Abstract

In October 2009, a new genogroup II, type 4 (GII.4) norovirus variant was identified in the United States. We collected norovirus outbreak data from 30 states to assess whether this new strain was associated with increased acute gastroenteritis activity. No increase in norovirus outbreaks was observed during the 2009–2010 winter.

Noroviruses are the most common cause of epidemic gastroenteritis worldwide, accounting for >90% of viral gastroenteritis outbreaks and ∼50% of all-cause gastroenteritis outbreaks annually [1]. In the past, periodic increases in norovirus outbreak activity have been associated with the emergence of new strains, probably owing to evasion of population immunity [2–4]. Since 1996, these increases have occurred approximately every 2–4 years, with the last occurring in 2006–2007. In October 2009, a new norovirus variant of genogroup II, type 4 (GII.4), named GII.4 New Orleans, was detected in the United States by a national laboratory-based norovirus strain surveillance system, CaliciNet [5]. This prompted concern that more norovirus outbreaks could occur nationwide compared with previous years. At approximately the same time, the Centers for Disease Control and Prevention (CDC) began to receive reports of a perceived increase in norovirus activity in several states.

Our aim was to examine trends in norovirus outbreak activity in the United States systematically from January 2007 through April 2010 to assess whether the 2009–2010 winter norovirus season was greater in magnitude than previous seasons. We also evaluated the routes of transmission, setting, and seasonality of norovirus outbreaks in the United States.

METHODS

Because no comprehensive, national surveillance data existed for norovirus outbreaks caused by all modes of transmission (ie, person-to-person, foodborne, and waterborne) for our period of interest, we queried state and territorial public health departments for acute gastroenteritis and norovirus outbreak data for the study period from January 2007 through April 2010. An acute gastroenteritis outbreak was defined as ≥2 cases of vomiting and/or diarrhea associated with a common exposure. A suspected norovirus outbreak was defined as an acute gastroenteritis outbreak with norovirus as the suspected cause, based on clinical and epidemiologic features as determined by the state or territorial public health department. For laboratory-confirmed outbreaks, norovirus was confirmed in stool samples by reverse transcription-polymerase chain reaction. Depending on the reporting state or territorial public health department, an outbreak was considered to be confirmed as norovirus if at least 1 or 2 stool samples were norovirus positive. Norovirus strain data were obtained from CaliciNet and other relevant studies published in the literature [3, 5].

We examined temporal trends in the numbers of reported acute gastroenteritis and norovirus (both suspected and confirmed) outbreaks. We calculated rates of suspected and confirmed norovirus outbreaks using the National Center for Health Statistics’ bridged-race 2008 postcensal population estimates for all states included in the analysis as the denominator [6] and compared aggregate and state-specific rates by annual periods from 1 May through 30 April and by norovirus season (defined as 1 November through 30 April). We determined the proportion of acute gastroenteritis outbreaks due to suspected or laboratory-confirmed norovirus and also specifically examined the proportion of suspected and confirmed norovirus outbreaks that occurred primarily through person-to-person transmission or occurred in long-term care facilities.

RESULTS

Thirty state public health departments provided outbreak data for the requested study period. Of these 30 departments, 6 were located in the Northeast US census region, 7 in the Midwest, 9 in the South, and 8 in the West. For states and US territories that were unable to provide outbreak data, reasons included lack of norovirus outbreak surveillance, lack of nonfood-borne enteric disease outbreak data, and incomplete data for the time period requested.

From January 2007 through April 2010, the 30 states included in the analysis reported a total of 7734 acute gastroenteritis outbreaks, of which 5737 (74%) were suspected or confirmed to be due to norovirus (Table 1). Of these 5737 outbreaks, 2866 (50%) were laboratory confirmed as norovirus. Also, 3749 (65%) occurred in long-term care facilities. Among 29 states that provided data for outbreaks primarily due to person-to-person transmission, 3733 (79%) of 4742 suspected or confirmed norovirus outbreaks resulted primarily from person-to-person transmission.

