Respiratory tract infections (RTIs) are common in childhood and represent one of the main causes of hospitalization in this population. In recent years, many studies have described the association between gut microbiota (GM) composition and RTIs in animal models. In particular, the “inter-talk” between GM and the immune system has recently been unveiled. However, the role of GM in human, and especially infantile, RTIs has not yet been fully established. In this narrative review we provide an up-to-date overview of the physiological pathways that explain how the GM shapes the immune system, potentially influencing the response to common childhood respiratory viral infections and compare studies analysing the relationship between GM composition and RTIs in children. Most studies provide evidence of GM dysbiosis, but it is not yet possible to identify a distinct bacterial signature associated with RTI predisposition. A better understanding of GM involvement in RTIs could lead to innovative integrated GM-based strategies for the prevention and treatment of RTIs in the paediatric population.

Respiratory Syncytial Virus (RSV) bronchiolitis is the leading cause of hospitalization in infants. The role of RSV load in disease severity is still debated. We present the interim results of a prospective monocentric study enrolling previously healthy infants hospitalized for RSV bronchiolitis, collecting nasopharyngeal aspirates every 48 h from admission to discharge, and evaluating RSV-load dynamics in relation to clinical outcome measures of bronchiolitis severity, including: need, type and duration of oxygen therapy, length of hospitalization, and the bronchiolitis clinical score calculated at admission. The results showed that the highest viral replication occurs within the first 48 hours after admission, with a significant decrease at subsequent time points (p < 0.0001). Moreover, higher RSV-RNA values were associated with the need for oxygen therapy (p = 0.03), particularly high-flow nasal cannulae type (p = 0.04), and longer duration of respiratory support (p = 0.04). Finally, higher RSV-load values were correlated with lower white blood cells, especially lymphocyte counts and C-reactive protein levels s (p = 0.03, p = 0.04, and p = 0.01, respectively), as well as with patients of a younger age (p = 0.02). These data suggest that RSV may actively contribute to the clinical severity of bronchiolitis, together with other potential non-viral factors.