Respiratory syncytial virus (RSV) is a common virus that causes infections of the upper and lower respiratory airways (including the nose, sinuses and lungs). Discovered in 1956, RSV is a ribonucleic acid (RNA) virus that belongs to the family Paramyxoviridae. It is classified into 2 subgroups, RSV-A and RSV-B. Both subgroups tend to co-circulate during each season and both can cause severe disease.

RSV is a leading cause of acute lower respiratory infections and associated hospitalizations in infants and children globally. Nearly all children will have experienced at least one RSV infection by their second year of life. In 2019, there were 33 million RSV-associated cases of acute lower respiratory airway infection, 3.6 million RSV-associated hospital admissions, and 101 400 RSV-attributable deaths in children under 5 years of age. Over 97% of pediatric RSV-related deaths occur in children living in low- and middle-income countries. 

RSV infection in adults is generally mild and presents as low-level upper respiratory airway infections. However, older adults and those with chronic respiratory and/or cardiac conditions are more likely to experience severe disease. RSV-related complications include bronchiolitis (inflammation of the smaller airways in the lung), pneumonia, acute otitis media (infection of the middle ear) and conjunctivitis.