Epidemiology Master Level Quiz

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Wade Hampton Frost

!  Epidemiology of infectious diseases !  Role of asymptomatic poliovirus infection !  Cyclical nature of influenza epidemics !  Tracking influenza outbreaks by excess pneumonia

!  Epidemiologic methods !  Index case !  Use of life tables to estimate secondary attack rates !  Use of life tables to express data as person-years !  Development of mathematical expression of the epidemic curve (with Lowell Reed)

Contributions of Wade Hampton Frost

2 Source: Lilienfeld AM. (1983). Am J Epi; 117:379–83.

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Patterns of Disease Transmission in Time and Space

Patterns in Time

Patterns in Space

The context of disease transmission

Epidemiological Triad: a Framework for Disease Transmission

Factors Determining Disease and Severity

Host Agent Environment

Age Biological Temperature

Sex Chemical Humidity

Race Physical Water

Occupation Air

Prior exposures Food

Housing

Radiation

Disease in the individual host

!  Class A, inapparent infection frequent: mycobacterium tuberculosis

!  Class B, clinical disease frequent; few deaths: measles virus

!  Class C, infections usually fatal: rabies virus

Distributions of Clinical Severity

10 Source: Mausner and Kramer. (1985).

Inapparent Mild Moderate Severe (nonfatal) Fatal

Outcomes of Poliovirus Infection: One Paralytic Case for Every 100 to 200 Infected Persons

Epidemics

Endemic and Epidemic Disease

Time

Number of cases

Endemic Epidemic

!  Endemic: the habitual presence of a disease within a given geographic area

!  Epidemic: the occurrence in a community or region of a group of illnesses of similar nature, clearly in excess of normal expectancy, and derived from a common or propagated source

!  Pandemic: a world-wide epidemic

Endemic, Epidemic, and Pandemic

How to recognize an epidemic

Influenza Pandemic, 1918 and 1919

!  Mode of transmission

!  Infectiousness

!  Latent and infectious periods

!  Incubation period

!  Herd immunity

!  Population mobility

What Determines the Course of an Epidemic?

Modes of Transmission

Section B

Modes of Disease Transmission

!  Direct transmission !  Person-to-person

•  Respiratory •  Fecal-oral •  Sexual •  Blood

!  Indirect transmission !  Common vehicle (e.g., food)

•  Single exposure •  Multiple exposures •  Continuous exposure •  Vector

Model of the Human Body

Droplet Dispersal Following a Violent Sneeze

Modes of Transmission and Prevention

!  Probability of a susceptible person coming in contact with an infected person !  Probability of being susceptible !  Frequency of infection in population

!  Probability that infectious agent is transmitted during the contact !  Characteristics of effective contact

Infectiousness

!  Compare risk of disease in groups with different exposures

!  Assumptions !  All persons in denominator were exposed !  All persons in denominator were susceptible !  All cases were detected

•  For example, no subclinical cases

Attack Rate

Number of cases among people at risk Total number of people at risk

Secondary Attack Rates: Spread of SARS-Coronavirus

Basic Reproduction Number and Effective Reproduction Number

!  Basic reproduction number (R o ): the

average number of secondary cases produced by an infective person in a fully susceptible population

!  Effective reproduction number (R): the average number of secondary cases produced by an infective person in a population not fully susceptible to infection

Latent Period

Infectious Period

Incubation Period: Interval from Infection to the Time of Onset of Clinical Illness

Observations Made During the Epidemic of Measles on the Faroe Islands in the Year 1846— Peter Ludwig Panum

“As to the length of the incubation period, accurate and satisfactory observations have hitherto been lacking … The isolated situation of the villages, and their limited intercourse with each other, made it possible in many, in fact most cases, to ascertain where and when the person who first fell ill had been exposed to the infection …”

The Incubation Period for Measles

Why an Incubation Period?

Foodborne Diseases

Distribution of Incubation Periods

Herd Immunity

Section C

!  The resistance of a group to attack by a disease to which a large portion of members are immune, thus lessening the likelihood of a patient with a disease coming into contact with a susceptible individual

Herd Immunity

The Size of an Epidemic Is Determined by the Number of Susceptible and Immune Persons

0 5 10 15 20 25 30 35 40 0

100

200

300

400

500

600

700

800

900

1000

Time

P eo pl e

Susceptibles

Immune

Infected

Based on the Reed-Frost model of the epidemic curve

Contact patterns

Social Interaction at Warwick University

Sexual Contact Network

Source: Colorado Springs Study 12

Influenza Virus Transmission Network

Population mobility

Global Disease Transmission

Ways to Describe an Epidemic

1.  Time of exposure

2.  Time of disease onset

3.  Incubation period

Three Important Variables in the Investigation of an Epidemic

!  Distribution of the times of onset of a disease

!  In a single exposure, common vehicle epidemic, the epidemic curve represents the distribution of incubation periods

Epidemic Curves

Epidemic Curve and Distribution of Incubation Periods

Diarrheal Illness in Passengers Aboard the M/S Viking Sun, By Onset, September 11–23, 1976

Common Source Outbreak

Source: CDC. (1976). 5

Epidemic Curve of a Propagated Epidemic

Epidemic Curves: Cycles and Seasonality

Outbreak investigation

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

Steps in Investigating an Acute Outbreak

1.  Define the epidemic

2.  Examine the distribution of the cases by: !  Time !  Place !  Person

3.  Look for combinations of relevant variables

4.  Develop hypotheses

5.  Test hypotheses

6.  Recommend control measures !  Control present outbreak !  Prevent similar outbreaks

!  Modes of disease transmission

!  Determinants of disease in the individual

!  Disease in populations !  Epidemics !  Herd immunity !  Incubation period !  Epidemic curves

!  Steps in outbreak investigation

Summary

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