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BACKGROUND: Noroviruses are the leading cause of epidemic gastroenteritis in the United States; most outbreaks are caused by GII.4 strains. Over the past decade, new GII.4 strains have emerged every 2–3 years and have sometimes led to increased outbreak activity. In 2012, global detection of a new norovirus strain, GII.4 Sydney, raised concerns about its potential impact.
METHODS: We analyzed norovirus outbreak data reported in near-real time by a network of five sentinel states (Minnesota, Ohio, Oregon, Tennessee, and Wisconsin) during the 2012–2013 seasonal year (August–July) and compared them with those of the two preceding seasonal years. Data on general outbreak characteristics, patient demographics, and clinical outcomes were collected through the National Outbreak Reporting System (NORS), while molecular typing data were collected through CaliciNet.
RESULTS: During August 2012–July 2013, 700 norovirus outbreaks were reported compared with 586 and 476 during the same period in 2011–2012 and 2010–2011, respectively. Oregon, Tennessee, and Ohio reported an increased number of outbreaks in 2012–2013, while Minnesota and Wisconsin reported a decrease. Of 403 outbreaks with sequence data, 249 (63%) were caused by GII.4 Sydney. There was no significant difference in the proportion of outbreaks caused by GII.4 Sydney between the states with an increased number of outbreaks in 2012–2013 compared with those without such an increase (61% versus 64%; p=0.55). GII.4 Sydney outbreaks were reported more frequently in healthcare settings than did non-GII.4 Sydney outbreaks (76% vs. 44%, respectively; rate ratio [RR] 1.75; 95% confidence interval [CI] 1.44–2.12). A higher proportion of cases in GII.4 Sydney outbreaks compared with non-GII.4 Sydney outbreaks required emergency department visits (3% versus 2%; RR 1.76; 95% CI 1.13–2.73) and hospitalization (2% versus 1%; RR 2.23; 95% CI 1.45–3.43).
CONCLUSIONS: GII.4 Sydney rapidly emerged during the 2012–2013 season to become the predominant norovirus outbreak strain in the United States; however, its emergence does not appear to have caused a substantial increase in outbreak activity compared with previous seasons. Further analysis of norovirus outbreak surveillance data and clinical data on endemic disease can help verify the findings of this preliminary assessment. Proper hand hygiene, environmental disinfection, and isolation of ill persons remain the mainstays of norovirus prevention and control.