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BACKGROUND: Influenza and other respiratory viruses cause substantial annual morbidity and mortality in the United States. The Influenza Incidence Surveillance Project (IISP) was initiated in 2009 to conduct population-based surveillance for influenza-like illness (ILI) and determine the contribution of influenza and other respiratory viruses.
METHODS: Public health department personnel in 8 jurisdictions enrolled 41 health care providers to conduct surveillance for patients presenting with ILI (fever with cough or sore throat). Providers counted weekly ILI and all-cause patient visits by age. Respiratory specimens were collected from the first 10 ILI patients per week and tested for influenza; in 5 sites, specimens were also tested for respiratory syncytial virus (RSV), adenovirus (ADV), rhinovirus/enterovirus (RV/EV), human metapneumovirus (MPV), coronaviruses (COV), and parainfluenza viruses 1, 2 and 3 (PIV) by PCR. Incidence was calculated by multiplying the number of ILI patients by the percent virus-positive patients each week, using the providers’ patient population size as the denominator.
RESULTS: From August 2013 through July 2014, 2.6% of 282,133 outpatient visits reported were for ILI. Influenza was detected in 28% of 1912 specimens throughout the year and among the 8 sites was detected in 50%-80% (median 69%) of specimens during the site’s peak week. Among influenza viruses detected, 70% were subtype A/2009 H1N1, 12% were A/H3N2, and 18% were influenza B viruses. Among sites testing for other respiratory viruses, 33% of specimens had at least one non-influenza virus detected, including RV/EV (12%), MPV (4.6%), RSV (4.2%), COV (3.9%), and ADV (3.3%), and PIV1 (2.3%), PIV2 (0.6%), and PIV3 (1.7%). Patient visits for ILI occurred 3.8 times more frequently among children aged <18 years than adults aged >18 years. We observed the highest incidence of PIV3 and RV/EV among children aged <1 year (2.4 and 10.0 per 1000 population, respectively), RSV and ADV among children aged 1 to 2 years (6.3 and 3.6 per 1000 population, respectively), PIV1, MPV and COV among children aged 2 to 4 years (2.9, 6.4, and 2.2 per 1000 population, respectively), and influenza among children aged 5 to 17 years and adults (11 and 3.3 per 1000 population, respectively).
CONCLUSIONS: Continuous, population-based surveillance established in the IISP enabled the calculation of respiratory virus-associated ILI rates throughout the year. Influenza was the most commonly detected pathogen, reflective of the predictive ILI case definition. Rates of influenza were highest among patients aged >5 years, while rates of other respiratory viruses were higher in younger age groups.