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The vast majority of healthcare-associated acute gastroenteritis (AGE) outbreaks in the U.S. are caused by . Although noroviruses typically cause uncomplicated, self-limiting disease, occur in long-term care facilities (LTCFs) where are at particularly high risk of complications and death.

To better understand the burden of AGE outbreaks, the CDC established two national surveillance systems in 2009. An analysis of merged epidemiologic and laboratory data from the first 10 years indicated that while reporting has improved, prevention of norovirus-associated AGE outbreaks in LTCFs still has a long way to go.

"Our finding of a relatively high burden of morbidity and mortality attributed to norovirus outbreaks in LTCFs aligns with ," wrote Laura E. Calderwood, MPH, Sara A. Mirza, PhD, MPH, both of the CDC in Atlanta, and colleagues in . "Therefore, norovirus remains a prime target for interventions in LTCFs, including standard infection control procedures, and LTCF residents and staff are an important risk group for targeted interventions such as vaccines."

The analysis revealed that from 2009 to 2018, there were 13,092 LTCF norovirus outbreaks and 416,284 associated LTCF cases. The researchers attributed 6,641 hospitalizations and 729 deaths to outbreak-associated norovirus AGE. This corresponded to a hospitalization rate of 21.6 per 1,000 cases and a death rate of 2.3 per 1,000 cases, they said.

was responsible for 90.4% of the norovirus outbreaks in LTCFs, with attack rates almost three times higher among LTCF residents than among staff (median 29.0% vs 10.9%, P<0.001). Most outbreaks were classified as GII.4 strains, which have been associated with prolonged viral shedding and .

"Vaccine protection specific to the predominant GII.4 norovirus strains is therefore an important consideration in ongoing vaccine development efforts," the authors said. "Continued resource support for the detection, investigation, and reporting of norovirus outbreaks is needed to understand the true burden of norovirus in LTCFs."

In the following interview, Mirza, who is an epidemiologist in the division of viral diseases at the CDC, discussed the findings in greater detail.

What does your analysis add to current knowledge about norovirus AGE outbreaks in LTCFs?

Mirza: The National Outbreak Reporting System [] is a powerful tool for understanding the burden of norovirus and other gastroenteric pathogens at the national level. Surveillance of norovirus outbreaks in LTCFs has improved, and we now have a baseline with which we can compare trends over time to understand the impact of non-pharmaceutical interventions, such as improvements in infection-control protocols and future vaccines.

What three factors might further improve AGE-outbreak reporting, and what role might clinicians have in this?

Mirza: First, laboratory confirmation of norovirus outbreaks can provide a better understanding of the burden of all types of gastroenteric viral pathogens, including norovirus, sapovirus, and rotavirus. Providing clinical samples to public health laboratories would allow for additional testing, such as genotyping, which is necessary to understand the differences in severity and transmission between strains.

In addition, clinicians should report outbreaks to local or state health departments so that interventions can be identified to reduce the spread of ongoing outbreaks or prevent future outbreaks.

Finally, adequate staffing of LTCFs -- including healthcare providers trained in infection control and with the ability to dedicate time to infection prevention -- is key. Working with public health officials to collect detailed information on outbreaks is often overly burdensome to staff who are already stretched thin with their daily activities.

How might lessons learned during the COVID-19 pandemic inform future reporting of AGE outbreaks?

Mirza: Although reporting of norovirus outbreaks may have been negatively impacted by the resource demands of the COVID-19 pandemic in recent years, we expect it may eventually lead to improvements in reporting of infectious disease outbreaks of all kinds, including norovirus. The pandemic has improved the lines of communication and reporting infrastructure between healthcare facilities and public health agencies, broadened public awareness of infectious disease issues, and increased testing capacity at public health laboratories.

Have you continued to track this data through the pandemic and if so, did you find any differences?

Mirza: We saw a sharp decrease in norovirus activity in LTCFs at the beginning of the pandemic, and levels have stayed lower than normal throughout the pandemic. This decrease can be seen in our interactive data tool, , which allows users to filter outbreak data through the 2020 calendar year by setting and pathogen. More recent trends in norovirus outbreak activity across all settings can be seen in our near-real time reporting system, .

We continue to track norovirus outbreaks and are working to learn more about the full impact of the pandemic on the future of norovirus within specific settings.

What is your take-away message from the study?

Mirza: LTCFs should implement plans to prevent norovirus outbreaks spread through person-to-person contact. Norovirus is highly contagious, especially in shared-living settings, and residents of LTCFs are particularly vulnerable to norovirus complications, often requiring medical support.

You can read the study abstract here.