Epidemic threshold

An epidemic threshold is the level of disease activity that separates the normal, expected baseline from an epidemic — the point at which case counts are high enough to declare that transmission has moved beyond what is usual and to trigger a defined public-health response. Below the threshold, cases are treated as endemic background; above it, agencies escalate to outbreak investigation, resource mobilization, or campaign interventions.

Thresholds are disease- and setting-specific and are usually expressed as a rate over a defined population and time window rather than a raw count. The best-known example is WHO's meningococcal-meningitis alert and epidemic thresholds in the African meningitis belt: an alert threshold of roughly 5 cases per 100,000 population per week, and an epidemic threshold of 10 cases per 100,000 per week, at which reactive vaccination campaigns are launched. Seasonal influenza surveillance uses a different construction — a statistical 'epidemic threshold' set a fixed number of standard deviations above a seasonal baseline curve, so the line moves week to week.

The concept only works where a reliable baseline exists. Setting a threshold requires historical surveillance data to know what 'normally expected' looks like for a given place and season — which is one reason ongoing surveillance is the foundation of outbreak detection. For a disease that is normally absent, the effective threshold is a single case: one case of a disease that should not be there (Ebola outside its endemic range, or polio in a certified polio-free country) is itself the alarm.