BACKGROUND: The Influenza Incidence Surveillance Project (IISP) is the only population-based surveillance system to systematically monitor year-round outpatient influenza-like illness (ILI) and associated respiratory viruses in the US. We present the age-specific incidence of ILI visits due to influenza and other respiratory viruses from seven surveillance seasons.
METHODS: From October 2009 to July 2016, 8 to 12 state/local public health jurisdictions participated in IISP, each recruiting approximately 5 outpatient clinics with enumerated patient populations. Clinics reported weekly ILI (reported fever with cough or sore throat) and all-cause patient visit counts and collected upper respiratory specimens from the first 10 ILI patients per week. Specimens were tested at the public health laboratories by RT-PCR for influenza, respiratory syncytial virus (RSV), rhinovirus/enterovirus (RVEV), adenovirus (ADV), human metapneumovirus (MPV), coronaviruses (COV) and parainfluenza viruses 1-3. We extrapolated the weekly number of patients positive for each virus from the tested subset and calculated incidence using the patient population as the denominator.
RESULTS: The number of participating clinics ranged by season from 39 to 74 with an average annual population of 384,211 persons and an average 459,953 outpatient visits. Incidence of ILI per 1000 population ranged from 14.2 in 2011-12 to 33.8 in 2014-15. Across all seasons, 20,844 specimens were tested for influenza; 17,677 (85%) were further tested by a respiratory panel. Incidence for each virus varied by season; the median influenza incidence was 7.0 per 1000 population (range 1.9 in 2011-12 to 11.0 in 2014-15). Non-influenza viruses with the highest median incidence per 1000 population were RVEV (4.3), RSV (1.7), COV (1.6) and ADV (1.3). Virus incidence varied by age: influenza incidence was highest each season among patients aged 5 to 17 years (median 13.5 per 1000 population), while rates of RVEV, RSV, ADV, MPV and COV were highest among children aged <5 years (median 12.6, 7.0, 5.9, 4.2 and 3.7 per 1000 population, respectively). Among patients with a viral detection, 6% to 10% were positive for >1 virus; of these patients, the most common co-detections were RVEV/ADV (16%), RVEV/RSV (12%), RVEV/MPV (6%), and RSV/ADV (5%).
CONCLUSIONS: The ILI definition is predictive of influenza and may not be optimal for detecting other respiratory viruses. Nevertheless, during seven surveillance seasons we found the highest incidence of outpatient ILI was associated with influenza, RVEV and RSV. Incidence of influenza was highest among older children as compared with other respiratory viruses. Rhinoviruses/enteroviruses were the most common viruses associated with viral co-detections.