BACKGROUND: With the recent decline in rotavirus disease following universal vaccination, norovirus has become the leading cause of medically-attended acute gastroenteritis (MAAGE) among children aged <5 years in the United States. However, the incidence of MAAGE and the proportion attributable to norovirus across the full age spectrum has not previously been assessed through active, population-based surveillance in the United States. Our primary objective is to estimate the incidence of MAAGE and medically-attended norovirus disease in the United States, including age group-specific estimates.
METHODS: We conducted active surveillance over a 12-month period (April 2014–March 2015) among all members of Kaiser Permanente Northwest (KPNW). MAAGE-associated healthcare encounters, comprised primarily of outpatient visits, but also including inpatient admissions, and telephone and email consultations, were identified daily from electronic health records and selected for recruitment through age-stratified random sampling. Study participants completed an initial survey, provided a stool specimen within seven days of their MAAGE encounter, and completed a 2-week follow-up survey. Stool specimens were tested for norovirus by RT-PCR.
RESULTS: During the 12-month study period, we identified 18,846 unique MAAGE episodes among KPNW members (annualized incidence of 3648 per 100,000 population). Of these, 3680 (20%) members were randomly selected for recruitment, screened, and eligible to participate, of whom 1467 (40%) provided stool specimens and were included in the final analysis. Norovirus was detected in 194 (13%) specimens. Overall incidence of norovirus-associated MAAGE was 452 per 100,000. Children aged <5 years had the highest incidence of norovirus-associated MAAGE (1468 per 100,000); incidence declined with increasing age through adults aged 45–64 years (295 per 100,000) then increased again through those aged ≥85 years (759 per 100,000).
CONCLUSION: This is the first active surveillance study of medically attended norovirus disease performed in the United States to cover the full age spectrum. Data from this study can provide key baseline information on the burden of norovirus disease in the United States and inform the potential impact of future norovirus vaccines.
