Inclusion Criteria:

• * ≤ 12 months of age at the time of presentation for evaluation for an acute respiratory infection (ARI).
• * Documentation of an ARI, which is defined as an acute onset (\<10 days) illness that includes: At least two of the following symptoms: fever (measured or subjective), chills, rigors, myalgia, headache, sore throat, nausea or vomiting, diarrhea, fatigue, congestion OR any one of the following: cough, shortness of breath, difficulty breathing, olfactory disorder, taste disorder, confusion, persistent chest pain, pale, gray, hypoxia, clinical or radiographic evidence of pneumonia or respiratory distress syndrome.
• * Residents of Connecticut

Exclusion Criteria:

• * Illness duration of \>10 days at the time of respiratory specimen collection, measured from the date of the first symptom of the current acute illness.
• * Parents/guardians are not able to provide informed consent
• To be eligible for inclusion in the study as a healthy control, an individual must meet all of the following criteria:
• * Immunized against RSV ≤ 12 months of age
• * Residents of Connecticut
• * Match an enrolled case that was a vaccine failure by date of birth (±1 month), sex, and immunoprophylactic agent received (i.e., maternal vaccine vs mAb).