Timely ascertainment of syphilis cases is critical to initiating disease-control measures. Epidemic curves typically use the report date and may introduce lag-time bias into assessment.
To reassess a large syphilis epidemic using an imputed infection date.
We compared 2 types of epidemic curves—1 based on report date and 1 on estimated infection date—using the large 1993–2003 Baltimore epidemic as our model.
In general, the shape of the report curves did not accurately reflect the shape of the corresponding infection curves during the growth period (period of largest increase in incidence); during the hyperendemic period (period of highest incidence), peaks in report curves did not follow peaks in the infection curve by the appropriate lag time. There was a tendency for reporting data to underestimate infections during the growth period and overestimate infections during the hyperendemic period. A sensitivity analysis showed similar trends regardless of the length of stage-specific incubation period used.
Lag-time bias may be present when using epidemic curves based on report dates. Health departments should consider using an estimated infection date.
Reassessment of a large-scale syphilis epidemic found that epidemic curves based on estimated infection date may better describe epidemics than traditionally derived report date curves.
From the *Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland; †Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, and Baltimore City Health Department, Baltimore, Maryland; and the ‡Johns Hopkins University School of Medicine, Division of Infectious Diseases, Baltimore, Maryland
This work was supported by NIH grants R01-AI45724 and K24-AI01633.
The authors would like to thank Vivian Go, Laura McGough, and Aaron Goodfellow for their insightful comments on and corrections to the manuscript.
Address for reprints: Anne M. Rompalo, MD, ScM, 1830 East Monument Street, Room 447, Baltimore, MD 21205. E-mail: firstname.lastname@example.org.
Received for publication June 4, 2004, and accepted April 4, 2004.