Reassessing a large-scale syphilis epidemic using an estimated infection date

Objectives: 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. Goal: To reassess a large syphilis epidemic using an imputed infection date. Study: 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. Results: 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. Conclusions: Lag-time bias may be present when using epidemic curves based on report dates. Health departments should consider using an estimated infection date.