Chapter 2

At the end of the chapter, the reader will be able to:

1. Understand and calculate incidence and prevalence of a disease

2. Discuss the application of incidence and prevalence of diseases in pharmacoepidemiology

3. Understand and calculate various mortality measures

4. Discuss the application of mortality measures in pharmacoepidemiology

5. Understand and calculate risk ratio and odds ratio

6. Understand and calculate relative and absolute risk reduction, number needed to treat, and number needed to harm

7. Discuss the application of various association and effect measures in pharmacoepidemiology

As discussed in Chapter 1, pharmacoepidemiology is the study of the use and the effects of drugs in large numbers of people.1 It is a relatively new discipline employing the methods of epidemiology in the study of drug use and drug effects in populations.

Epidemiology is the study of the factors that determine the occurrence and distribution of diseases in populations.2 Specifically, epidemiology focuses on who is likely to develop a disease under what circumstances. This reflects the basic principle in epidemiology that disease does not occur randomly in a population; rather, certain people are at higher risks to develop certain conditions compared to others. Individual genetic characteristics, behaviors, socioeconomic status, environmental milieu, and probably the interaction among these factors have an impact on the development of disease. Epidemiologic studies are conducted to examine the frequency or distribution of disease in groups of people, to determine the cause of or risk factors for a disease, and to evaluate the effectiveness of preventive and therapeutic measures to control the disease.

### Infectious Disease Epidemiology

In history, mankind had long been plagued by infectious diseases. For example, in 1900, the leading causes of death in the U.S. were pneumonia and influenza; tuberculosis; and diarrhea, enteritis, and ulceration of the intestines (Table 2-1). As a fundamental medical science to study the frequency and occurrence of disease, historically, epidemiology has been considered the study of infectious diseases in large populations.3 From this perspective, epidemiology has focused on how infectious disease is distributed in populations and the factors that determine this distribution. Infectious diseases have some special features:3

• There is usually the presence of a single, known, identifiable cause. Infectious disease can be caused by bacteria, viruses, fungi, or parasites. For example, cholera, an acute diarrheal infection, is caused by ingestion of the bacterium Vibrio cholera. The 2009 H1N1 flu is caused by the swine-origin influenza A (H1N1) virus.
• Infectious disease has the potential of transmission from one person or species to another. Some infectious diseases may be spread from person to person by means of direct or indirect contact or airborne small particles, some may be transmitted via animal or insect bites, and some may be transmitted through contaminated food or water.
• Individuals with infectious disease, sometimes even without being recognized as a case, may become a risk factor for other people. For example, ...

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