Twenty-seven participants, including 25 singers, 1 conductor and 1 accompanist attended a choir practice on March 12, 2020. The practice was indoor and took place in a non ventilated space of 45 m2. No choir member reported having been symptomatic for COVID-19 between March 2 and March 12.The mean age of the participants was 66.9 (range 35-86) years. The secondary attack rate was 70%: 19/27 participants were diagnosed with COVID-19 from 1 to 12 days after the rehearsal with a median of 5.1 days. Thirty-six percent of the cases needed a hospitalization (7/19), and 21% (4/19) were admitted to an ICU. The index cases were asymptomatic and possibly multiple. CONCLUSION: In the absence of valid barrier measures to prevent COVID-19 transmission, indoor choir practice should be suspended during the SARSCoV-2 surging phases. Transmission of the virus among gatherings from asymptomatic cases is a crucial issue and a main challenge to COVID-19 control.