Viruses associated with acute respiratory infections and influenza-like illness among outpatients from the Influenza Incidence Surveillance Project, 2010-2011

Abstract

Background: The Influenza Incidence Surveillance Project (IISP) monitored outpatient acute respiratory infection (ARI; defined as the presence of ≥ 2 respiratory symptoms not meeting ILI criteria) and influenza-like illness (ILI) to determine the incidence and contribution of associated viral etiologies.

Methods: From August 2010 through July 2011, 57 outpatient healthcare providers in 12 US sites reported weekly the number of visits for ILI and ARI and collected respiratory specimens on a subset for viral testing. The incidence was estimated using the number of patients in the practice as the denominator, and the virus-specific incidence of clinic visits was extrapolated from the proportion of patients testing positive.

Results: The age-adjusted cumulative incidence of outpatient visits for ARI and ILI combined was 95/1000 persons, with a viral etiology identified in 58% of specimens. Most frequently detected were rhinoviruses/enteroviruses (RV/EV) (21%) and influenza viruses (21%); the resulting extrapolated incidence of outpatient visits was 20 and 19/1000 persons respectively. The incidence of influenza virus-associated clinic visits was highest among patients aged 2-17 years, whereas other viruses had varied patterns among age groups.

Conclusions: The IISP provides a unique opportunity to estimate the outpatient respiratory illness burden by etiology. Influenza virus infection and RV/EV infection(s) represent a substantial burden of respiratory disease in the US outpatient setting, particularly among children.

Keywords: acute respiratory illness; epidemiology; influenza; influenza-like illness; respiratory virus; seasonality.

Figure 1.

Age-specific proportions of outpatient visits for acute respiratory infections (ARIs) and influenza-like illnesses (ILIs) in the Influenza Incidence Surveillance Project, August 2010 through July 2011. Influenza-like illness (ILI) was defined among patients aged <2 years as fever and >1 of the following symptoms: cough, sore throat, rhinorrhea, and nasal congestion. We defined ILI among patients aged ≥2 years as fever with cough or sore throat. Patients with presenting with ≥2 of the following symptoms who did not also meet the ILI case definitions were defined as having ARI: fever, cough, sore throat, rhinorrhea, and nasal congestion. The graphs present ARI and ILI cases as mutually exclusive groups.

Figure 2.

Seasonal distribution of respiratory viruses detected by reverse-transcription polymerase chain reaction among patients with influenza-like illnesses (ILIs), Influenza Incidence Surveillance Project, August 2010 through July 2011. The bars indicate the number of virus detections, and the lines indicate the percentage of patients with ILI who tested positive.

Figure 3.

Age-specific cumulative incidence of respiratory virus-associated acute respiratory infections (ARIs) and influenza-like illnesses (ILIs) by age group from August 2010 through July 2011, Influenza Incidence Surveillance Project. Incidence was extrapolated from the population rate of ARI and ILI and the proportion testing positive for each virus. ILI was defined among patients aged <2 years as fever and >1 of the following symptoms: cough, sore throat, rhinorrhea, and nasal congestion. We defined ILI among patients aged ≥2 years as fever with cough or sore throat. Patients with presenting with ≥2 of the following symptoms who did not also meet the ILI case definitions were defined as having ARI: fever, cough, sore throat, rhinorrhea, and nasal congestion. The graphs present ARI and ILI cases as mutually exclusive groups. Abbreviations: ADV, adenovirus; COV, coronavirus; EV, enterovirus; MPV, human metapneumovirus; PIV, parainfluenza virus; RSV, respiratory syncytial virus; RV, rhinovirus.