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Vol. 16,
No. 1
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March
1995
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Planning a Public Health Surveillance
System Steven M. Teutsch, M.D., M.P.H. Epidemiology Program Office Public health surveillance is the ongoing systematic collection, analysis,
and interpretation of outcome-specific data for use in the planning, implementation,
and evaluation of public health practice1.
A surveillance system includes the functional capacity for data collection
and analysis, as well as the timely dissemination of these data to persons
who can undertake effective prevention and control activities. While the
core of any surveillance system is the collection, analysis, and dissemination
of data, the process can be only understood in the context of its usefulness
for addressing specific health outcomes. Development of the Concept of Surveillance Langmuir defined the term "disease surveillance" as the collection,
analysis, and dissemination of data on specific diseases2.
This construct did not encompass direct responsibility for control activities.
In 1965, the Director General of the World Health Organization (WHO) established
the epidemiological surveillance unit in WHO’s Division of Communicable
Diseases3, and included "the epidemiological
study of disease as a dynamic process." In 1968, the 21st World Health
Assembly, through the leadership of Langmuir and Raska, focused on national
and global surveillance of communicable diseases and affirmed the three
main features of surveillance: a) the systematic collection of pertinent
data, b) the orderly consolidation and evaluation of these data, and c)
the prompt dissemination of results to those who need to know—particularly
those in position to take action4. The Assembly’s
discussion also addressed the application of the concept to public health
problems other than communicable disease5.
In addition, "epidemiologic surveillance" was said to imply "…the
responsibility of following up to see that effective action has been taken."
Since that time, a wide variety of health events, such as childhood lead
poisoning, leukemia, congenital malformations, abortions, injuries, and
behavioral risk factors have been placed under surveillance6. Surveillance in Public Health Practice Public health practice can be divided into surveillance; epidemiologic,
behavioral, and laboratory research; service (including program evaluation);
and training. Surveillance data should be used to identify research and
service needs, which, in turn, help to define training needs. Unless data
are provided to those who set policy and implement programs, their use is
limited to archives and academic pursuits, and the material is therefore
appropriately considered to be health information rather than surveillance
data. However, surveillance does not encompass epidemiologic research or
service, which are related but independent public health activities that
may or may not be based on surveillance. Thus, the boundary of surveillance
practice excludes actual research and implementation of delivery programs. Because of this separation of surveillance from research, "epidemiologic"
should not be used to modify surveillance1.
The term "public health surveillance" describes the scope (surveillance)
and indicates the context in which it occurs (public health). It also obviates
the need to accompany any use of the term "epidemiologic surveillance"
with a list of all the examples this term does not cover. Surveillance is
correctly—and necessarily—a component of public health practice, and should
continue to be recognized as such. Because of its close link to public health
action and the focus on providing information for action, surveillance is
directly tied to officials responsible for health of the population. Uses in Public Health Surveillance Data Surveillance has many uses Table 1. The natural history
of disease can be demonstrated by the surveillance of malaria rates in the
United States since 1930. In the 1940s, malaria was still an endemic health problem in the southeastern
United States. Rates of malaria decreased until the early 1950s, when military
personnel involved in the conflict in Korea returned to the United States
with malaria. The general downward trend in reported cases of malaria continued
into the 1960s until, once again, numbers of cases of malaria rose, this
time among veterans returning from the war in Vietnam. Since that time,
we have continued to see increases in numbers of reported cases of malaria
involving immigrant population, as well as among U.S. citizens traveling
abroad (Figure 1). The uses of surveillance
Stephen B. Thacker, M.D., M.Sc.
Centers for Disease Control and Prevention (CDC)
Atlanta, Georgia – U.S.A.
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w Portrayal of the natural history of disease w Detection of epidemics w Documentation of the distribution and spread of a health event w Facilitating epidemiologic and laboratory research w Testing of hypotheses w Evaluation of control and prevention measures w Monitoring of changes in infectious agents w Monitoring of isolation activities w Detection of changes in health practice w Planning |
Figure 1
Malaria rates, by year – United States, 1930-1988
Surveillance data can also be used to detect epidemics. For example, during the swine influenza immunization program in 1976, a surveillance system was established to detect adverse sequelae related to the program7. Working with state and local health departments, CDC was able to detect an epidemic of Guillain-Barré syndrome, which rapidly led to the termination of a program in which 40,000,000 U.S. citizens had been vaccinated. However, most epidemics are not detected by such analysis of routinely collected data but are identified through the astuteness and alertness of clinicians and public health officials within the community. When someone notes an unusual occurrence in the health of a community, the existence of organized surveillance efforts in the health department provides the infrastructure for conveying information to facilitate a timely and appropriate response.
