Tamil Nadu: 15 chickenpox outbreaks in the 4 weeks to week 9 of 2018, against a median of 2 for the same weeks in 2015–17.

A single week of outbreak reports holds only a handful of events, too few to compare. So the observatory adds up the four weeks to this one and compares the total with the median of the same four weeks in up to three earlier years; the baseline is short because reporting itself has grown. Only rises are flagged, and only when the total is at least 5 above that median, because a fall in reported outbreaks mostly means that reports did not arrive. These are outbreaks reported to IDSP, so they reflect reporting as well as disease.

For chickenpox outbreaks, neither a rise nor a fall is better in itself, so the observatory marks this movement as unusual rather than good or bad.

Same four weeks: 2017 2, 2016 5, 2015 2. The preceding four weeks: 5. Counts are outbreaks reported to IDSP, which reflect reporting as well as disease.

Full history of chickenpox outbreaks in Tamil Nadu