Outbreaks of norovirus in health care settings and outbreaks caused by a particular genotype of the virus are more likely to make people seriously ill, according to a new study (“The Norovirus Epidemiologic Triad: Predictors of Severe Outcomes in US Norovirus Outbreaks, 2009–2016”) by the Centers for Disease Control and Prevention (CDC) in The Journal of Infectious Diseases. Based on an analysis of nearly 3,800 U.S. outbreaks affecting more than 100,000 people from 2009 to 2016, the research highlights several factors that can make norovirus outbreaks more severe and may help guide efforts to develop a vaccine to prevent this highly contagious disease.

Each year in the U.S., norovirus causes an estimated 19 to 21 million cases of vomiting and diarrhea, between 56,000 and 71,000 hospitalizations, and 570 to 800 deaths, with much of the disease burden falling on young children and older adults, according to the CDC. The disease spreads through direct contact with an infected person, consuming contaminated food or water, or touching contaminated surfaces.

In the new study, researchers linked, for the first time, data from a national outbreak reporting system and a laboratory surveillance network that collects data about norovirus genotypes associated with confirmed outbreaks. Severe outcomes, including hospitalizations and deaths, were more frequent in outbreaks caused by genogroup II type 4 (GII.4) and in outbreaks in health care settings, including hospitals, long-term care facilities, and outpatient facilities.

“Noroviruses are the leading cause of acute gastroenteritis outbreaks worldwide. Clarifying the viral, host, and environmental factors (epidemiologic triad) associated with severe outcomes can help target public health interventions. Acute norovirus outbreaks reported to the National Outbreak Reporting System (NORS) in 2009–2016 were linked to laboratory-confirmed norovirus outbreaks reported to CaliciNet. Outbreaks were analyzed for differences in genotype (GII.4 vs non-GII.4), hospitalization, and mortality rates by timing, setting, transmission mode, demographics, clinical symptoms, and health outcomes,” write the investigators.

“A total of 3747 norovirus outbreaks were matched from NORS and CaliciNet. Multivariable models showed that GII.4 outbreaks (n = 2353) were associated with healthcare settings (odds ratio [OR], 3.94 [95% confidence interval {CI}, 2.99–5.23]), winter months (November–April; 1.55 [95% CI, 1.24–1.93]), and older age of cases (≥50% aged ≥75 years; 1.37 [95% CI, 1.04–1.79]). Severe outcomes were more likely among GII.4 outbreaks (hospitalization rate ratio [RR], 1.54 [95% CI, 1.23–1.96]; mortality RR, 2.77 [95% CI, 1.04–5.78]). Outbreaks in healthcare settings were also associated with higher hospitalization (RR, 3.22 [95% CI, 2.34–4.44]) and mortality rates (RR, 5.65 [95% CI, 1.92–18.70]).

“Severe outcomes more frequently occurred in norovirus outbreaks caused by GII.4 and those in healthcare settings. These results should help guide preventive interventions for targeted populations, including vaccine development.”

In a related editorial commentary that appears with the study, Geoffrey A. Weinberg, M.D., of the University of Rochester School of Medicine and Dentistry, noted that this research shows “that the notion of noroviruses simply being ‘a cruise ship virus’ or an occasional foodborne winter vomiting illness is outdated.”

The findings confirm previous research about the severity of GII.4 norovirus outbreaks and suggest that future vaccines against norovirus should include these genotypes, according to the lead author of the study, Rachel M. Burke, Ph.D., of the CDC. The study results also suggest that targeting these vaccines for use in people in health care settings may help reduce hospitalizations and mortality associated with norovirus, added Dr. Burke. Although there is no currently available vaccine that protects against norovirus, several candidate vaccines are in the development pipeline.

“Linking data from these two different sources gives us a really powerful tool, a different way to look at norovirus outbreaks in the U.S., and a better understanding of some of the interactions between what is going on with the virus versus the host versus the environment,” Dr. Burke said.

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