Health Surveillance is a process of monitoring the health status of the population through the systematic collection, orderly consolidation and evaluation, and prompt dissemination of relevant data. This process encompasses more than just disease reporting and analysis; it also involves identifying the need for and initiating control activities.
A Health Surveillance system consists of the processes, procedures and infrastructure that support a system for responding to threats to public health. Depending on the nature of the threat, the response might include public education, containment strategies, quarantine, vaccination, or other intervention measures. The primary purpose of surveillance is to identify and respond to disease threats in a timely fashion. This response should maximize the number of lives saved while minimizing the negative impact on the economy, social or cultural fabric, and other important societal values.
Typically, the responsibility for health surveillance rests with local and state health departments, although federal jurisdiction is used where a problem crosses state boundaries. Notifiable-disease systems are often mandated by state law, and such health-care providers as clinicians, hospitals and laboratories are obligated to report on disease cases. A significant amount of surveillance is carried out by syndromic (symptom-based) systems that do not require clinical or laboratory diagnosis and collect data on the occurrence of specific symptoms rather than specific diseases (e.g., “rash illness” versus measles).
Health surveillance is not a separate entity but an integral part of the public health system, and it depends on the astuteness and alertness of local clinicians, epidemiologists, and other health-care professionals. Often, existing data sets provide the basis for health surveillance, and such data include vital records, medical-management information and billing systems, hospital discharge data, police records of violence, health-interview surveys, risk-factor survey data, and school records for disabilities or injuries among children.
In general, surveillance is outcome oriented and seeks to measure the frequency of a disease or condition in terms of incidence, prevalence, or mortality; the severity of a disease or injury, measured by case-fatality ratio, hospitalization rates, or disability levels; and the cost of a disease or injury, as measured by health-care expenditures. Surveillance can also be conducted using qualitative methods, such as focus groups or mailed questionnaires.
A major objective of health surveillance is to identify clusters of disease, which may suggest a particular etiology or method of spread. This recognition might lead to further epidemiologic or laboratory research, or the identification of individuals who might be good candidates as subjects of intervention studies.
Another useful tool in analyzing health-surveillance data is Bayesian networks, which are graphs that graphically represent the cause-effect relationships between a number of factors. The networks can also be used for sensitivity analysis, quantifying the impact that a small change in one of the factors will have on the others. Moreover, this statistical procedure can help to evaluate whether a new data stream or analytical technique is worth incorporating into the system. This is not usually considered when evaluating new clinical interventions, but it is frequently employed in the evaluation of changes to surveillance systems.