Say No to Norovirus

Abstract & Commentary

By Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford, Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.

Source: Johnston CP, et al. Outbreak management and implications of a nosocomial norovirus outbreak. Clin Infect Dis 2007; 45:534-540.

Synopsis: Norovirus outbreaks, many caused by newly emerged strains, are increasing in frequency in healthcare facilities and other settings, are difficult to control and are costly.

In the midst of multiple outbreaks of norovirus infection in Maryland in 2004, infection control personnel at Johns Hopkins Hospital were notified that 2 healthcare workers (HCW) had acute gastroenteritis, resulting in the initiation of active surveillance of gastrointestinal illness among patients and staff. Between January 7th and May 1st, 265 HCW and 90 patients had the new onset of vomiting and/or diarrhea, thus meeting the case definition. Clustering of cases occurred in the coronary care unit (CCU) and psychiatry units. It was noted that one of the first affected HCWs in the CCU had vomited in the bathroom used by the entire staff, while another and vomited into a trash basket on the unit. The attack rate was 5.3% (7 of 133) among patients and 29.9% among HCWs in the CCU, where the outbreak had a bimodal temporal distribution and lasted a total of 8 weeks. In the psychiatry units, the attack rates were 16.7% (29 of 233) for patients and 38.0% (76 of 200) among HCWs; the outbreak continued on these units for 16 weeks. Norovirus was identified in 2 of 10 samples tested by the Maryland Department of Health and one of 6 tested at the National Institutes of Health. The virus belonged to genogroup II.4 and had 98%-99% nucleotide sequence identity with the Farmington Hills and other new-variant viruses that first circulated in the U.S. and Europe in 2002-2004. The prolonged transmission eventually succumbed to the implementation of aggressive infection control measures, which included unit closures and disinfection with sodium hypochlorite. An economic analysis estimated the total cost of the outbreak to be $657,644.


Norovirus has become a scourge of healthcare facilities in the U.S.1 The CDC received notices toward the end of 2006 suggesting an increase in the number of outbreaks of acute gastroenteritis (AGE), particularly in long-term care facilities. Although baseline data was not available since acute gastroenteritis is not reportable, further investigation confirmed this apparent increase. More detailed information indicated that the North Carolina Division of Public Health received 17 reports of outbreaks consistent with norovirus infection in 2006 compared to only 6 in 2005 and 3 in 2004. Wisconsin had 106 AGE outbreaks reported in 2006, a more than 4-fold increase from the previous year while New York also reported a 4-fold increase from 76 to 333. Molecular confirmation confirmed norovirus as the cause of outbreaks in cruise ships, long-term care and assisted living facilities, restaurants, catered events, parties, and a variety of other settings. Three-fourths of the noroviruses studied belonged to 2 new GII.4 variants, Minerva and Laurens.

Noroviruses may be foodborne, but are also transmitted directly from person-to-person. In addition, transmission may result from contact with contaminated environmental surfaces, on which the virus can persist for a prolonged period. The infectious dose is < 10 viral particles, while patients shed the virus in very high concentrations and may continue to shed for relatively prolonged periods. Shedding, perhaps at lower levels is also prolonged. Furthermore, fecal shedding is reported to be frequent in asymptomatic individuals during outbreaks, having been found in 26% of clinically unaffected HCW and 33% of unaffected patients.2

The prolonged duration of the Johns Hopkins outbreak is not unusual. I can vouch from personal experience at one institution with which I am affiliated that it can be very frustrating to have implemented aggressive control measures, such as those in the accompanying BOX, and to have the outbreak continue, nonetheless. The optimal approach is early recognition and to immediately dealing with the affected units as if they were cruise ships, extending this approach to the entire healthcare facility and to do so sooner rather than later. I can also confirm that these outbreaks are enormously costly to institutions especially because of the need to close units to further admissions and because the high frequency of involvement of healthcare workers.


  1. CDC. Norovirus activity - United States, 2006-2007. MMWR Morb Mortal Wkly Rep 2007; 56:842-846.
  2. Gallimore CI, Cubitt D, du Plessis N, Gray JJ. Asymptomatic and symptomatic excretion of noroviruses during a hospital outbreak of gastroenteritis. J Clin Microbiol 2004; 42:2271-2274.