Table 1.

Characteristics of Acute Gastroenteritis (AGE) and Suspected Norovirus Outbreaks Reported by 30 States, January 2007–April 2010

Outbreak description Outbreaks in all 30 states, no. (% of total) State-specific outbreaks, range, no. (% of total) 
All AGE outbreaks 7734 (100) 17–1032 
Suspected norovirus outbreaks 5737 (74) 7–995 (23–100) 
    Laboratory confirmed 2866 (50) 5–454 (8–100) 
    Occurring in long-term care facilities 3749 (65) 3–874 (12–94) 
    Primarily person-to-person transmissiona 3733 (79) 6–646 (28–100) 
Norovirus not suspected 1997 (26) 0–354 (0–77) 
Outbreak description Outbreaks in all 30 states, no. (% of total) State-specific outbreaks, range, no. (% of total) 
All AGE outbreaks 7734 (100) 17–1032 
Suspected norovirus outbreaks 5737 (74) 7–995 (23–100) 
    Laboratory confirmed 2866 (50) 5–454 (8–100) 
    Occurring in long-term care facilities 3749 (65) 3–874 (12–94) 
    Primarily person-to-person transmissiona 3733 (79) 6–646 (28–100) 
Norovirus not suspected 1997 (26) 0–354 (0–77) 
a

One state was excluded because of a lack of reporting for outbreaks primarily due to person-to-person transmission; a total of 4742 suspected norovirus outbreaks were reported by the 29 states included.

Similar seasonality was observed for acute gastroenteritis and suspected and confirmed norovirus outbreaks, with distinct winter-spring peaks (Figure 1); ∼82% of all suspected and confirmed norovirus outbreaks occurred between November and April, inclusive.

Figure 1.

Total numbers of nonnorovirus acute gastroenteritis (AGE) and suspected and laboratory-confirmed norovirus outbreaks reported by 30 states, from January 2007 through April 2010. GII4, genotype II, type 4.

Figure 1.

Total numbers of nonnorovirus acute gastroenteritis (AGE) and suspected and laboratory-confirmed norovirus outbreaks reported by 30 states, from January 2007 through April 2010. GII4, genotype II, type 4.

The number of suspected and confirmed norovirus outbreaks that occurred from 1 May 2009 through 30 April 2010 (n = 1241) was comparable to the number that occurred during the same May–April annual period in 2007–2008 (n = 1258) and 2008–2009 (n = 1999) (Table 2). Similarly, looking at the number of suspected and confirmed norovirus outbreaks by norovirus seasons from 1 November through 30 April, the number of outbreaks that occurred during 2009–2010 (n = 1079) was comparable to the number that occurred during 2007–2008 (n = 998) and 2008–2009 (n = 1595) (Table 2). The number of outbreaks in early 2007 during the 2006–2007 season, the first season after the emergence of GII.4 Minerva, was larger than the numbers for the same period in other surveyed years, including 2010 (Figure 1). Nine states (30%) reported increased outbreak activity in 2009–2010 compared with 2007–2008 and 2008–2009, 20 states reported decreased outbreak activity, and 1 state reported no change.

Table 2.

Population-Based Rates and Numbers of Suspected and Confirmed Norovirus Outbreaks Reported by 30 States, by Year and Norovirus Season from 2007–2008 through 2009–2010

