Public Health Surveillance - a dynamic tool

Alexander Langmuir, expert in public health surveillance and intelligence  
Alexander Langmuir, the man behind epidemiologic intelligence, gave a crystal clear definition of surveillance as it applies to public health. He defined it as the continued watchfulness, systematic collection, consolidation and interpretation of public health data, and dissemination of this information to people who are in a position to take action.

Surveillance is not static, but dynamic. Surveillance system must be flexible. It should adapt itself to the changing epidemiology of the problem under surveillance. There was an era when measles in India was under event-based surveillance. But it is now realized that in order to achieve elimination of measles, we should move on to case-based surveillance where every case of measles has to be investigated and control measures promptly instituted.

Though not in the literal sense, surveillance can afford to be less rigorous when the disease has a low killing power (case fatality ratio), high prevalence and large number of subclinical cases. When the same disease epidemiology changes towards the opposite extreme of these conditions, the surveillance system needs to be tightened and made more sensitive.

Surveillance is an ongoing process. It is continuous, and not episodic. This differentiates it from other overseeing methods such as monitoring and evaluation. However, there could be an instance when surveillance may be stopped. Smallpox surveillance was rolled back by the World Health Organization in 1980 when the disease was no longer perceived as a public health problem. Why continue surveillance when the problem ceases to exist?

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