Background: In the context of the circulation of the SARS-CoV-2 B.1.617.2 (Delta) variant, vaccination re-authorised mass indoor gatherings. The "Indoor Transmission of COVID-19" (ITOC) trial (ClinicalTrials.gov, NCT05311865) aimed to assess the risk of transmission of SARS-CoV-2 and other respiratory viruses during an indoor clubbing event among participants fully-vaccinated against COVID-19.Methods: ITOC, a randomised, controlled trial in the Paris region (France), enrolled healthy volunteers aged 18-49 years, fully-vaccinated against COVID-19, with no co-morbidities or symptoms, randomised 1:1 to be interventional group "attendees" or control "non-attendees". The intervention, a 7-hour indoor event in a nightclub at full capacity, with no masking, prior SARS-CoV-2 test result or social distancing required. The primary-outcome measure was the numbers of RT-PCR-determined SARS-CoV-2-positive subjects on self-collected saliva 7 days post-gathering in the per-protocol population. Secondary endpoints focused on 20 other respiratory viruses.Results: Healthy participants (n = 1,216) randomised 2:1 by blocks up to 10, 815 attendees and 401 non-attendees, yielding 529 and 287 subjects, respectively, with day-7 saliva samples. One day-7 sample from each group was positive. Looking at all respiratory viruses together, the clubbing event was associated with an increased risk of infection of 1.59 [95% CI 1.04-2.61].Conclusions: In the context of low Delta-VOC circulation, no evidence of SARS-CoV-2 transmission among asymptomatic and vaccinated participants was found, but the risk of other respiratory virus transmission was higher.