Risk factors and outcomes among children admitted to hospital with pandemic H1N1 influenza

Background

Limited data are available on disease characteristics and outcomes of children with 2009 pandemic influenza A(H1N1) virus infection (pandemic H1N1 influenza) who have required hospital admission.

Methods

We reviewed the charts of 58 children with pandemic H1N1 influenza admitted to a large pediatric hospital in Ontario, Canada, between May 8 and July 22, 2009. We compared risk factors, severity indicators and outcomes of these children with those of 200 children admitted with seasonal influenza A during the previous 5 years (2004/05 to 2008/09).

Results

Children with pandemic H1N1 influenza were significantly older than those with seasonal influenza (median age 6.4 years v. 3.3 years). Forty-six (79%) of the children with pandemic H1N1 influenza had underlying medical conditions; of the other 12 who were previously healthy, 42% were under 2 years of age. Children admitted with pandemic H1N1 influenza were significantly more likely to have asthma than those with seasonal influenza (22% v. 6%). Two children had poorly controlled asthma, and 6 used inhaled medications only intermittently. The median length of stay in hospital was 4 days in both groups of children. Similar proportions of children required admission to the intensive care unit (21% of those with pandemic H1N1 influenza and 14% of those with seasonal influenza) and mechanical ventilation (12% and 10% respectively). None of the children admitted with pandemic H1N1 influenza died, as compared with 1 (0.4%) of those admitted with seasonal influenza.

Interpretation

Pandemic H1N1 influenza did not appear to cause more severe disease than seasonal influenza A. Asthma appears to be a significant risk factor for severe disease, with no clear relation to severity of asthma. This finding should influence strategies for vaccination and pre-emptive antiviral therapy.Influenza causes significant morbidity and mortality in childhood.¹ Infants, young children and people 65 years of age and older account for the highest rates of influenza-related hospital admission.² Earlier case series of 2009 pandemic influenza A(H1N1) virus infection (pandemic H1N1 influenza) reported small numbers of children^3,4 or did not present data on children separately.⁵ A recently published series that included 122 children confirmed typical influenza-like presentation, reported a high prevalence of underlying medical conditions (60%, including asthma in 29%) and described the need for intensive care in 20% and mechanical ventilation in 10%.⁶ A previous comparison of children with pandemic H1N1 influenza and those in previous years with seasonal influenza included only children considered to have died of influenza.⁷In this article, we present our experience with children admitted to hospital with pandemic H1N1 influenza. Our primary goal was to describe the demographic characteristics, clinical features and markers of severity of illness of these children. Our secondary goal was to identify risk factors for severe disease or poor outcome by comparing these children with those who had been admitted in previous years with seasonal influenza.