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Patient-to-Patient Spread of Norovirus is Probably Overestimated

A recent study suggests that patient-to-patient transmission of norovirus is potentially overestimated within clinical settings.

Stephanie Kampmeier, MD, from the University Hospital, Muenster, Germany, and colleagues published the results of their study in the Journal of Clinical Virology.
Norovirus infection is the most frequent cause of acute gastroenteritis outbreaks worldwide, spreading by the fecal-oral route. It is the most common cause of food poisoning in the United States, and outbreaks of norovirus infection are frequently reported in healthcare systems.

As a consequence, studies analyzing secondary attack rates during hospital outbreaks have placed special emphasis on patient-to-patient transmission, the authors note. However, overall transmission rates outside outbreak settings have not been investigated.
Dr. Kampmeier and colleagues therefore conducted a study to examine the overall rates of norovirus transmission between patients, both within and outside outbreak situations. The researchers identified patients with norovirus infection that were confirmed during their stay in a tertiary care center from 2012 to 2015. Next, they followed asymptomatic individuals who were exposed to the norovirus-infected patients, monitoring the asymptomatic individuals for development of symptoms of acute gastroenteritis that are typical of norovirus infection.

The researchers identified 94 patients with confirmed norovirus infection, 85 (90.4%) of whom developed symptoms of acute gastroenteritis, while 9 were identified as chronic carriers. According to the authors, 102 asymptomatic individuals had contact with these 94 infected patients; 25 of the asymptomatic exposed individuals were associated with an outbreak—of these 25 individuals, only 11 developed symptoms of norovirus infection, while 14 remained unaffected. The remaining 77 exposed patients also did not develop symptoms of norovirus infection.

The outbreak secondary attack rate was 44.0%, which is consistent with previous findings, the authors note. However, they stress that the overall secondary attack rate was quite low—only 10.8%: This is in contrast with most assumptions in which various factors are considered to contribute to increased transmission rates, they say. These include virus-associated factors, infection control interventions, and host factors (including genetic factors and degree of immunity), they say.
Although noroviruses can cause large outbreaks, on the basis of the results of their study, the authors emphasize that cases involving individual norovirus-infected patients do not automatically result in outbreaks.
Dr. Kampmeier and colleagues acknowledge that the ease with which norovirus spreads is a significant problem that healthcare facilities must continue to address. However, they suggest that, because not every norovirus-infected patient is source of an outbreak, future studies should examine host factors that lead to increased susceptibility.
“An instant risk stratification of exposed patients could be helpful to concentrate on vulnerable patients to prevent norovirus outbreaks,” the authors conclude.
Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee. 
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