The distribution and spread of disease can be documented from surveillance data, such as intra- and inter-country specific data on cholera, though one must be cognizant of differences in data collection systems in different jurisdictions. U.S. cancer mortality statistics have also been mapped at the county level to identify a variety of geographic patterns that suggest hypotheses on etiology and risk8. Recognition of such clusters can lead to further epidemiologic or laboratory research, sometimes using individuals identified in surveillance as subjects in epidemiologic studies.
Surveillance data can also be used to test hypotheses. For example, in 1978 the U.S. Public Health Service announced a measles elimination program that included an active effort to vaccinate school-age children. Because of this program and the state laws that excluded students from school if they had not been vaccinated, CDC anticipated a change in the age pattern of persons reported to have measles. Before the initiation of the program, the highest reported rates of measles were for children 10-114 years of age. As predicted, almost immediately after the school exclusion policy was implemented, the number of cases not only decreased, but the peak occurrence also shifted from school-age to preschool-age children.
With routine collected data, one can examine—without special studies—the effect of a health policy. For example, the introduction of inactivated poliovirus vaccine in the United States in the 1950s was followed by a dramatic decrease in the number of reported cases of paralytic poliomyelitis, and the subsequent introduction in the 1960s of oral poliovirus vaccine was followed by an even greater decline.
Efforts to monitor changes in infectious agents have been facilitated by the use of surveillance data. The National Nosocomial Infections Surveillance System, has been used to monitor changes in antibiotic-resistance patterns of infectious agents associated with hospitalized patients.
The increasing use of various technologies and practices in health care has come to be an issue of growing concern over the past decade; surveillance data are useful both in planning research to learn the causes of these changes and in monitoring the impact of such changes in practice and procedure on outcomes and costs associated with health care9.
Finally, surveillance data are useful for planning. With knowledge about changes in the population structure or in the nature of conditions that might affect a population, officials can plan more effectively the appropriate health services and preventive activities.
Objectives of Surveillance System
Planning a surveillance system begins with a clear understanding of the purpose of surveillance Table 2. In the context of public health, surveillance may be established to meet a variety of objectives, including assessment of public health status, establishment of public health priorities, evaluation of programs, and conduct of research.
Steps in planning a surveillance system
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2. Develop case definitions 3. Determine data sources data-collection mechanism (type of system) 4. Determine data-collection instruments 5. Field-test methods 6. Develop and test analytic approach 7. Develop dissemination mechanism 8. Assure use of analysis and interpretation
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Surveillance is inherently outcome oriented and focused on various outcomes associated with health-related events or their immediate antecedents. These include the frequency of an illness or injury, usually measured in terms of numbers of cases, incidence, or prevalence; the severity of the condition, measured as a case-fatality ratio, hospitalization rate, mortality rate, or disability; and the impact of the condition, measured in terms of cost. Where health outcomes are incontrovertibly linked to risk factors or specific procedures, it is often useful to measure the latter because they are more frequent (and hence more precisely ascertainable for small populations) and may be more close linked to public health interventions. For example, mammography with suitable follow-up is the major prevention strategy for reducing mortality associated with breast cancer.
The level of utilization of mammography by women can be monitored regularly and should be a more timely indicator of the impact of public health prevention programs than measurement of mortality from breast cancer. Surveillance data should also improve targeting of prevention efforts toward populations with low utilization. In addition, over-utilization by some parts of the population (e.g., women <35 years of age who do not have other risk factors) might stimulate efforts to reduce unnecessary procedures.
High-priority health events should be under surveillance. Both quantitative and qualitative approaches can be used in a selections process. Some quantitative factors are shown in Table 3. In addition, criteria based on a consensus process to identify high-priority problems may identify emerging issues or problems that might otherwise not be considered. The consensus process leading to the Year 2000 Health Promotion and Disease Prevention Objectives in the United States is an example of a mechanism for identifying high-priority conditions, types of behavior, and interventions that require ongoing monitoring10.
