Bronchiolitis is a very common winter disease that normally affects children less than one year of age. It is a common reason for parents and carers to bring their child to an Emergency Department (ED) and the frequent need for hospital admission means that paediatric units are at their capacity each winter. During the COVID19 pandemic the virus that causes bronchiolitis (Respiratory Syncytial Virus; RSV) disappeared meaning this winter there have been virtually no cases of bronchiolitis in the United Kingdom. This phenomenon has been observed in many other countries around the world. Evidence from Australia suggests as restrictions such as social distancing for COVID19 are relaxed bronchiolitis returns, even in the summer. At the release of lockdown the return has been so dramatic in some areas of Australia the summer time numbers are above a typical winter. There is also evidence it may affect older children up to 2 years of age. It is likely the Australian experience will be mirrored in the UK. The ability to track, anticipate and respond to a surge in bronchiolitis is important. There is a need to understand: 1. the onset of RSV spread at the earliest opportunity. This is important as some children are at higher risk of hospitalisation, intensive care admission or death if they contract RSV; knowing when to passively immunise these children is a public health priority. 2. whether the population at risk is a wider age range than normal and whether disease severity is greater as these will both effect service planning; There are currently no existing studies or surveillance systems fully able to address these questions. This study will use staff in Emergency Department to report, in real time, case of bronchiolitis that they see and record essential, but non-identifying, information about them. In 2022/23 the study evolved from a prospective surveillance study into a genomic analysis study with sites collecting positive respiratory samples for RSV genomic review. In 2024/25 the study was amended in light of the introduction of the Bivalent Prefusion F Vaccine in Pregnancy in the United Kingdom to undertake a Vaccine Effectiveness study. The main inclusion criteria remains the same but mothers who would have been eligible for the vaccine, and deliver an infant during the RSV season, were consented to obtain vaccination details so these could be linked to BronchSTART records.