Effects of seasonal variability on Respiratory Syncytial Virus (RSV) in children under five years of age. A systematic review The increase in morbidity and mortality of patients with RSV in the winter season is a well-known phenomenon, where the causes in children under five years of age are associated with changes in temperature and humidity (maximum and minimum extremes), constituting a significant factor in respiratory infections, which are one of the most frequent pathologies in children of this age. In developing countries such as Colombia, there is an incidence of 95% of reported cases, where their mortality rates are high, resulting that 72.4% occurs in children under one year. RSV is highly contagious; it can survive up to seven hours in the host; 90% of children are infected with RSV during the first five years of life, and up to 40% show compromised lower airways. In order to carry out this review, national and international electronic databases were used, reflecting the evolving concepts of seasonal variability and the impact of RSV in children under five years of age. Key words: Respiratory Syncytial Virus, RSV Morbidity, Seasonal variability, Epidemiology.