New Strain Causing Most Norovirus Outbreaks in United States

A new strain of norovirus called GII.4 Sydney was the leading cause of norovirus outbreaks in the United States from September to December 2012, according to a study published in Morbidity and Mortality Weekly Report released today by the Centers for Disease Control and Prevention (CDC). The new strain was detected in Australia in March 2012, and caused outbreaks in that country and several other countries.

CDC researchers analyzed 2012 data collected through CaliciNet on norovirus strains associated with outbreaks in the United States. They found that of the 266 norovirus outbreaks reported during the last four months of 2012, 141 were caused by the GII.4 Sydney strain.

The new strain spread rapidly across the United States from September to December 2012, says Dr. Aron Hall, epidemiologist, CDCs Division of Viral Diseases (DVD). The proportion of reported outbreaks caused by this strain increased dramatically from 19 percent in September to 58 percent in December.

Norovirus is very contagious. In the United States, norovirus is the number one cause of acute gastroenteritis, which leads to diarrhea and vomiting. Each year, more than 21 million people in the United States get infected and develop acute gastroenteritis; approximately 800 die. Young children and elderly adults have the highest risk for severe illness.

Norovirus spreads primarily from infected people to others through direct contact. It also spreads through contaminated food, water, and surfaces. Norovirus infections are common during this time of the year. Most outbreaks occur from November to April, and activity usually peaks in January.

New norovirus strains often lead to more outbreaks but not always, says Jan Vinjé, director of CaliciNet. Over the past decade, new strains of GII.4 have emerged about every two to three years. We found that the new GII.4 Sydney strain replaced the previously predominant GII.4 strain.

Better surveillance in the United States and abroad have helped to detect new strains of norovirus sooner. Early identification of new strains helps to alert the public and health professionals to better prevent infections and control outbreaks.

Health professionals should remain vigilant to potential increases in norovirus infection this season due to GII.4 Sydney. They should follow standard prevention and control measures for norovirus. People should know that the best ways to help prevent norovirus infection include washing hands with soap and water, disinfecting surfaces, rinsing fruits and vegetables, cooking shellfish thoroughly, and not preparing food or caring for others while ill.

Right now, its too soon to tell whether the new strain of norovirus will lead to more outbreaks than in previous years. However, CDC continues to work with state partners to watch this closely and see if the strain is associated with more severe illness, says Hall.