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

Abstract

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.

Figure 1

Epidemic curve of seasonal influenza A and pandemic H1N1 influenza at a large pediatric hospital in Toronto, Canada. Specimens originated from inpatients, outpatients and patients seen in the emergency department. Pandemic H1N1 polymerase chain reaction (PCR) testing was introduced on Apr. 28, 2009. Some cases of influenza A detected by means of direct fluorescent antibody assay (DFA) or viral culture before this date may have been pandemic H1N1 influenza.

Figure 2

Age distribution of children admitted to hospital with seasonal influenza A (2004/05 to 2008/09) and pandemic H1N1 influenza (2009).