HealthDay News — A novel norovirus strain, GII.4 Sydney, first detected in Australia last year, was responsible for the majority of contagious intestinal illness in the United States from September through December 2012, the CDC reports.

During that time period, the CDC received 266 reports of norovirus via CaliciNet, a national electronic laboratory surveillance network. More than half, 53%, were caused by the GII.4 Sydney strain.

The proportion of reported outbreaks caused by this strain increased dramatically from 19% in September to 58% in December, replacing the previously predominant GII.4 strain as the leading cause of norovirus. 

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Norovirus causes gastrointestinal illness, with symptoms including diarrhea, abdominal pain, vomiting, headache, fever and chills. Compared with other norovirus genotypes, GII.4 noroviruses have been associated with increased rates of hospitalizations and deaths during outbreaks, according to the FDA.

Health care providers and public health practitioners should remain vigilant to the potential for increased norovirus activity in the ongoing season related to the emergent GII.4 Sydney strain, the agency advises. A change in the predominant strain of a circulating norovirus can sometimes lead to an increase in illness.

“Right now, it’s too soon to tell whether the new strain of norovirus will lead to more outbreaks than in previous years,” Aron Hall, DVM, MSPH, DACVPM, an epidemiologist in the CDC’s Viral Gastroenteritis Team said in a press release. “However, CDC continues to work with state partners to watch this closely and see if the strain is associated with more severe illness.”


  1. Centers for Disease Control and Prevention. MMWR Morb Mortal Wkly Rep. 2013 Jan 25;62:55.