Epidemiology

Bronchiolitis is one of the most common acute illnesses in infancy and the leading cause of hospitalisation in this age group.[5][6][7]

In 2019, an estimated 33.0 million episodes of respiratory syncytial virus (RSV) acute lower respiratory infection (ALRI) resulted in about 3.6 million hospital admissions, 101,400 overall RSV-attributable deaths, and 26,300 in-hospital deaths, globally in children younger than 5 years.[8] In children younger than 6 months, 1.4 million hospital admissions, and 13,300 in-hospital deaths, were due to RSV-ALRI.[8]

Investigators studying registry data of 2.72 million children and their families from two countries over two decades identified 16 predictors for RSV-related hospitalisations in the first year of life.[9]​ At the population level, proximity of birth month to the next epidemic peak, having siblings <4 years old, and gestation age at birth were the most important. At the patient level, both complex and simpler types of congenital heart disease, trisomy 21, and oesophageal malformations greatly increased risk. Family characteristics that also increased risk of RSV hospitalisation included asthma diagnosis in a sibling, and psychiatric diagnoses and substance use disorders among parents.

Bronchiolitis-related mortality is disproportionally greater in low-income and middle-income countries compared with industrialised countries.[1][8]​​​​ Mortality is also greater among those children who require admission to an intensive care unit.[10]

The incidence of bronchiolitis displays a distinct seasonal pattern, with most cases in the US occurring from November to April.[11] 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. The social distancing and non-pharmacological protective measures (i.e., use of masks) employed during the coronavirus disease 2019 (COVID-19) pandemic interrupted the usual seasonality of viruses that cause bronchiolitis, initially reducing the number of children hospitalised with the disease. As those measures were relaxed, however, several out-of-season rebound RSV epidemics were reported.[12]

Bronchiolitis is almost exclusively an infantile disease, and by age 2 years, essentially all children have serological evidence of having been infected with RSV.[13]​ 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.[8] Infants with underlying risk factors for severe infection, such as prematurity, congenital heart disease, or chronic lung disease, have a greater risk of hospitalisation, but the majority of hospitalisations are in infants with no underlying risk factors.[5]

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