•Criteria for identifying high-priority health
events for surveillance
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Incidence Prevalence Mortality Years of potential life lost w Severity Case-fatality ratio Hospitalization rate Disability rate w Cost Direct and indirect costs w Preventability w Communicability w Public interest |
Because public health surveillance is driven by the need to be cognizant of diseases and injuries in the community and to respond appropriately, surveillance is inherently an applied science. Therefore, as surveillance has evolved, it is generally undertaken only when there is reasonable expectation that control measures will be taken as appropriate. For many conditions, the link between surveillance and action is obvious (e.g., meningococcal meningitis prophylaxis for contacts of patients diagnosed as having meningitis.) For emerging conditions, such as eosinophilia-myalgia syndrome, there is a compelling public health need to identify cases to delineate the magnitude of the problem, identify the mode of spread, and take appropriate action.
Surveillance data are usually augmented by additional studies to determine more precisely the cases, natural history, predisposing factors, and modes of transmission associated with the health problem. Yet, undertaking surveillance exclusively for research purposes is not warranted. Research needs are often better served by other, more precise (and often more costly) methods of case identification (e.g., registries), which facilitate more detailed data collection and tracking of cases. For example, registries of type I diabetes may have surveillance value, but are justified primarily because they fill research needs. The ongoing public health application of these data is more limited. Scarce public health resources and efforts of health-care providers to report cases need to be focused on problems for which the public health importance and the need for public health action can be readily recognized.
A primary role of surveillance is the assessment of the overall health status of a community. One approach to this issue is the development and identification of a set of indicators that measure major components of health status. Such a set has been developed in the United States for use at national, state, and local levels11.
Methods
Once the purpose of and need for a surveillance system has been identified, methods for obtaining, analyzing, disseminating, and using the information should be determined and implemented.
Surveillance systems are ongoing and required the cooperation of many individuals. The system must be acceptable to those who will contribute to its success and it should be flexible enough to meet the continually evolving needs of the community and to accommodate changes in patterns of disease and injury12. It must provide information that is timely enough to be acted upon. All of these considerations must be carefully balanced in order to design a system that can successfully meet identified needs without becoming excessively costly or burdensome.
Case Definitions
Practical epidemiology is heavily dependent on clear case definitions that include criteria for person, place, and time as well as clinical, laboratory, and epidemiologic features13,14.
While high sensitivity and specificity are both desirable, generally one comes at the expense of the other. A balance must be struck between the desire for high sensitivity and the level of effort required to track down false-positive cases. In addition, case definitions evolve over time. During periods of outbreaks, cases that are epidemiologically linked to the outbreak may be accepted as cases, whereas in non-epidemic periods, serologic or other more specific information may be required.
Data Collection
Information on diseases, injuries, and risk factors can be obtained in may ways. Each mechanism has characteristics that must be balanced against the purpose of the system. Timeliness is of the essence for frequently fatal conditions such as plague, rabies, or meningococcal meningitis. Notifiable-disease systems are most appropriate for such potentially catastrophic conditions that require urgent action. Conversely, detailed information on influenza strains or Salmonella serotypes must come from laboratory-based systems.
Often, existing data sets can provide surveillance data. Such sets include vital records, administrative systems, and risk-factor or health-interview surveys. Among administrative systems, hospital-discharge data, medical-management-information and billing systems, police records for violence, and school records for disabilities or injuries among children can all provide needed data. In addition, with some modification, an existing system might provide needed data more economically or efficiently than a newly initiated system. While existing data systems can be used for surveillance, they are not surveillance systems in and of themselves. Surveillance is a larger process that requires analysis, interpretation, and use of the data. These steps are not components of most data systems.
Existing registries or ongoing surveys may collect information on defined populations. To the extent that the condition of interest is uniformly distributed, the population under study is reasonably representative, and the information collected is available on a timely basis, such systems can be valuable data sources. Although many registries are established for research purposes, they often provide valuable data for surveillance purposes. In particular, cancer registries have been widely used15.
A network of sentinel providers can also collect data on common conditions, such as influenza; more specialized providers can provide data on less common conditions, e.g., ophthalmologists who provide information on treatment of patients for diabetic retinopathy.
Standardization
Data-collection instruments should use generally recognized and, where suitable, computerized formats for each data element to facilitate analysis and comparison with data collected in other systems, e.g., census and other surveillance data. Careful consideration should be given to using identifiers. Although additional assurances of confidentiality and privacy considerations will be required, the ability to link data to other systems enhances the value of the system.
