To compare risk factors for severe disease as measured by admission to hospital and intensive care unit (ICU) and other clinical outcomes in children with pandemic H1N1 (pH1N1) versus those with seasonal influenza.
Retrospective analysis of children admitted to hospital with pH1N1 versus seasonal influenza A.
Canadian tertiary referral children's hospital.
All laboratory-identified cases of pH1N1 in children younger than 18 years admitted to hospital in 2009 (n=176) and all seasonal influenza A cases admitted to hospital from influenza seasons 2004–2005 to 2008–2009 (n=200). Children with onset of symptoms more than 3 days after admission were excluded.
Primary and secondary outcome measures
Primary outcomes include admission to hospital and ICU and need for mechanical ventilation. Secondary outcomes include length of stay in hospital and duration of supplemental oxygen requirement.
Children admitted with pH1N1 were older than seasonal influenza A admissions (hospital admission: 6.5 vs 3.3 years, p<0.01; ICU admission: 7.3 vs 3.6 years, p=0.02). Children hospitalised with pH1N1 were more likely to have a pre-existing diagnosis of asthma (15% vs 5%, p<0.01); however, there was no difference in the severity of pre-existing asthma between the two groups. After controlling for obesity, asthma (OR 4.59, 95% CI 1.42 to 14.81) and age ≥5 years (OR 2.87, 95% CI 1.60 to 5.16) were more common risk factors in admitted children with pH1N1. Asthma was a significant predictor of the need for intensive care in patients with pH1N1 (OR 4.56, 95% CI 1.16 to 17.89) but not in patients with seasonal influenza A.
While most pH1N1 cases presented with classic influenza-like symptoms, risk factors for severe pH1N1 disease differed from seasonal influenza A. Older age and asthma were associated with increased admission to hospital and ICU for children with pH1N1.
Young age and underlying medical conditions have traditionally been considered risk factors for severe influenza in children.
Children admitted with pH1N1 influenza are more likely to have asthma; however, the impact of asthma severity is unknown.
The presence of asthma and increased age, but not severity of asthma, were more common risk factors for hospitalisation with severe H1N1 influenza than with seasonal influenza A.
These results suggest that in future pandemics, certain high-risk groups may be more adversely affected than expected with seasonal influenza.
Treatment of pH1N1 influenza with oseltamivir did not appear to be associated with differing outcomes or severity of disease.
Strengths and limitations of this study
The strength of this study is that it compares a large number of children admitted with microbiologically confirmed pH1N1 to those admitted over 5 years with seasonal influenza A. For each admitted child with suspected asthma, at least two physicians reviewed the case to confirm a diagnosis of pre-existing asthma and to grade the asthma as mild, moderate or severe.
The main limitations of this study include its retrospective design, single-centre site, the inability to calculate population-based rates and that the number of admitted patients with asthma, particularly to ICU, was small.