

Epidemiologic Linkage

Travel to a country or region with known ZIKV transmission, OR
Sexual contact with a laboratory confirmed case of ZIKV infection, OR
Receipt of blood or blood products within 30 days of symptom onset; OR
Organ transplant recipient within 30 days of symptom onset; OR
Association in time and place with a confirmed or probable case.
Zika Virus Disease Case Definition

Case classification


Meets clinical criteria AND

resides in or has recently traveled to an area with ongoing ZIKV transmission, OR
has direct epidemiologic linkage to a person with laboratory evidence of recent ZIKV infection (e.g. sexual contact, in utero or perinatal transmission, blood transfusion, organ transplantation), OR
association in time and place with a confirmed or probable case.

AND meets the following laboratory criteria:
positive ZIKV-specific IgM antibodies in serum or cerebrospinal fluid (CSF); AND
negative dengue virus-specific immunoglobulin M (IgM) antibodies; AND
No neutralizing antibody testing performed; OR
Less than four-fold difference in neutralizing antibody titers between ZIKV and dengue or other flaviviruses endemic to the region where exposure occurred.


Meets clinical criteria AND
Has laboratory evidence of recent ZIKV infection by:
Detection of ZIKV by culture, viral antigen or viral ribonucleic acid (RNA) in serum, CSF, tissue, or other specimen (e.g. amniotic fluid, urine, semen, saliva); OR
ZIKV IgM antibodies in serum or CSF with ZIKV neutralizing antibody titers 4-fold or greater than neutralizing antibody titers against dengue or other flaviviruses endemic to the region where exposure occurred.
Here is an example of a CDC case definition for Zika.
I've included it to familiarize you with case definitions and classifications (next frame) - you don't need to memorize these criteria. That's what the Manual of Communicable Diseases is for!
Now back to rates...
I think this bathtub analogy is a good way to remember prevalence and incidence. On the exam when you get a question about this - think bathtub!
OK, now that you understand the the basic idea of incidence and prevalence, watch this video to go a little deeper....
MedMastery. (2016) Incidence and Prevalence - Everything you need to know. (6:23)
Epi is lit! Take a look at this video about a career in epidemiology....
Here's an example....
Here are some more incubation periods. Why do you think incubation periods are
important when trying to control travel-related diseases?
Ashley Conway, Instructor
Principles of Public Health
Fall 2016
That's it for the Week 4 presentation. Hope you have a great week!
What does the "Exam Alert" icon mean?
I'll be inserting this icon when presenting course content that students in past semesters tended to get wrong on the exam.
Measles incubation period and period of communicability.
Period of communicabilty
The period of communicability is the time during which an infectious agent is transmissible, directly or indirectly, from an infected person or animal to a susceptible person
Here's an example...
Full transcript