Norovirus Not a Winter Virus

Recent nationwide wastewater surveillance reveals an average norovirus trend, with rates rising in nearly 50 areas. In California and the Northeast, WastewaterSCAN recorded an upward trend to high levels of norovirus.

From Aug. 1, 2025, to May 7, the Centers for Disease Control and Prevention’s NoroSTAT program reported 1,194 outbreaks, compared with 2,534 during the same period last year. The CDC said current numbers are average compared with past data. However, the agency’s tracking is based on confirmed cases reported by state agencies and may miss many illnesses.

Several norovirus strains are currently circulating, including GII.4, a common strain for years, and GII.17, a more mutated strain that partially evades prior immunity because people have had less exposure, NBC News reported. During the 2024–25 season, GII.17 surpassed GII.4 as the predominant strain in the U.S., causing about 75% of outbreaks.

The newer variant isn’t more contagious itself, but experts said the virus spreads more easily because fewer people have partial immunity to it.

Most norovirus outbreaks happen when infected people spread the virus to others through direct contact, the CDC said. Norovirus is also the leading cause of acute gastroenteritis-related vomiting and diarrhea among people of all ages in the U.S.

Norovirus outbreaks occur year-round but are most common from November to April, the CDC said. In years when a new strain emerges, norovirus illness can increase by 50%.

Each year in the U.S., norovirus causes, on average, 900 deaths (mostly among adults aged 65 and older), 109,000 hospitalizations, 465,000 emergency department visits (mostly in young children), and 19 to 21 million illnesses.