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Background: COVID-19 (coronavirus disease 2019) outbreaks in workplace settings have been investigated to understand how transmission occurred. However, there is limited data looking at COVID-19 transmission in conference settings in England, particularly where an outbreak did not occur. The aim of this work was to investigate COVID-19 infection risk factors and control measures at a large conference, with a known case but no reported onward transmission to inform prevention of future outbreaks of respiratory infections in conferences and similar settings. Methods: This cross-sectional study was part of a wider COVID-19 Outbreak Investigation to Understand Transmission (COVID-OUT) study. A two-day in-person conference on SARS-CoV-2 transmission and environment was held at a university conference centre on 17–November 18, 2021, in England, with about 100 delegates. A questionnaire survey was conducted among 50 conference attendees to identify any confirmed cases and understand transmission, history of COVID-19 symptoms, testing and vaccination. Results: One person met the definition of a confirmed case at the conference. This case was most likely infectious when attending the conference, however there were no known secondary cases. All respondents reported receiving at least two doses of a COVID-19 vaccine before the conference and an increased frequency of handwashing/sanitising hands during the study period in comparison to before the pandemic. Prior to the conference, a COVID-19 risk assessment including a review of the ventilation at the site was completed. All attendees were advised to take an LFD test before travelling to the conference, wear face coverings, and maintain 1-m distance during the conference. Conclusion: A multipronged approach, encouraging attendee behaviours (regular hand washing, mask wearing, being vaccinated against COVID-19) and introducing control measures at the conference site (ventilation, sufficient spacing capacity, combined with prior knowledge of COVID-19 transmission, were effective in limiting the spread of COVID-19 in this setting.