THRIVE - Trial of Passive Humoral RSV Immunity for Value and Effectiveness

Inclusion Criteria: 1. Aboriginal and/or Torres Strait Islander infant ≥ 6 calendar months old and \< 9 calendar months old. 2. Parent/caregiver is willing for their infant to participate in the study and informed consent for the infant's participation in the study has been given. 3. Parent/caregiver is willing to comply with all study procedures outlined in the protocol, including review of maternal/ infant immunisation records, electronic medical records and public health notifications, for the duration of the study. Exclusion Criteria: 1. Infants with a contra-indication to RSV-SMA per the Australian Immunisation Handbook (i.e. anaphylaxis to a prior dose). 2. Infants who have received a prior dose of RSV-SMA at ≥ 3 calendar months old. 3. Previously enrolled in this trial. Temporary Exclusion Criteria 1. Infants who have received a prior dose of RSV-SMA between ≥ 1 calendar months old and \< 3 calendar months old will be excluded until at least 150 days have passed since their most recent dose. Randomisation can be delayed until participants meet this criterion. 2. Acute illness at the time of assessment (e.g. fever ≥ 38.5°C, acute respiratory or other infection as determined by trained and delegated study staff) is temporarily excluded until they are recovered and/or symptom-free for ≥ 24 hours.