Incidence of Norovirus-Associated Acute Gastroenteritis in Four Veteran’s Affairs Medical Center Populations in the United States, 2011–2015

Background In the USA, norovirus is an important cause of epidemic acute gastroenteritis (AGE) as well as a leading cause of pediatric AGE. However, the burden of sporadic norovirus disease in US adults has not been well-documented. Our objective was to estimate the incidence of outpatient visits and hospitalizations for community-acquired norovirus AGE at four Veterans Affairs Medical Centers (VAMCs) and their associated outpatient clinics in Atlanta, GA; Bronx, New York; Houston, TX; and Los Angeles, CA. Methods From November 2011 to September 2015, stool specimens collected for clinician-requested diagnostic testing within 7 days of AGE symptom onset and with reported vomiting or diarrhea were tested for norovirus by real-time RT-PCR and positive samples were genotyped by Sanger sequencing. Incidence of norovirus-associated outpatient visits and hospitalizations were calculated by multiplying the prevalence of norovirus among tested specimens by AGE-coded outpatient encounters and inpatient discharges, and dividing by the unique patients served at each VAMC. Results 1,620 stool specimens were tested from all 4 sites. Seven percent of outpatient (n = 795) samples (annual range: 3%–10%; range by site: 3%–10%) and 6% of (n = 825) samples from hospitalized patients tested positive for norovirus (annual range: 3%–8%; range by site: 3%–10%). Forty-four percent of norovirus-positive specimens were typed as GII.4 Sydney. Seventy-four percent of norovirus-positive specimens were collected between November and April. From 2011 to 2015, outpatient norovirus incidence was 250/100,000 population (annual range: 129 to 426/100,000; range by site: 87 to 428/100,000), and the incidence of norovirus hospitalizations was 28/100,000 population (annual range: 19 to 39/100,000; range by site: 14 to 57/100,000). By age group and setting, the highest incidence was observed among 45- to 64-year-old outpatients (370/100,000 population), and 85+-year-old inpatients (63/100,000 population). Conclusion This study provides estimates of the incidence of norovirus AGE outpatient visits and hospitalizations across multiple years among a geographically distributed VA population, highlighting the substantial burden of norovirus in US adults. Disclosures All authors: No reported disclosures.