CaliciNet: Which Norovirus Strains Caused U.S. Outbreaks, 2024–2026

What CaliciNet is

CaliciNet is CDC's laboratory network for tracking norovirus. Participating public health laboratories submit the genotype of the norovirus from each confirmed outbreak, with basic epidemiologic data, to the CaliciNet database, where the strains are compared with the norovirus sequences already on file. CDC posts the confirmed outbreaks monthly, by state, by genogroup and by genotype.

The strains, year by year

September 1, 2025 – August 31, 2026 (153 outbreaks, as posted in September 2026):

"Other" covers genotypes each causing less than 5% of outbreaks: GI.1[P1] (2.6%), GI.2[P2] (1.3%), GI.3[P3] (3.9%), GI.3[P13] (2.0%), GI.5[P5] (0.7%), GI.6[P11] (2.0%), GI.7[P7] (1.3%), GII.1[P33] (2.6%), GII.2[P2] (0.7%), GII.3[P12] (2.0%), GII.6[P7] (2.0%) and GII.9[P7] (2.6%).

September 1, 2024 – August 31, 2025 (411 outbreaks):

In 2024–2025, GII.17[P17] caused most confirmed outbreaks; in 2025–2026, GII.4 Sydney[P16] was the most common.

Dual typing

CaliciNet laboratories are adopting dual typing: a single RT-PCR reaction that amplifies part of both the polymerase gene and the capsid gene (region B-C) for genogroup I or II noroviruses. More about CaliciNet.

Sources

Based on "CaliciNet Data," Norovirus, Centers for Disease Control and Prevention, as updated September 3, 2026; a work of the United States government in the public domain. The page is updated monthly; the figures here are those posted then.