Introduction - On July 21, 2020, a cluster of COVID-19 cases was reported among young adults. The Brittany regional unit of Santé publique France, in collaboration with the Regional Health Agency (ARS), conducted an epidemiological investigation aimed at breaking the chains of transmission. Method - A primary case or contact was defined as any person present in Quiberon from July 17 to August 5, 2020, who underwent a SARS-CoV-2 RT-PCR (reverse transcription-polymerase chain reaction) test, who had been in contact with the index case, or who had visited the bars, the index case’s campground, or the beaches at night. Secondary, tertiary, or quaternary cases or contacts were contacts of the previous cases who underwent a SARS-CoV-2 RT-PCR test. Factors associated with the risk of SARS-CoV-2 infection were identified using multivariate logistic regression. Results - For the analysis, 633 individuals (cases or contacts) were included out of 716 identified individuals. The SARS-CoV-2 infection attack rate was 60.9% among primary cases, 7.6% among secondary cases, and 4.9% among tertiary cases. Symptoms were present in 54.8% of primary cases. Age (median = 19.4) was identified as a risk factor for SARS-CoV-2 infection (primary and secondary cases) (odds ratio: OR = 0.97; 95% confidence interval (95% CI): [0.95–0.99]). Attending the implicated nightclub was associated with a 3.07-fold [1.06–9.45] higher risk of developing this infection. Furthermore, individuals in tier 1 had a 20.3-fold [12.09–34.08] higher risk of infection than those in higher tiers. Discussion - These results support the hypothesis that the occurrence of COVID-19 cases is linked to visits to the implicated bar among young adults attending summer parties. In the face of this epidemic, young adults continue to be heavily affected due to a failure to adhere to preventive measures, particularly in enclosed spaces (bars, nightclubs), and at parties where alcohol is often consumed and social distancing is rarely observed.