Background: The rhinovirus etiology of acute wheezing and allergic sensitization are important risk factors for asthma. Early interventions with oral corticosteroids have shown a possible potential to decrease asthma risk in some rhinovirus affected subgroups. However, information about the first wheezing episode overall and the effect of the risk factors diagnosed during the first wheezing episode on asthma development and lung function remain limited. 

Aims: The aims of this thesis were to study 1) the virus etiology of the first severe wheezing episode and the associations among the virus etiology, atopic characteristics and vitamin D status; 2) the efficacy of prednisolone during the first severe wheezing episode concerning the time to initiation of asthma control medication and 3) lung function 4 years after the first severe wheezing episode. 

Methods: In children aged 3-23 months, virus etiology and patient characteristics of the first wheezing episode were studied using laboratory diagnostics, standard parental questionnaires and patient charts. The efficacy of prednisolone was studied in a randomized placebo-controlled trial. During a 4-year follow-up using impulse oscillometry with exercise and bronchodilation tests, lung function was investigated. 

Results: Rhinovirus was the most common etiology (76%) of the first wheezing episode and positively associated with atopic characteristics and prolonged coughing. Vitamin D levels of the children were normal and were not associated with virus etiology or atopic characteristics. Children with a high rhinovirus genome load benefitted from prednisolone in terms of longer time to initiation of asthma control medication. Early allergic sensitization was associated with increased airway reactivity at preschool age. 

Conclusions: Rhinovirus is a common etiologic agent in the first severe wheezing episode and linked to atopic characteristics. These findings about the efficacy of prednisolone create a basis for planning the early intervention strategies to secondary prevention of asthma. Diagnosing allergic sensitization early is important for predicting the risk of asthma and compromised lung function development.