E3 Easy Epidemiology for Everyone

Indiana Epidemiology Newsletter
October 2007

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.


Surveillance is defined as “the systematic ongoing collection and analysis of data and the timely dissemination of the information to those who need it to take action” (Last, 2001).  Surveillance is what local and state health officials do every day to monitor the pulse of health in a community.  Public health officials generally use two types of surveillance methods: passive and active.

Passive surveillance is used most frequently, since it is the least costly and least labor intensive.  Passive surveillance relies on outside sources to provide information about disease to public health officials.  Reportable disease surveillance is an example of a passive surveillance system.  State law requires health care providers, hospitals, and laboratories to report certain conditions or laboratory results to the local health department (LHD) for investigation.  Upon receiving these reports, LHD officials will conduct a case investigation to determine if there is any link to other cases of the same disease within their jurisdiction.  If there is an associated link, the local health officials can conduct active surveillance to identify additional cases. 

Active surveillance is used when public health officials actively seek information regarding specific cases of illness.  This may involve contacting hospitals, emergency departments, clinics, private providers, long-term care facilities, schools, and even places of employment, depending on the disease and the suspected exposure.  Active surveillance is more time-consuming and costly to maintain and, thus, is generally used for shorter periods of time, such as outbreak investigations.

Other types of public health surveillance that are commonly used include syndromic surveillance and sentinel surveillance. 

Syndromic surveillance collects health information prior to actual disease diagnosis (e.g., emergency department chief complaint data) to detect disease outbreaks earlier.  Syndromic surveillance categorizes data into syndrome categories, such as gastrointestinal or respiratory, and is monitored daily by local and state health officials for increases in certain syndromes that may indicate increased illness in the community. 

Sentinel surveillance is used to monitor conditions that are not reportable or have a specific seasonality.  Sentinel surveillance enlists information from health care providers who volunteer to participate in the surveillance program to indicate the level of a particular illness in the community.  In Indiana, one of the most successful sentinel surveillance programs is the influenza sentinel physician program.  Sentinel physicians enrolled in the program report levels of influenza-like illness each week year-round and submit specimens for testing at the ISDH Laboratory.  The ISDH then forwards this information to the Centers for Disease Control and Prevention (CDC).  Summary information is reported back to the sentinels and to local health departments.  Sentinel influenza data have been collected in Indiana for over 10 years.


Next month’s topic: Descriptive Epidemiology