Characteristics of Children and Young Adults with Influenza A Infections Concurrent with the Presence of H1N1 Influenza Virus

Background: During the novel H1N1 influenza outbreak in April 2009, children and young adults under 24 years of age were noted to have the highest attack rate and the highest rate of H1N1-related complications, including severe pneumonia and death.  To properly manage influenza and to prevent its spread within a healthcare facility, it is essential for healthcare providers to rapidly recognize and isolate individuals with influenza infections.

Objective: To describe characteristics of children and young adults hospitalized for influenza A infections during the novel H1N1 influenza outbreak period, and to identify characteristics distinct from patients with seasonal influenza A.

Methods:  The study was conducted at Children's National Medical Center in Washington, D.C.  It included patients hospitalized for a laboratory-confirmed influenza A infection during the novel H1N1 outbreak period (Cohort 1) and in the 2008-2009 influenza season (Cohort 2). Clinical and epidemiological features as well as hospital course were compared.

Results: During the novel H1N1 outbreak period, the Emergency Department visits for influenza-like illness increased by 77.7% compared to the same time period in 2008 and 29% of these patients required hospitalization. As shown in the Table, Patients in both cohorts had similar gender and race distributions, while patients in Cohort 1 (n=111) were older than those in Cohort 2 (n=44) (median age Cohort 1: 4.7; Cohort 2:1.6 years, p=0.02). Preexisting asthma (Cohort 1: 21.6%; Cohort 2: 9.0%, p=0.07) and sickle cell disease (Cohort 1: 13.5%; Cohort 2: 6.8%, p=0.24) were more prevalent in Cohort 1. In both Cohorts, 44.5% of patients presented with gastrointestinal (GI) symptoms (Cohort 1: 40.5%; Cohort 2: 54.5%, p=0.11). The median hospital length of stay was comparable (Cohort 1: 3 days; Cohort 2: 4 days, p=0.07). Of the 18.7% of patients admitted to the intensive care unit (Cohort 1: 16.2%; Cohort 2: 25.0%, p=0.21), most (68.7%) had preexisting medical conditions (Cohort 1: 72.2%; Cohort 2: 54.6%, p=0.33).

Conclusions: Compared to those hospitalized during a typical influenza season, children with influenza A during the H1N1 outbreak period tended to be older and more likely to have preexisting asthma or sickle cell disease. Almost half of the patients had GI symptoms.  These early findings, if substantiated with a larger study, could be invaluable in understanding the epidemiology associated with the emerging novel H1N1 influenza virus.