Epidemic curve

An epidemic curve is a histogram of cases against a chosen event time, commonly symptom onset. Its axes must specify the case definition, event date and bin width. It helps compare an observed outbreak with common-source outbreak and propagated outbreak mechanisms.^[ref]

Counts per bin differ from an incidence rate: a rate additionally needs the population or person-time at risk. Replacing daily bins by weekly bins also changes bar heights and can hide short peaks.

The displayed timing is delayed from infection. In a model with completely observed symptomatic infections, infection intensity $\lambda (t)$ and an independent, stationary incubation period density $f(u)$, the expected symptom-onset intensity isThis convolution follows by adding infections at each earlier time weighted by their onset-delay probability. Reporting delay would introduce another observation process. A peak in recorded reports therefore need not coincide with a peak in infections; reconstructing transmission requires the relevant delay and ascertainment model.