Epidemiology

Bronchiolitis is one of the most common acute illnesses in infancy and the leading cause of hospitalization in this age group.[3][4][5]

In 2015, an estimated 33.1 million episodes of respiratory syncytial virus (RSV) acute lower respiratory infection (ALRI) resulted in about 3.2 million hospital admissions, and 59,600 in-hospital deaths, globally in children younger than 5 years.[6] In children younger than 6 months, 1.4 million hospital admissions, and 27,300 in-hospital deaths, were due to RSV-ALRI.[6]

The incidence of bronchiolitis displays a distinct seasonal pattern, with most cases in the US occurring from November to April.[7] The peak incidence of the disease usually occurs in January or February. In the southeast, the onset and peak of infections is slightly earlier. Other temperate areas generally show a similar pattern of annual midwinter epidemics. In contrast, parainfluenza-1 infections (causing croup) display a biennial incidence pattern.

Bronchiolitis is almost exclusively an infantile disease, and by 3 years of age essentially all children have serologic evidence of having been infected with RSV. However, primary infection with RSV in infants does not confer protective immunity, so repeat infections are common. Although in most infants the disease is mild and self-limited, severe disease can occur, especially in infants under 6 months of age.[6] Infants with underlying risk factors for severe infection, such as prematurity, congenital heart disease, or chronic lung disease, have a greater risk of hospitalization, but the majority of hospitalizations are in infants with no underlying risk factors.[3]

In addition to the acute effects of bronchiolitis, studies have demonstrated that a significant proportion of infants with RSV bronchiolitis go on to develop recurrent wheezing; rhinovirus has been increasingly studied and shown to have an association with recurrent wheezing and a diagnosis of asthma.[8][9][10][11][12] Risk factors such as family history of asthma increase the risk of a future asthma diagnosis.[9][13]

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