Field Testing
The careful development and field testing of surveillance systems and procedures is important to facilitate the implementation of feasible systems and to avoid making changes as systems are implemented on a broad scale. The data to be collected by a surveillance system, the data sources and collection methods, and the procedures for handling the information should be developed and tested. The frustration engendered by a new and poorly executed system may undermine efforts to improve or use existing systems for the same or other conditions. These field-test projects can demonstrate how readily the information can be obtained and can detect difficulties in data-collection procedures or in the content of specific questions. Analyses of this test information may also identify problems with the information collected. Model surveillance systems may facilitate the examination and comparison of a variety of approaches that would not be feasible on too large a scale and may identify methods suitable for other conditions or other settings.
Data Analysis
A determination of the appropriate analytic approach to data should be an integral part of the planning of any surveillance system. The data needed to address the salient questions must be assessed to assure that the data source or collection process is inadequate. Analyses may prove to be as simple as an ongoing review of all cases of rare but potentially devastating illnesses, such as plague. For most conditions, however, an assessment of the crude number of cases and rates is followed by a description of the population in which the condition occurs (person), where the condition occurs (place), and the period over which the condition occurs (time). These basic analyses require decisions as to the kind of information that needs to be collected. The level of detail required varies substantially from condition to condition. For instance, one may need more detailed information regarding the population that is not receiving prenatal care than on the one that is exposed to meningococcal disease, because the nature of the intervention for the former is likely to be more complex and requires an understanding of socioeconomic factors. Similarly, how one will collect data on geographic areas may depend on whether the data will be examined at the county, state, or census-tract level.
Most contemporary surveillance systems are maintained electronically. The type of hardware and software needed depends on the types of analyses to be performed and the size of the data bases. As personal computers become more powerful, the capacity of data-sharing systems such as local-and wide-area networks become more widely available, more surveillance systems can be operated on personal computers. Software to meet most basic analytic needs for surveillance, including mapping and graphing, is now widely available. The analytic approach often suggests a basic set of analyses that are performed on a regular basis. These analyses should be designed early in the development of the system and incorporated into an automated system, which can then be run by support personnel.
Interpretation and Dissemination
Data must be analyzed and presented effectively so that decision makers at all levels can readily see and understand the implications of the information. Knowledge of the characteristics of the audiences for the information and how they might use it may dictate any of a variety of communications systems. Routine, public access to the data—consistent with privacy constraints—should be planned for and provided. This access can be facilitated with various electronic media, ranging from systems with structured-analysis features suitable for general users to files of raw data for persons who can do special or more detailed analyses themselves.
The primary users of surveillance information, however, are public health professionals and health-care providers. Information directed primarily to those individuals should include the analyses and interpretation of surveillance results, along with recommendations that stem from the surveillance data. Graphs and maps should be used liberally to facilitate rapid review and comprehension of the data. Communications media represent a valuable secondary audience that can be used to amplify the messages from surveillance information. The media play an important role in presenting and reinforcing health messages. Innovative methods for presenting information capitalizing on current audiovisual technology should be explored.
Evaluation
Planning, like surveillance itself, is an iterative process requiring the regular reassessment of objectives and methods12. The fundamental question to be answered in evaluation is whether the purposes of the surveillance system have been met. Did the system generate needed answers to problems? Was the information timely? Was it useful for planners, researchers, health-care providers, and public health professionals? How was the information used? Was it indeed worth the effort? Would those who participated in the system be willing to continue to do it? What could be done to enhance the attributes of a system (timeliness, simplicity, flexibility, acceptability, sensitivity, predictive-value positive, and representativeness)?
Answers to these questions will direct subsequent efforts to revise the system. Changes might be minor (e.g., the addition of data elements to existing forms) or major (e.g., the need to obtain information from entirely different data sources). For example, a system to determine utilization of mammography might be based on administrative billing systems. Yet, problems with reports of multiple mammography examinations for the same individual might require the addition of unique patient identifiers or the addition of questions on mammography use from self reports on health-interview surveys.
Periodic rigorous evaluation assures that surveillance systems remain vibrant. Systems that assess problems of only historical interest should be discontinued or simplified to reduce the reporting burden. Contemporary systems should take advantage of the emergence of new technology for information collection, analysis, and dissemination. They should capitalize on new information systems. For example, sentinel surveillance systems have become more flexible to allow the inclusion of an array of topics. Electronic medical records and standardized clinical data bases all provide opportunities to obtain data that have been burdensome or difficult to secure16. These information sources may also provide more timely data and may allow individuals to be tracked, an option that would be virtually impossible without such electronic systems.
