BronchStop Study Protocol, Season 2.

Introduction: In 2021 we launched the BronchStart study, which collected information on 17,899 hospital attendances in children with serious respiratory tract infections following the release of lockdown restrictions. Our study informed the Joint Committee on Vaccination and Immunisation's (JCVI) decision to recommend the introduction of maternal respiratory syncytial virus (RSV) vaccination, which was rolled out in the United Kingdom in August/September 2024 for all pregnant women at a gestation of 28 weeks or more. That winter we performed the BronchStop study, which examined vaccine effectiveness in its first season, conducted a survey of mothers to understand factors affecting vaccine uptake, and collected RSV positive samples for molecular epidemiology. Methods and analysis: In the winter season of 2025-2026 we will conduct a UK-wide, multi-centre, prospective, test-negative case control study. The aim is to assess the effectiveness of maternal RSV vaccination against hospitalisation for RSV-associated acute lower respiratory tract infection (ALRI) amongst infants under the age of 6 months born to vaccine-eligible pregnant mothers. A survey designed in partnership with our public and patient involvement (PPI) group will be administered to mothers of recruited infants to understand factors affecting maternal vaccine uptake. RSV-positive samples will undergo whole genome sequencing, and all samples will undergo real-time, reverse transcriptase polymerase chain reaction (rRT-PCR) testing for a panel of respiratory viruses to understand residual causes of severe infant respiratory disease in the post-vaccination era. Ethics and dissemination: Participants recruited to the study will be asked for informed consent to participate in the maternal survey, for researchers to access their vaccination records, and for routinely collected virological samples from their infants to undergo rRT-PCR testing. Regular reports to advisory groups, including JCVI and the World Health Organisation, and for peer-reviewed publications are planned to disseminate findings and inform decision-making.