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.