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Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States. We estimated the basic (R0) and effective (Re) reproduction numbers for 7,094 norovirus outbreaks reported to the National Outbreak Reporting System (NORS) during 2009-2017 and used regression models to assess whether transmission varied by outbreak setting. The median R0 was 2.75 (interquartile range [IQR] 2.38-3.65), and median Re was 1.29 (IQR 1.12-1.74). Long-term care and assisted living facilities had an R0 of 3.35 (95% CI 3.26-3.45), but R0 did not differ substantially for outbreaks in other settings, except for outbreaks in schools, colleges, and universities, which had an R0 of 2.92 (95% CI 2.82-3.03). Seasonally, R0 was lowest (3.11 [95% CI 2.97-3.25]) in summer and peaked in fall and winter. Overall, we saw little variability in transmission across different outbreaks settings in the United States. [ABSTRACT FROM AUTHOR]

Although recognized as the leading cause of epidemic acute gastroenteritis across all age groups, norovirus has remained poorly characterized with respect to its endemic disease incidence. Use of different methods, including attributable proportion extrapolation, population-based surveillance, and indirect modeling, in several recent studies has considerably improved norovirus disease incidence estimates for the United States. Norovirus causes an average of 570--800 deaths, 56,000--71,000 hospitalizations, 400,000 emergency department visits, 1.7--1.9 million outpatient visits, and 19--21 million total illnesses per year. Persons ≥65 years of age are at greatest risk for norovirus-associated death, and children <5 years of age have the highest rates of norovirus-associated medical care visits. Endemic norovirus disease occurs year round but exhibits a pronounced winter peak and increases by ≤50% during years in which pandemic strains emerge. These findings support continued development and targeting of appropriate interventions, including vaccines, for norovirus disease. [ABSTRACT FROM AUTHOR]

The duration of immunity to norovirus (NoV) gastroenteritis has been believed to be from 6 months to 2 years. However, several observations are inconsistent with this short period. To gain better estimates of the duration of immunity to NoV, we developed a mathematical model of community NoV transmission. The model was parameterized from the literature and also fit to age-specific incidence data from England and Wales by using maximum likelihood. We developed several scenarios to determine the effect of unknowns regarding transmission and immunity on estimates of the duration of immunity. In the various models, duration of immunity to NoV gastroenteritis was estimated at 4.1 (95% CI 3.2-5.1) to 8.7 (95% CI 6.8-11.3) years. Moreover, we calculated that children (<5 years) are much more infectious than older children and adults. If a vaccine can achieve protection for duration of natural immunity indicated by our results, its potential health and economic benefits could be substantial. [ABSTRACT FROM AUTHOR]

The article focuses on a study which described foodborne norovirus outbreaks in the U.S. during the 2001-08 period. A definition of a foodborne disease is provided, along with the scope of the data collected for each outbreak. The categories of food preparation settings in outbreaks are commercial, institutional, private and other. Results of the study showed that norovirus was the confirmed or suspected cause in 46% of 6,355 foodborne outbreaks. It also found that outbreaks were more frequent during the winter season than the rest of the year. It cites food handling, consumption of contaminated raw food and inadequate cooking as factors that contributed to contamination.

The article discusses the emergence of the GII.4 Sydney norovirus strain in the U.S. in 2012. The GII.4 strain has caused 53% or 141 of the 266 norovirus outbreaks reported through the CaliciNet during September-December. Majority of the GII.4 outbreaks resulted from direct person-to-person transmission, 29 were foodborne, and one was waterborne.