Time period Rate for all 30 states (no. of outbreaks) State-specific rate, median (range) 
Entire year (May–April)   
    2007–2008 8.8 (1258) 8.5 (0–34.8) 
    2008–2009 14.0 (1999) 12.6 (0.4–57.7) 
    2009–2010 8.7 (1241) 8.0 (1.1–26.3) 
Norovirus season (November–April)  
    2007–2008 7.0 (998) 6.3 (0–28.0) 
    2008–2009 11.2 (1595) 9.4 (0.4–52.4) 
    2009–2010 7.6 (1079) 6.5 (1.1–20.7) 
Time period Rate for all 30 states (no. of outbreaks) State-specific rate, median (range) 
Entire year (May–April)   
    2007–2008 8.8 (1258) 8.5 (0–34.8) 
    2008–2009 14.0 (1999) 12.6 (0.4–57.7) 
    2009–2010 8.7 (1241) 8.0 (1.1–26.3) 
Norovirus season (November–April)  
    2007–2008 7.0 (998) 6.3 (0–28.0) 
    2008–2009 11.2 (1595) 9.4 (0.4–52.4) 
    2009–2010 7.6 (1079) 6.5 (1.1–20.7) 

NOTE. Rates represent outbreaks per 1000000 population, based on 2008 census data.

DISCUSSION

Our findings suggest that the emergence of the novel GII.4 New Orleans variant in October 2009 was not associated with an increase in the overall number or rate of norovirus outbreaks in the United States, as had been observed in 2006–2007 with the emergence of the GII.4 Minerva variant [3]. Thus, a widespread increase in disease activity in a presumably immunologically naive population did not occur. The reasons for this are unclear, although factors including the relative pathogenicity and transmissibility of this variant and the degree of cross-protection conferred by previous norovirus exposure, as well as innate host factors, may all play a role in determining the fitness of a new variant [7, 8].

Our findings also highlight the value of national enteric disease outbreak surveillance that covers all modes of disease transmission. For ∼4 decades, the CDC has coordinated national surveillance for foodborne and waterborne enteric disease outbreaks [9, 10]. However, as observed in our study, norovirus accounts for the majority of acute gastroenteritis outbreaks in the United States, with person-to-person transmission reported in more than three-fourths of these outbreaks [1, 11, 12]. National surveillance for enteric disease outbreaks primarily due to person-to-person transmission only began in February 2009 through the National Outbreak Reporting System (NORS) [2]. Moving forward, NORS data can be used to monitor trends in norovirus outbreaks due to all modes of transmission, identify priority settings and populations for interventions (such as long-term care facilities), and more accurately estimate the burden of epidemic norovirus disease.

A new national laboratory-based surveillance system for detection of norovirus outbreak strains, CaliciNet, has also been implemented recently [5]. In this system, state and public health laboratories electronically report to CDC the nucleic acid sequences of norovirus strains identified in gastroenteritis outbreaks. In fact, CaliciNet data first identified the emergence of the new GII.4 New Orleans variant in 2009, prompting this investigation. In the future, laboratory data from CaliciNet will be linked to clinical and epidemiologic information from NORS, allowing timely investigation of norovirus outbreaks and virologic trends.

This study had some limitations. First, our results are not from nationwide surveillance but from states that included 39% of the total US population. Second, criteria for suspected and confirmed norovirus outbreaks and reporting practices for outbreaks were not standardized across states. Thus, our aggregate analysis of outbreak trends was subject to reporting practices of individual states that may have varied over time owing to factors such as availability of resources, staff, and time dedicated to enteric disease outbreak surveillance. However, guidance for confirmation of norovirus outbreaks has been recently been published by the CDC [2].

In conclusion, the emergence of a new GII.4 variant in October 2009 does not appear to have caused an increase in overall norovirus outbreak activity in the United States. However, norovirus outbreaks continue to account for the majority of acute gastroenteritis outbreaks, with person-to-person transmission implicated in most outbreaks and two-thirds of outbreaks occurring in long-term care facilities. Together, these findings highlight the importance of comprehensive gastroenteritis surveillance in assessing the true burden of norovirus disease.

These findings were based on data contributed by participating state health partners. We are extremely grateful for their collaboration on this project.

The findings and conclusions of this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.

Potential conflicts of interest. All authors: No reported conflicts.

All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed in the Acknowledgments section.

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