Near Real-Time Surveillance of U.S. Norovirus Outbreaks by the Norovirus Sentinel Testing and Tracking Network — United States, August 2009–July 2015

Norovirus is the leading cause of endemic and epidemic acute gastroenteritis in the United States (1). New variant strains of norovirus GII.4 emerge every 2-4 years (2-4) and are often associated with increased disease and health care visits (5-7). Since 2009, CDC has obtained epidemiologic data on norovirus outbreaks from state health departments through the National Outbreak Reporting System (NORS) (8) and laboratory data through CaliciNet (9). NORS is a web-based platform for reporting waterborne, foodborne, and enteric disease outbreaks of all etiologies, including norovirus, to CDC. CaliciNet, a nationwide electronic surveillance system of local and state public health and regulatory agency laboratories, collects genetic sequences of norovirus strains associated with gastroenteritis outbreaks. Because these two independent reporting systems contain complementary data, integration of NORS and CaliciNet records could provide valuable public health information about norovirus outbreaks. However, reporting lags and inconsistent identification codes in NORS and CaliciNet records have been an obstacle to developing an integrated surveillance system.

CaliciNet by U.S. Census intercensal yearly population estimates (10), and were expressed as outbreaks per 1,000,000 person-years. Report timeliness for NORS was assessed by determining the interval in days between the first report of an outbreak to a state health department and the date a NORS report was submitted. Report timeliness for CaliciNet was assessed as the difference in days between the date of receipt of a stool specimen at a public health laboratory and the date a CaliciNet report was submitted. The proportion of reports submitted within 7 business days was evaluated.
A complete NORS report was defined as a report that contained complete information for all fields required for NoroSTAT participation: date the first person in the outbreak became ill, primary transmission mode, exposure setting, and the number of ill persons. Linking ability was defined as the ability to link CaliciNet reports manually with a NORS report by user-submitted identification variables (automated linking was not possible during the period of analysis). The proportion of linked reports was assessed for NoroSTAT states and for other states. Norovirus genotypes reported to CaliciNet by NoroSTAT states and by other states were evaluated pre-and post-implementation of NoroSTAT. The primary transmission modes and outbreak settings reported to NORS by states participating in NoroSTAT were compared with other states and were similarly evaluated pre-and post-NoroSTAT implementation. CaliciNet-NORS linked reports were further analyzed to determine the distribution of reported transmission modes, outbreak settings, and norovirus genotypes from each reporting system. All analyses were completed using statistical software. Wilcoxon's signed-rank test was used for numerical comparisons of reporting lag and outbreak size, and chi-square tests or Fisher's exact tests were used for categorical comparisons, with a p-value <0.05 considered statistically significant.
The median outbreak reporting rates to NORS from NoroSTAT states before and after NoroSTAT introduction were similar (17.3 and 21.0 per 1,000,000 person-years, respectively) ( Table 1). The median reporting rate to NORS from non-NoroSTAT states increased, from 3.0 per 1,000,000 person-years (pre-NoroSTAT) to 4.1 (post-NoroSTAT). The median reporting rate to CaliciNet from NoroSTAT states increased significantly from 4.9 (pre-NoroSTAT) to 9.0 (post-NoroSTAT). The median reporting rate to CaliciNet from non-NoroSTAT states was similar pre-(2.6) and post-NoroSTAT (2.1).
The median reporting interval to NORS significantly declined in NoroSTAT states from 22 to 2 days and to CaliciNet from 21 to 3 days (Table 1). No change in reporting interval to NORS occurred in non-NoroSTAT states (median = 26 days pre-and post-NoroSTAT); however, there was a significant decline in the median reporting interval to CaliciNet from 21 to 11 days. The percentage of NORS reports submitted within 7 business days increased significantly, from 26% to 95% among NoroSTAT states, and increased marginally in non-NoroSTAT states (from 12% to 13%). The percentage of CaliciNet reports submitted within 7 business days increased from 29% to 95% among NoroSTAT states; a more modest, but still significant increase (10% to 30%) occurred among non-NoroSTAT states.
NORS reports with all NoroSTAT-required fields completed increased significantly, from 87% to 99.9% among NoroSTAT states, and from 43% to 64% in non-NoroSTAT states. The percentage of CaliciNet reports that were linkable to NORS reports also increased significantly among both NoroSTAT states (from 86% to 95%) and all other states (from 29% to 33%). Over the entire 6-year period, 2,861 CaliciNet reports linked to NORS reports across all states, providing more complete data on the outbreaks than either system alone. The NORS reports provided the transmission mode for all 1,106 (100%) linked CaliciNet records without an identified transmission mode, and the outbreak setting for 669 (60%) CaliciNet records without an identified setting. In addition, CaliciNet reports provided the genotype for all 719 linked NORS records without a reported genotype.
The genotypes of norovirus outbreaks reported to CaliciNet by NoroSTAT states were similar to those reported from non-NoroSTAT states ( Table 2). In the pre-NoroSTAT period, GII.4 New Orleans was the dominant genotype in both NoroSTAT and non-NoroSTAT states, accounting for 54% and 65% of outbreaks in these states, respectively. The dominant genotype shifted to GII.4 Sydney in the post-NoroSTAT period in both NoroSTAT and non-NoroSTAT states, accounting for 61% and 68% of outbreaks, respectively. Similarly, the transmission modes and outbreak settings reported to NORS by NoroSTAT states were representative of national data, both pre-and post-NoroSTAT implementation ( Table 2). Personto-person transmission was the predominant transmission mode, and long-term care facilities were the most common outbreak setting.

