Severe and recurrent infections of the respiratory tract in early childhood constitute major risk factors for the development of bronchial hyper-responsiveness and obstructive respiratory diseases in later life. In the first years of life, the vast majority of respiratory tract infections (RTI) leading to wheezing and asthma are of a viral origin and severity and recurrence are the consequence of a greater exposure to infectious agents in a period when the immune system is still relatively immature. Therefore, boosting the efficiency of the host immune response against viral infections seems to be a rational preventative approach. In the last decades it has been demonstrated that living in farm environments, i.e. early-life exposure to microbes, may reduce the risk of allergic and infectious disorders, increasing the immune response efficacy. These findings have suggested that treatment with bacterial lysates could promote a nonspecific immunomodulation useful in the prevention of recurrent RTIs and of wheezing inception and persistence. Experimental and clinical studies showing the reduction of RTI frequency and severity in childhood and elucidating the involved mechanisms can support this hypothesis.

Following the most recent modification by the American Academy of Pediatrics, based on American studies on RSV epidemiology, the Italian Drug Agency (AIFA) decided to limit the total financial coverage of the palivizumab prescription by the National Health Service only to the < 29 wGA group and age ≤ 12 months at the beginning of the RSV epidemic season. However, the vulnerability of otherwise healthy premature infants ≥ 29 wGA has been demonstrated in Italian analyses. We retrospectively reviewed records from children ≤ 1 years of age admitted for RSV-induced ALRI at the Gaslini Hospital, over three consecutive RSV epidemic seasons (RES) (2014–2017). We found that the prescription limitation on RSV immunoprophylaxis in preterms was associated in the 2016–2017 RES with: a) a high proportion of admission for the < 36 wGA infants, the great majority born at 33- < 36 wGA and a chronological age of < 6 months; b) a high proportion of preterms treated with high flow nasal cannula ventilation. These results strongly point to a need to reevaluate the role of palivizumab prophylaxis in the >= 29 wGA subpopulation when specific risk factors are present.

Respiratory tract infections (RTIs) are common in childhood because of the physiologic immaturity of the immune system, a microbial community under development in addition to other genetic, physiological, environmental and social factors. RTIs tend to recur and severe lower viral RTIs in early childhood are not uncommon and are associated with increased risk of respiratory disorders later in life, including recurrent wheezing and asthma. Therefore, a better understanding of the main players and mechanisms involved in respiratory morbidity is necessary for a prompt and improved care as well as for primary prevention. The inter-talks between human immune components and microbiota as well as their main functions have been recently unraveled; nevertheless, more is still to be discovered or understood in the above medical conditions. The aim of this review paper is to provide the most up-to-date overview on dysbiosis in pre-school children and its association with RTIs and their complications. The potential role of non-harmful bacterial-derived products, according to the old hygiene hypothesis and the most recent trained-innate immunity concept, will be discussed together with the need of proof-of-concept studies and larger clinical trials with immunological and microbiological endpoints.