Involvement of Interested Parties in Surveillance
Virtually all surveillance systems involve networks of organizations and individuals. Surveillance of notifiable disease relies on health-care providers including clinicians, hospitals, and laboratories to report to local health departments, who have the initial responsibility for responding to reports and amassing data.
Programs for injuries and chronic and environmental diseases also may have complex organizational structures and may involve a wide array of external professional and voluntary interest groups whose needs must be addressed. Some basic surveillance information can be gleaned from such ongoing information systems as vital records, hospitalization programs, and registries. Many, however, require surveillance systems to be established in unique places (e.g., rehabilitation units and emergency medical services for spinal-cord injuries or radiology centers for mammography). The support and interest of these groups of constituents are valuable in establishing the systems; these groups can provide key input regarding purposes of systems and users systems, as well as assistance in developing the systems themselves.
The complex relationships among these organizational units and their constituents requires open communication to establish priorities and methods consistent with the needs and resources of each group. The conflicting desire for more detailed information must be balanced against the associated burden and cost, as well as against the utility of collecting extensive amounts of data. For example, electronic systems that may facilitate higher quality, more complete, and more timely data also involve the commitment of equipment, training, and changes in day-to-day activities that may permeate all levels of the system. One must understand the needs of each recipient group for the information and assess and assure their commitment to the system. It is also critical to be attentive to how components of the system can best be integrated into the overall system in terms of day-to-day operations.
The success of surveillance depends heavily on the quality of the information entered into the system and on the value of the information to its intended users. A clear understanding of how policy makers, voluntary and professional groups, researchers, and others might use surveillance data is valuable in garnering the support of these audiences for the surveillance system.
The future of Public Health Surveillance
As we approach the year 2000, several activities are expected to contribute to the evolution of public health surveillance. First, use of the computer—particularly the microcomputer—has revolutionized the practice of public health surveillance. We can anticipate computer networks that will link local and provincial health departments with national and international officials. These networks will facilitate not only the sharing of data, but graphical analyses. The Minitel system currently in use in France has already demonstrated the utility of office-based surveillance of various conditions of public health importance17.
The second area of renewal activity associated with surveillance is that of epidemiologic and statistical analysis. A by-product of the use of computers is the ability to make more effective use of sophisticated tools to detect changes in patterns of occurrence of health problems. In the 1980s, applications and methods of time series analysis and other techniques have enabled us to provide meaningful interpretation of data collected in surveillance efforts18. More sophisticated techniques will doubtless continue to be applied in the area of public health as they are developed.
Until recently, surveillance data were usually disseminated as written documents published periodically by government agencies. While paper reports will continue to be produced, and public health officials will continue to refine the use of print media, they are also beginning to use electronic media for the dissemination of surveillance data. More effective use of the electronic media and all the other tools of communications should facilitate the use of surveillance data for public health practice. However, ready access to detailed information on individuals will continue to cause ethical and legal concerns that my constrain access to data of potential public health importance.
Also the development of methods for chronic disease, environmental and occupational health, and injury control will continue to be a major challenge in public health.
A more fundamental principle that will underlie the ongoing development of surveillance is the increasing ability of people to look at public health surveillance as a scientific endeavor19. A growing appreciation of the need for rigor in surveillance practice will improve the quality of surveillance programs and will therefore facilitate the analysis and use of surveillance data. This more vigorous approach to surveillance practice will result in the increased frequency and quality of the evaluation of the practice of surveillance12.
Finally, and probably most important, surveillance must be used more consistently and thoughtfully by policy makers. Epidemiologists not only need to improve the quality of their analysis, interpretation, and display of data for public health use, they also need to listen to persons empowered to set policy in order to understand what stimulates the policy markers’ interest and action. This assessment allows surveillance information to be crafted so that it is presented in its most useful form to the appropriate audience and in the necessary time frame. In turn, as we maximize the utility of data for decision making and better understand what is essential to that process, we will elevate public health surveillance to a new and higher level of importance.
The critical challenge in public health surveillance today, however, continues to be the assurance of its usefulness. To do this properly, the principles of surveillance and its role in guiding epidemiologic research and influencing other aspects of the overall mission of public health must be fully understood. Analytic methods for public health surveillance must be developed; computer technology for efficient data collection, analysis, and graphic display must be applied; ethical and legal concerns must be addressed effectively; the use of surveillance systems must be reassessed on a routine basis; and surveillance principles must be applied to emerging areas of public health practice.
References
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