A backlog can create a false-looking surge
If several days of reports arrive together, a report-date chart may show one large bar. An onset-date chart may spread those same cases across earlier days.
Both charts can be accurate representations of their date fields, but they answer different questions.
Recent onset counts are often incomplete
People whose symptoms began yesterday may not yet have sought care, been tested, or been reported. The right-hand edge of an onset curve can therefore appear to decline before the trend is known.
Check four features
- Which date is used?
- How wide are the time bins?
- Which case definition and population are included?
- How much reporting delay affects recent periods?
Bin width can reveal or hide patterns. It should fit the disease and question rather than simply make the chart visually smooth.
A shape suggests hypotheses
A sharp peak or repeated waves can guide investigation, but the curve alone does not prove a transmission route or source. Exposure histories, laboratory information, and other evidence are needed.
Use the curve to organize time and generate questions. Avoid treating its newest bar as complete or its visual shape as a substitute for an outbreak investigation.
