The relationship between RSV (human respiratory syncytial virus) infections and invasive infections or pneumonia caused by Streptococcus pneumoniae has been observed epidemiologically for several years. Few fundamental data exist to support the epidemiological relationship. This study will investigate the proportion of pneumococcal carriage in RSV-infected infants and noninfected infants (proportion, serotype profile).

The relationship between RSV (human respiratory syncytial virus) infections and Streptococcus pneumoniae pneumonia has been suggested for many years based on a similar seasonal pattern and cohort studies, but it could not be confirmed due to the circulation of several viruses concurrently. The post-COVID-19 period was marked by a temporal association between RSV infections and invasive pneumococcal disease (and pneumonia) in children under five, highlighting the potential role of RSV in the dynamics of pneumococcal disease in infants.

Furthermore, the carriage rate of S. pneumoniae in infants under 2 years of age in healthy populations is described as around 25%. In the case of RSV infection, this rate rises to 50%. Some data from animal models have shown that RSV infection increases S. pneumoniae transmission within a group of individuals, partly explaining this difference. However, data remain limited, particularly in human samples, and further research is needed to better understand the role of the RSV-pneumococcal relationship in the occurrence of severe RSV infections (pneumonia) and invasive pneumococcal infections. This study will examine pneumococcal carriage in bronchiolitis in infants aged 6-18 months, through nasopharyngeal swabs from infants with bronchiolitis and healthy infants. Pneumococcal carriage rates and serotype profiles will be investigated.

Inclusion Criteria:

• Infants aged 6 months to 18 months inclusive
• With bronchiolitis during RSV epidemic season (bronchiolitis group) or without evidence of RSV viral infection (healthy group)
• No chronic illness
• No history of bronchiolitis
• Signed consent from parents or legal guardians
• Patient affiliated to a social security scheme

Exclusion Criteria:

• Chronic respiratory illness
• Medical history of bronchiolitis or newborn asthma
• Treatment with immunosuppressants
• Patient on AME

The included population will consist of infants aged 6 to 18 months, divided into two groups. On one hand, infants with RSV bronchiolitis (regardless of severity) and on the other hand, RSV-negative infants (consulting for other reasons, such as trauma, jaundice, etc.). The recruitment age is justified by the peak occurrence of invasive pneumococcal infections, which typically occurs around 12 months of age.