Characterizing Norovirus Transmission from Outbreak Data, United States.

Molly K Steele,Katia Koelle,Lance A Waller,Ben A Lopman,Andreas Handel,Aron J Hall,Mary E Wikswo,Karen Levy

doi:10.3201/eid2608.191537

Abstract

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.

Highlights

Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States
National Outbreak Reporting System (NORS) data consist of web-based reports of all foodborne, waterborne, and enteric disease outbreaks transmitted by contact with environmental sources, infected persons or animals, or unknown modes of transmission reported by state, local, and territorial public health agencies
CaliciNet data consists of sequence-derived genotypes and epidemiologic data from norovirus outbreaks submitted from local, state, and federal public health laboratories

Summary

Introduction

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 effective reproduction number (Re) quantifies the average number of secondary cases that arise from a primary case in a population that is not completely susceptible. Several transmission modeling studies in different settings have estimated R0 and Re of norovirus, but a large variation in these estimates occurs and R0 ranges from 1.1–7.2 [10]. The variability of estimates from models that use outbreak data likely are driven by context; outbreaks might occur in populations that are not representative of the population as a whole and transmission likely is higher in these settings than in the community [4]. We assessed whether norovirus was suspected or confirmed as the cause of the outbreak

Methods

Results

Conclusion