Surveillance should be directly relevant to control needs, and as such the criteria for deciding whether to undertake surveillance of a particular hazard, exposure or event are closely aligned to those for deciding whether to undertake control measures in respect of that hazard, exposure or event. The major criteria for determining the case for surveillance are therefore:
There are additional considerations in respect of decisions about what type or form of surveillance might be undertaken, and these will be discussed later.
The public health importance of a problem, and the priority for control and prevention, is determined by a variety of factors, the most important of which is usually the overall burden on the population. Other factors, such as the potential for epidemic spread, political or media interest, the availability and cost-benefit ratio of interventions, and national or international disease control targets may also be influential in determining the priority given to the control and surveillance of a public health problem. An example of how priorities can change, and as a result investment in surveillance can be influenced, can be seen in respect of sexually transmitted infections in the UK in the second half of the 20th century. Investment in the control, and surveillance, of sexually transmitted infections in the UK declined during the 1950's and 1960's, as it was perceived that the post war peak in gonorrhoea and syphilis had been successfully controlled. Despite rises in the incidence of these diseases in the 1960's and 1970's it was not until the advent of AIDS in the 1980's that significant investment in improved surveillance, as part of a major new programme of control and prevention, took place . These improved surveillance systems provided much of the information, and identified cases for research, that enabled targeted and effective control measures to be implemented before HIV had been identified. More recently, the rise in antimicrobial resistance has resulted in significant investment in many countries in surveillance systems for monitoring MRSA , antibiotic resistant gonorrhoea  and other important resistant bacterial infections.
The decision to undertake surveillance, and more particularly the decision as to what type of surveillance should be undertaken, should take into account the strategy for prevention and control of the disease, hazard or exposure that is to be monitored, and also the types of intervention that are to make up the control programme. Potential points for intervention to control or prevent infectious disease are outlined in figure 1.
The development of a new intervention may introduce the requirement for new surveillance systems to inform and monitor the application of that intervention. Thus the introduction of pneumococcal vaccination in the UK, initially among high risk groups and subsequently as part of the routine childhood immunisation programme, has prompted the development of enhanced surveillance systems for pneumococcal infection, including monitoring of serotypes causing invasive illness and monitoring of vaccine uptake among high risk groups, which has informed policy (in respect of choice of vaccine) and enabled programme delivery to be assessed .
New opportunities for the development of surveillance may also arise as a result of changes in the feasibility of collecting data that would inform control and prevention activities. These opportunities might arise because of the introduction of a new health service that is able to provide information that would previously not have been available or prohibitively costly to obtain, or because of the introduction of new technology that significantly reduces the costs, and hence the cost-benefit, of data collection for surveillance purposes. An example of the former are the surveillance systems that have been developed to make use of data recorded by telephone health advisory services, in the UK  and the USA . Examples of surveillance that has been established because of opportunities provided by new technology include primary care surveillance systems recently introduced in the UK .
1. Catchpole MA. The Role of Epidemiology and Surveillance Systems in the Control of Sexually Transmitted Diseases. Genitourinary Medicine 1996; 72: 321
2. Griffiths C, et al. Trends in MRSA in England and Wales: analysis of morbidity and mortality data for 1993-2002. Health Stat Q 2004;21:15-22
3. Paine T, Herring A, Turner A, et al. GRASP: a new national sentinel surveillance initiative for monitoring gonococcal antimicrobial resistance in England and Wales. Sex Transm Infect 2001; 77:398-401
4. Health Protection Agency. Surveillance of the impact of the pneumococcal conjugate (PCV) vaccination programme for children in England and Wales and protocol for investigation of vaccine failures. (July 2006). http://www.hpa.org.uk/infections/topics_az/pneumococcal/PneumococcalGuidanceSurveillance.htm
5. Cooper DL, Smith GE, Baker M, Chinemana F, Verlander N, Gerard E, Hollyoak V, Griffiths R. A national symptom surveillance system in the UK using calls to a telephone health advice service. MMWR 2004: 53(Suppl);179-183.
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7. Health Protection Agency. Primary Care Surveillance - QResearch (July 2006) http://www.hpa.org.uk/infections/topics_az/primary_care_surveillance/QResearch.htm