Figure 4.

Reasons why defining ‘events’ to be detected by syndromic surveillance are complex.

Reason definition is complex	Example
Little or no historical data may be available	Bioterrorism, newly emerging diseases
Simulated data is sensitive to modelling assumptions	Patients’ health-seeking behaviour is difficult to predict
Event may not be routinely monitored by non-syndromic systems	Seasonal hay fever
Exposure may be clearly defined but impact on public health is still uncertain	Heat waves
Laboratory ‘gold-standard’ for independent verification may not exist	Newly emerging pathogen
Precise start and end date of exposure might be uncertain	Seasonal influenza
Events causing similar symptoms may occur at the same time	Air pollution and seasonal respiratory illness
Control period without events may be unavailable	Syndromic baseline data is rarely zero