Establishing a human co-infection challenge study of Streptococcus pneumoniae (Spn) and live attenuated influenza vaccine (LAIV) in older healthy adults: Exploring Co-infection with Live Attenuated Influenza Vaccine and PneumococcuS in healthy oldEr adults (ECLIPSE), a UK-based single-centre, single-blind randomised controlled trial

Introduction Secondary bacterial pneumonia following influenza is a significant cause of morbidity and mortality in older adults. The Exploring Co-infection with Live Attenuated Influenza Vaccine and PneumococcuS in healthy oldEr adults study investigates how the order of exposure to Streptococcus pneumoniae serotype 6B (Spn6B) and live attenuated influenza vaccine (LAIV) affect mucosal immunity, pathogen load and respiratory symptoms in adults over 55 years of age. Immunisation with LAIV is a safe alternative to nasal challenge with live influenza virus because it is cold-adapted and cannot replicate in the lower respiratory tract, reducing the risk of disease in older study participants.Methods and analysis This is a single-centre, single-blind randomised controlled human co-infection study (protocol V.4.2, 20 January 2026). Up to 60 healthy adults aged between 55 and 80 years will be randomised 1:1 for intranasal immunisation with LAIV first followed by inoculation with Spn6B or intranasal inoculation with Spn6B first followed by immunisation with LAIV with a 3-day interval between exposures. Throughout the study, we will collect samples from the nose, throat, saliva and blood to analyse how the immune system responds. Participants will also complete a daily symptom diary. Participants will collect saliva and nasosorption samples at home.A control cohort of up to 20 participants (up to 10 adults aged 18–49 years and up to 10 adults aged 55–80 years) will enable age-stratified comparisons of viral load and mucosal immune responses to immunisation with LAIV only. This cohort will not be randomised.The primary outcome is feasibility of the study and the safety of participants. The secondary outcomes include respiratory symptoms, how the order of exposure affects pneumococcal carriage, LAIV viral load and nasal immune responses over time as well as the impact of asymptomatic infection with other respiratory pathogens at baseline on these parameters. The study will improve our understanding of immune responses to co-infection with bacterial and viral pathogens in the upper respiratory tract in older adults and why they are at increased risk of pneumonia.Ethics and dissemination Ethical approval for this study has been obtained from the East Midlands–Leicester South Research Ethics Committee (REC reference 24/EM/0200). The results will be disseminated via publication in a peer-reviewed journal and presentation at international conferences.Trial registration number ISRCTN13284643.