Virulent Genotype of Norovirus Linked with Longer, More Severe Outbreaks in Health Care Settings


Investigators have determined that norovirus outbreaks occur most frequently in health care settings and are often caused by strains of a virulent genotype.

Investigators from the US Centers for Disease Control and Prevention (CDC) have linked data from 2 outbreak reporting systems in the United States to determine trends of norovirus outbreaks, including the most virulent genotype.

Past research has found that of the 3 genogroups and 29 genotypes of noroviruses, genogroup II type 4 (GII.4) occurs most frequently in outbreaks throughout the world. In a new study published in the Journal of Infectious Diseases, CDC investigators set out to determine how the epidemiological triad of viral, host, and environmental factors contribute to severe norovirus outcomes.

The study is the first of its kind, noted the authors, and links data from 2 US surveillance systems—acute norovirus outbreak data reported by state health departments from 2009 to 2016 to the National Outbreak Reporting System (NORS), and epidemiological and sequencing information from a national system of laboratories known as CaliciNet.

This combination of data provided the investigators with robust epidemiologic information specific to norovirus genotypes, helping to define the characteristics of outbreaks that result in more severe health outcomes. Looking at a total of 3747 norovirus outbreaks linked between both systems, investigators found that GII.4 strains were reported in 2353 (62.8%) of outbreaks. Of the GII.4 outbreaks, 1991 (84.6%) occurred during the months of November to April, while 980 of 1394 (70.3%) outbreaks from non-GII.4 genotypes occurred during the same time period.

The outbreaks related to GII.4 were found to be larger, with a median number of 26 cases per outbreak, compared with non-GII.4 outbreaks, which had a median of 21 cases. The GII.4 outbreaks were also longer in duration, lasting a median of 9 days, compared with a median of 6 days in non-GII.4 outbreaks.

In addition, the investigators observed that 72.2% of outbreaks related to the GII.4 strain occurred in health care settings versus only 40.4% of non-GII.4 related outbreaks.

The investigators also observed that adults ages 75 years and older were significantly more likely to be infected with a GII.4 strain, which is consistent with previous research.

“These findings confirm previous research about the severity of GII.4 norovirus outbreaks,” Rachel M. Burke, MPH, PhD, an investigator in the Division of Viral Diseases at the CDC and the first author of the study told Contagion®. “However, for the first time, we were able to look at different outbreak characteristics in the same analysis, using surveillance data from the epidemiologic and laboratory perspectives. We found that GII.4 outbreaks were independently associated with higher rates of hospitalization and death, even after taking into account other factors such as the age of cases or location of the outbreak. This study provides further evidence that these GII.4 noroviruses are inherently more virulent than other norovirus genotypes.”

Dr. Burke noted that although there are no approved vaccines currently on the market, there are several in development, including at least 5 vaccines in preclinical trials. It is unlikely that a norovirus vaccine will be approved and available in the next 5 years, but it is critical to develop a vaccine that will protect against GII.4 genotypes.

There are an estimated 56,000 to 71,000 hospitalizations in the United States each year from endemic norovirus illness, and 570 to 800 related deaths.

Although most cases of norovirus pass within 1 to 3 days, young children, the elderly, and individuals with certain medical conditions can be susceptible to more severe infections which can result in severe dehydration, hospitalization, and death. People with norovirus can remain contagious for up to 2 weeks after recovery and continue to spread the virus through their stool or vomit.

With norovirus season officially underway, the investigators indicate that prevention is the best method to avoid norovirus.

“The best ways to protect against norovirus are to wash hands often; clean and disinfect surfaces with bleach; rinse fruits and vegetables and cook shellfish thoroughly before eating; and when sick, stay home and don’t prepare food for others until 2 days after symptoms stop,” Dr. Burke said.

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