Surveillance

The World Health Organisation (WHO) define “surveillance” as the systematic ongoing collection, collation and analysis of data and the timely dissemination of information to those who need to know so that action can be taken. In other words, the epidemiological surveillance is necessary to plan, single out, manage and evaluate the actions relevant for the management of population sanitary status. In particular, the main tasks of surveillance systems may be summarised as follows:

• to monitor the sanitary status dynamics with the aim of intervening with timely preventive measures,
• to evaluate measures already implemented in relation to disease prevention and control,
• to optimize the use of available resources.

Surveillance is first and foremost a process for producing information that will trigger, inform or be used to evaluate defined public health (or clinical) action. If there is no clear and immediate link between the information output of an activity and existing or planned public health action then it is unlikely that the activity is surveillance.

Surveillance Types

1. Passive 

• definition - the most common form of surveillance, health authorities take no action while waiting for report forms to be submitted. It is also a potentially misleading term, since case reporting is not a passive activity for the reporter, who must complete the form.
• advantage - less costly than other reporting systems, data collection is not burdensome to health officials, and the data may be used to identify trends or outbreaks if providers and laboratories report.
• Limitations - non-reporting or under-reporting, which can affect representativeness of the data and thus lead to undetected trends and undetected outbreaks. A positive test may not be reported to prevent the stigmatization associated with STD, because of a lack of awareness of reporting requirements by health care providers, or the perception on the part of the health care provider that nothing will be done. 
• Incomplete reporting -  (i)reflect lack of interest, surveillance case definitions that are unclear or have recently changed, or changes in reporting requirements. (ii) Result of the patient not being willing to provide the information or hardware/software systems that cannot capture the information in databases.

2. Active surveillance

• definition - involves outreach by the public authority, such as regular telephone calls or visits to laboratories, hospitals, and providers to stimulate reporting of specific diseases.
• it places intensive demands on resources, implementation of active surveillance should be limited to brief or sequential periods of time and for specific purposes.
• It is a reasonable method of surveillance for: (i)conditions of particular importance - to document a suspected outbreak, or to augment timely disease intervention or epidemiologic investigation (e.g., for congenital syphilis in certain jurisdictions); (ii) episodic validation of representativeness of passive reports and as a departure point for enhancing completeness and timeliness of reporting (e.g., lab visitation programs to ensure all reactors reported);( iii) diseases targeted for elimination or eradication (e.g., smallpox, syphilis).
• Operationally, active surveillance includes visits or telephone calls to such key reporting sources as clinicians or laboratories by public health authorities on a regular or episodic basis to elicit (or verify) case reports and/or reviewing medical records and other alternative sources to identify diagnoses that may not have been reported. It is generally employed when it is expected that more disease is in the community than is shown in the passive surveillance systems.

3. Sentinel Surveillance

• Sentinel surveillance involves the collection of case data from only part of the total population (from a sample of providers) to learn something about the larger population, such as trends in disease. 
• advantages - (i) less expensive to obtain than those gained through active surveillance of the total population, and the data can be of higher quality than those collected through passive systems. This is because it is logistically easier to obtain higher quality information from a smaller population.(ii) Sentinel surveillance systems may be useful in identifying the burden of disease for conditions that are not reportable, or behavioral characteristics that are of sufficient public health importance to merit monitoring. 
• vulnerability - not being able to ensure the representativeness of the sample selected 
• Candidates for sentinel systems might include: human papilloma virus, herpes simplex primary infection, congenital infection, or other adverse outcomes of STDs. 

4. Lab-based reporting

• a surveillance system in which the reports of case come from clinical lab instead of healthcare practitioners/hospital

5. Universal case report 

• a surveillance system in which all cases of disease are supposed to be reported 
• Additionally, case reports received by the public health authority may require further action to ensure completeness, proper case classification, and partner management.

Ideal Surveillance System

Information Cycle
1. Collection 

• long or short term - Long-term data although in some cases essential, e.g. in time series analysis, is often prohibitively expensive in terms of both labour and monetary cost. However, long-term data collection is critical, if the modeller is to attempt to perform a time series analysis to account for seasonal, cyclic or secular trend in the respective populations and infection patterns. Time series analysis is also useful in quantifying parasite or vector development and maturation (Mullens and Lii, 1987).
• longitudinal or cross sectional
• retrospective or prospective in nature. 

2. Collation 

• the assembly of written information into a standard order. One common type of collation is called alphabetisation, though collation is not limited to ordering letters of the alphabet.
• Although computers have been available and heavily used in the developed world for several years, they have been in limited supply in many of the lesser developed countries. As a result, the data from these lesser developed countries typically exist on paper.

3. Analysis & Interpretation

• For epidemiological modelling will depend on the question of interest. Appropriate analyses could include case-control or cohort studies, or they may focus on the quantification of the estimation and significance testing of specific population and disease dynamics parameters. Analyses may also be made in order to specify the appropriate distribution of these parameters to be used in a simulation model.

4. Dissemination/Utilisation

• Once the appropriate data have been collected, collated and analysed, what is the appropriate form(s) in which they should be disseminated? Meaningful dissemination of these data and analytic results could occur in at least two ways. The first is to make the data available, in a collated from, to all interested scientific researchers. This method has recently been adopted by the National Animal Health Monitoring System, which is a livestock health surveillance system operated by the United States Department of Agriculture (USDA-APHIS/VS).

Purpose and Uses of Surveillance

• estimate magnitude of the program 
• determine geographic distribution of illness 
• portray natural history of a disease 
• detect epidemics/ define a problem 
• generate hypothesis and stimulate research 
• evaluate control measure 
• monitor changes in infectiuos agent 
• detect changes in health practice 
• facilitate planning

Refferences :
1. http://www.cdc.gov/std/program/surveillance/4-PGsurveillance.htm
2. Lecture note Week 2, Surveillance, Response and the Role of Public Health Informatics by dr Luthfan Lazuardi