Recurrent upper respiratory tract infections in children have an important socioeconomic impact, with consequences on both the quality of life of the children, the possible medical sequelae and the inherent direct and indirect costs. The possibility to prevent these infections is limited in the absence of specific vaccines against microorganisms responsible for most of the respiratory tract infections (i.e. respiratory syncitial virus, adenovirus, rhinovirus). Immunoactive bacterial extracts that stimulate the nonspecific component of the immune system may protect against a large variety of microorganisms that enter the body by the oral and respiratory pathway; they may, therefore, play an important role with regard to this preventive action. OM-85 BV is an IBE that has been used in children who suffer from repeated infections to prevent the occurrence of new episodes (secondary prevention). In this condition, the drug has been shown to be effective in protecting children against recurrent airway infections. Its use as a primary preventive agent to prevent the development of repeated infections in children attending day-care centers (a very high-risk environment for repeated infections), however, did not show a similar efficacy. The risk of having > 4 episodes of upper respiratory tract infections over a period of 7.5 months was 26.7% in the verum group and 33.8% in the placebo group (relative risk 0.79, confidence interval 0.59-1.06]. In an exploratory analysis concentrating on the 3-month treatment period, however, a 48% reduction of the risk of presenting > 3 episodes was observed. Furthermore, this exploratory analysis showed a strong correlation between drug efficacy and age of the children. Three reasons could possibly explain the discrepancy between primary and secondary prevention: first, the environment is very different in day-care and at home. Second, evaluating the efficacy in primary prevention inevitably includes a number of children that are not prone to repeated infections, thus influencing the overall figures by diluting the effect. And third, the efficacy of OM-85 BV in children younger than 1 year could be limited, probably due to the immaturity of their immune system. These questions will have to be further investigated.