Characterizing Norovirus Transmission from Outbreak Data, United States.

Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States. We estimated the basic (R ₀ ) and effective (R _e ) 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 R ₀ was 2.75 (interquartile range [IQR] 2.38-3.65), and median R _e was 1.29 (IQR 1.12-1.74). Long-term care and assisted living facilities had an R ₀  of 3.35 (95% CI 3.26-3.45), but R ₀ did not differ substantially for outbreaks in other settings, except for outbreaks in schools, colleges, and universities, which had an R ₀  of 2.92 (95% CI 2.82-3.03). Seasonally, R ₀ 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.