Asthma is a chronic inflammatory disorder of the airways characterized by lower respiratory tract (LRT) symptoms such as wheeze, cough and airway obstruction. Patients with asthma frequently suffer from exacerbations, which can be triggered by allergens and, in particular, viral respiratory infections. It has recently been shown that mepolizumab, a humanized monoclonal antibody that neutralizes interleukin(IL)-5, markedly reduces the exacerbation rate in asthma patients with eosinophilic airway inflammation. Previous studies have indicated that in a mixed population (eosinophilic and non eosinophilic) of mild asthma patients, mepolizumab did not have an impact on lung function and asthma symptom scores upon allergen provocation, although it did on markers such as sputum and blood eosinophils. Together, these observations led to the hypothesis that mepolizumab treatment reduces the exacerbation rate by limiting virus-induced asthma exacerbations.