SAN DIEGO – Norovirus infections send about 1.6 million people to the doctor every year in the United States, according to the first active surveillance study to cover all age groups here.
“This provides a baseline burden of norovirus disease to assess the potential impact of future vaccines,” said Dr. Aron Hall of the Centers for Disease Control and Prevention in Atlanta. Estimated infection rates generally resembled those from past studies, but were more robust and granular, Dr. Hall said at an annual scientific meeting on infectious diseases.
Courtesy CDC/Charles D. Humphrey
This transmission electron micrograph (TEM) reveals norovirus virions, or virus particles.
About 19-21 million Americans suffer acute gastroenteritis because of norovirus infection every year, Dr. Hall noted. About 400,000 of these patients seek urgent care, at least 56,000 are hospitalized, and 570-800 ultimately die from associated complications. However, several factors have impeded large epidemiologic studies of norovirus disease, he added. Most infected patients do not seek care, those who do often do not undergo stool testing, there is no national norovirus case reporting system, turnaround times and the sensitivity of clinical assays remain suboptimal, and there are no specific ICD codes for norovirus gastroenteritis, he said.
To overcome these obstacles, Dr. Hall and his associates spent a year studying 480,000 Kaiser Permanente Northwest enrollees in the Portland, Ore. area. Enrollees seek care almost entirely in-network and demographically resemble other residents in the region, Dr. Hall said. The researchers used automated software to identify about 19,000 patients with ICD-9 codes for acute gastroenteritis. They called about half of these patients to request stool samples within 3 days of their appointments, while they were likely to still be shedding pathogens. They aimed to test all patients who were younger than 5 years or older than 65 years, and called a random sample of 35% of the other patients.
In all, the researchers tested stool specimens for 84% (1,467) of patients whom they reached within the 3-day window, said Dr. Hall. About 13% of these patients tested positive for norovirus, of which 85% were genotype II, while the rest were genotype I. The overall incidence of medically attended acute gastroenteritis due to norovirus was 5 cases per 100,00 person-years, but rates were four to five times higher among infants and children under the age of 2 years (22.7 and 29.1 cases per 100,000 person-years, respectively), and about 50% higher among adults aged 85 years and older (7.4 cases per 100,000 person-years).
“Extrapolation to the U.S. population would yield 1.6 million norovirus-associated MAAGE [medically attended acute gastroenteritis] encounters per year,” Dr. Hall concluded. The research team is now sequencing norovirus genotypes, testing stool samples for other enteric pathogens, and studying high-risk subgroups, clinical severity, and cases of potential household transmission, he added.
Dr. Hall spoke at the combined annual meetings of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association, and the Pediatric Infectious Diseases Society. He had no disclosures. One of his coauthors reported receiving research support from GlaxoSmithKline.