Epidemiology

Respiratory syncytial virus (RSV) is one of the most important pathogens in early childhood and is the most common cause of bronchiolitis and pneumonia in infancy worldwide.[5][6][7]

RSV usually is transmitted by direct or close contact with contaminated secretions, which may occur from exposure to large-particle droplets at short distances or by self-inoculation after touching contaminated surfaces.[8]

One meta-review of 98 studies noted that, prior to the COVID-19 pandemic era, there had been no discernible change in RSV hospitalisations over the past 20 years.[1] Data suggest that social distancing and other lockdown strategies associated with the pandemic significantly reduced the spread of RSV in the community, and led to a large, temporary decrease in diagnosed RSV and hospitalisation.[9][10][11]​​​ Following the easing of COVID-19 control measures, RSV incidence initially increased during the off-season in many places but has since returned to pre-pandemic epidemic seasonality patterns.[11][12][13][14]​​​​​​

RSV produces significant morbidity and mortality, especially among high-risk infants and those with prematurity, chronic lung disease, complex congenital heart disease, or immune deficiency.[15]

Prematurity (<35 weeks), male sex, age <6 months, birth during the first half of the RSV season, multiple siblings, or daycare exposure have been associated with higher hospitalisation rates.[1]​​​[16][17][18]

In Scotland, a mean of 1976 children per year for the period 2001 to 2003 were admitted to hospital with the principal diagnosis of bronchiolitis.[19] In the US, RSV accounts for 18% of hospital emergency visits and 20% of admissions to hospital due to acute respiratory infections in children under 5 years of age.[20] Among patients with acute respiratory tract infections in China, RSV accounted for 18.7% (95% CI 17.1% to 20.5%) of infections.[21]

In 2015, a global estimate of 33.1 million (uncertainty range [UR] 21.6-50.3) episodes of RSV-acute lower respiratory infection resulted in about 3.2 million (UR 2.7-3.8) hospital admissions and 59,600 (UR 48,000-74,500) in-hospital deaths in children younger than 5 years. In children younger than 6 months, 1.4 million (UR 1.2-1.7) hospital admissions and 27,300 (UR 20,700-36,200) in-hospital deaths were due to RSV-acute lower respiratory infection.[22]

Seasonal outbreaks occur worldwide during the winter months and continue onto early spring.[8][23]​ In the US and throughout the northern hemisphere, epidemics generally begin each November, with a peak in January or February. Cases then decline over the next 2 months with sporadic cases occurring throughout the remainder of the year. Regional variability also occurs but is less predictable.[23]​ In the southern hemisphere, seasonal outbreaks occur from May through September.[23] Tropical regions often have more prolonged circulation in association with the rainy season.

By 2 years of age, nearly all children have been infected with RSV, and half of those have been infected twice. Long-lasting immunity does not occur and re-infection is common, usually with diminished severity.

Adults and older people

There is increasing recognition of the burden of RSV infection in adults and older people.[24][25][26]​ There is growing literature reporting on the impact of RSV infection and other viral disease in long-term care settings, adult day-care centres, and nursing homes.[27][28]​

Although less than 1% of adults affected by RSV are estimated to require hospital care, it is the causative agent in up to 12% of acute respiratory illness requiring medical assistance in the US.[25][29]

Risk factors for progression to viral pneumonia and complications from RSV in adults include immunodeficiency (e.g., patients taking chemotherapy or immunotherapy), underlying lung disease (e.g., asthma, tobacco use, COPD), transplant recipients, heart disease, older age, living in a long term care facility, and frailty.[30]

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