How the United States counts a norovirus season

Norovirus outbreaks in the United States are tracked by NoroSTAT, which
combines a subset of two systems — the **National Outbreak Reporting System
(NORS)** and CaliciNet — and is updated monthly.

Twelve state health departments participate: Colorado, Michigan,
Minnesota, Nebraska, New Hampshire, North Carolina, Ohio, Oregon, South
Carolina, Tennessee, Virginia and Wisconsin. They report suspected and
confirmed outbreaks to the CDC within 7 business days, counted by the week
illness began.

How you tell a bad season from a normal one

The count on its own means nothing — outbreaks are seasonal and vary every
year. NoroSTAT plots the current season against **every season from 2012–13
onward**, using four reference lines:

**A season whose line consistently falls outside the blue area — especially
during the December-to-March peak — may indicate unusually high or low
norovirus activity.**

That is the whole method. "A bad norovirus year" is not a number of
outbreaks; it is a line sitting outside the middle half of fourteen previous
years for several weeks running.

The current picture

Between 1 August and 4 September 2026, participating states reported **27
norovirus outbreaks**. In the same period a year earlier there were 29.

The total for the 2026–27 seasonal year so far sits within the middle 50%
of the same period across the 2012–2026 seasons — an ordinary season, by the
test above.

For the genotypes currently circulating, the data reported to CaliciNet is the
place to look. A seasonal year here runs August through July, not January
to December, because that is how a norovirus season runs.

Source: Centers for Disease Control and Prevention.