Differences between asthmatics and nonasthmatics hospitalised with influenza A infection

Viral respiratory infections are the major cause of asthma exacerbations [1], while subjects with asthma are more likely to suffer significant morbidity when infected [2]. The severity and duration of respiratory insult depend on complex viral-host interactions. Abnormalities of the innate and adaptive immune response have been demonstrated in humans and in animal models of asthma [3]. These phenomena may explain the increased vulnerability of asthmatics to viral respiratory infections. Although less common than rhinovirus, which is responsible for 33–55% of asthma exacerbations in children and adults, seasonal influenza infection is a well-recognised cause of asthma exacerbations (3–23%) [1]. Dawood et al. [4] reported on a systematic surveillance programme for influenza-related hospitalisations of children in the USA. The proportion of children with asthma during seasonal influenza was 32% from 2003 to 2009, while 44% of children hospitalised with H1N1 in 2009 had asthma. O’Riordan et al. [5], in a paediatric hospital-based series, also found asthma to be more frequent during the H1N1 influenza pandemic of 2009 than in previous years of seasonal influenza (22% versus 6%). The remarkable increase in the risk of hospital admission in asthmatics with pandemic H1N1, compared with both seasonal influenza A and rhinovirus, may be explained by a lack of adaptive immunity to the H1N1 virus in many young asthmatic patients. Children with asthma were also more likely to require intensive care unit admission during the pandemic than with seasonal influenza A (22% versus 16%) [4]. In a prospective study during the autumn of 2009, Kloepfer et al. [6] collected nasal swabs for viral identification on a weekly basis. They found that children with asthma were more likely to become infected with H1N1 than children without …