Indiana Epidemiology Newsletter
|E3 is a new feature of the Indiana Epidemiology Newsletter dedicated to exploring the fundamentals of epidemiology. Each month, a different epidemiology concept will be explored to enhance understanding of basic epidemiology.|
In previous articles, we have talked about surveillance and descriptive epidemiology, so what happens when there is an “outbreak”? An outbreak would best be defined as more cases of disease in a given population at a given time than expected. For example, we expect to see a lot of influenza cases in February, especially in those with weakened or compromised immune systems. On the other hand, we do not expect to see many cases of influenza in July and August. If we did, we would call it an outbreak.
In an outbreak, an epidemiologist is going to begin by asking questions and gathering data about the people that are becoming ill (e.g., are they all elderly, are they all young). Epidemiologists also gather information about where these people live or where they were exposed to the illness (e.g., a restaurant or other common gathering place). Finally, epidemiologists gather information about what time these events occurred (e.g., how long were people ill, what time were they exposed, when did their first symptoms appear).
Once information is gathered, there are three types or methods of investigation that can assist in determining the cause of the illness and then control measures can be implemented to prevent others from becoming ill.
One of the most utilized methods that you have probably heard the most about is a Case Control study. A Case Control study is utilized when the population in which an outbreak occurs is so large that it is not possible to interview everyone. An example is the recent salmonella in peanut butter, where a large number (hundreds of thousands or millions) were possibly exposed, making it impossible to interview everyone. In a Case Control study, epidemiologists interview cases, (people who are ill with the disease of interest) as well as controls (those who are not ill, but may have been exposed). Epidemiologists then compare the two groups to determine if there is an exposure that the cases had and the controls may not have had. Using the peanut butter example: Those who ate the peanut butter became ill. Those who did not eat the peanut butter did not become ill. When epidemiologists compared the two groups, the peanut butter stood out.
Another common study design is the Cohort Study. A cohort is a group of people. This could be a prospective cohort that is exposed and then followed over time to determine if they develop illness. This would be a prospective study because the cohort is followed into the future. There is also a retrospective cohort. The retrospective study involves a cohort that is already ill. Epidemiologists work backward to determine the exposure that made the group of people ill. One of the most famous ongoing cohort studies is the Framingham, Massachusetts, heart study. This study, begun in 1948 with more than 5,000 people enrolled, has led to the identification of major risk factors for heart disease including high blood pressure and high cholesterol.
The last method of investigation is a Cross-sectional Study. A Cross-sectional Study is not used frequently, but it is most useful when the illness does not involve a large number of people. This gives a “snapshot” of the situation at one period in time. A Cross-sectional Study compares the presence or absence of disease and the presence or absence of the variables that could potentially cause illness.
Each of these methods of investigation will be explored individually in upcoming issues.
Next Month: Cohort Studies