3 Epidemiologic Surveillance and Epidemic Outbreak Investigation
I Surveillance of Disease
B Creating a Surveillance System
These objectives and methods should be developed with the aid of the investigators charged with collecting, reporting, and using the data. A pilot test should be performed and evaluated in the field, perhaps in one or more demonstration areas, before the full system is attempted. When it is operational, the full system also should be continually evaluated. The CDC has extensive information on surveillance at its website, www.cdc.gov.
C Methods and Functions of Disease Surveillance
The percentage of patients with reportable diseases that are actually reported to public health authorities varies considerably.1 One group estimated that the percentage reported to state-based passive reporting systems in the United States varied from 30% to 62% of cases.
Sometimes a change in medical care practice uncovers a previously invisible disease surveillance issue. For example, a hospital in Connecticut began reporting many cases of pharyngeal gonorrhea in young children. This apparently localized outbreak in one hospital was investigated by a rapid response team, who discovered that the cases began to appear only after the hospital started examining all throat cultures in children for gonococci and for beta-hemolytic streptococci.2
2 Evaluation of Time Trends
Secular (Long-term) Trends
Seasonal Variation

Figure 3-3 Incidence rates of varicella (chickenpox) in the United States, by month of report, 1986-1992.
(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:53, 1992.)
Figure 3-5 shows a late-summer peak for aseptic meningitis, which is usually caused by viral infection spread by the fecal-oral route or by insects. Figure 3-6 shows a pattern that is similar but has sharper and narrower peaks in late summer and early autumn. It describes a known arthropod-borne viral infection caused by California-serogroup viruses of the central nervous system.

Figure 3-5 Incidence rates of aseptic meningitis in the United States, by month of report, 1986-1992.
(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:20, 1992.)
3 Identification and Documentation of Outbreaks

Figure 3-8 Epidemic threshold, seasonal baseline, and actual proportion of deaths caused by pneumonia and influenza in 122 U.S. cities, 1994-2000.
The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from pneumonia and influenza since 1983.
(Data from Centers for Disease Control and Prevention: Update: influenza activity—United States and worldwide, 1999-2000. MMWR 49:174, 2000.)
Surveillance for Bioterrorism
For at least a century, epidemiologists have worried about the use of biologic agents for military or terrorist purposes. The basic principles of disease surveillance are still valid in these domains, but there are special concerns worth mentioning. The most important need is for rapid detection of a problem. With regard to bioterrorism, special surveillance techniques are being developed to enable rapid detection of major increases in the most likely biologic agents3 (Box 3-1). Detection is made more difficult if the disease is scattered over a wide geographic area, as with the anthrax outbreak in the United States after terrorist attacks in late 2001.
A technique developed for more rapid detection of epidemics and possible bioterrorism is syndromic surveillance.3 The goal of this surveillance is to characterize “syndromes” that would be consistent with agents of particular concern and to prime the system to report any such syndromes quickly. Rather than trying to establish a specific diagnosis before sounding an alert, this approach might provide an early warning of a bioterrorism problem.
4 Evaluation of Public Health and Disease Interventions

Figure 3-9 Incidence rates of paralytic poliomyelitis in the United States, by year of report, 1951-1991.
(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1991. MMWR 40:37, 1991.)
Figure 3-9 shows that after the inactivated vaccine was introduced in 1955, the rates of paralytic disease declined quickly. The public tended to think the problem had gone away, and many parents became less concerned about immunizing newborns. Because the inactivated vaccine did not provide herd immunity, however, the unimmunized infants were at great risk. A recurrent poliomyelitis spike occurred in 1958 and 1959, when most of the new cases of paralytic poliomyelitis were in young children who had not been immunized. The rates declined again in 1960 and thereafter because the public was shaken out of its complacency to obtain vaccine and because a newer oral vaccine was introduced. This live, attenuated oral vaccine provided both herd immunity and individual immunity (see Figure 1-2).
A lack of change in disease rates
An increase in disease rates after an initial decrease, as in the previous example of the polio vaccine
An increase in disease rates in a recently vaccinated group, as occurred after the use of defective lots of inactivated polio vaccine in the 1950s.
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