Discussion
Substantial improvements in norovirus outbreak reporting, measured by the volume, timeliness, and completeness of epidemiologic and laboratory reports have been observed since the introduction of NoroSTAT in participating states, likely because of stringent reporting requirements and enhanced communication between epidemiologists and laboratorians in both state health departments and at CDC. NoroSTAT participating states are providing near real-time norovirus outbreak surveillance data to CDC, with 95% of NORS and CaliciNet reports submitted by these states within 7 business days. States not participating in the NoroSTAT network saw more modest improvements in report timeliness and completeness, likely because of general improvements in the report submission user interface, and increased engagement of state health departments and laboratories with annual meetings, workshops, and newsletters.
Ninety-five percent of CaliciNet reports submitted by NoroSTAT states were linked to NORS reports, fostering better integration and coordination of epidemiologic and laboratory data. These linked reports provide more complete and accurate reporting of norovirus outbreaks, and their value is illustrated by the large proportion of reports for which data from one system supplement those from the other. Report linkages were completed manually and retrospectively during the period of this analysis; automated, prospective linking is currently being implemented.
The findings in this report are subject to at least two limitations. First, both NORS and CaliciNet collect data on norovirus outbreaks; because norovirus outbreaks are defined as two or more cases of illness with a common exposure, these results might not be generalizable to endemic norovirus illnesses. Second, variations in reporting practices among both NoroSTAT and non-NoroSTAT states can affect the quality and internal comparability of surveillance data. Despite these limitations, the NoroSTAT network was shown to be valuable in early identification and better characterization of norovirus outbreaks across the country. Near real-time surveillance data from NoroSTAT improved public health response and preparedness when the GII.4 Sydney variant emerged during 2012-2013. Early reporting of data from NoroSTAT states allowed a timely assessment showing no increase in norovirus outbreak activity in the United States associated with emergence of the GII.4 Sydney variant, in contrast to data from other countries (4). Similar strain-specific attribution analyses can be useful to rapidly detect the impact of the emergence of novel norovirus strains every few years.
The NoroSTAT network has built upon the initial success demonstrated by the first five states, expanding to seven states in August 2015 and to nine states in August 2016. The more rapid, complete, and integrated reporting by NoroSTAT-participating states demonstrates a key advancement in norovirus outbreak surveillance, providing near real-time monitoring of norovirus outbreak activity and emerging new strains.

Summary
What is already known about this topic?
Norovirus is the most common cause of acute gastroenteritis in the United States. Norovirus outbreaks are reported to CDC by state and territorial health departments. Reporting lags and incomplete reporting have been limitations to norovirus outbreak surveillance systems.
What is added by this report?
The initial five sentinel states that participated in the NoroSTAT network (Minnesota, Ohio, Oregon, Tennessee, and Wisconsin) during the first 3 years reduced the median reporting interval from 22 days to 2 days for epidemiologic data, and from 21 days to 3 days for laboratory data. These states also had more complete reports that better linked epidemiologic and laboratory data.
What are the implications for public health practice?
The NoroSTAT network provides near real-time surveillance of norovirus outbreak activity and emerging new strains. Data collected by NoroSTAT-participating states are representative of national trends and can help inform public health response.