text: SARS CoV 2 nosocomial infection acquired in a French university hospital during the 1st wave of the Covid 19 pandemic a prospective study In healthcare facilities nosocomial transmissions of respiratory viruses are a major issue SARS CoV 2 is not exempt from nosocomial transmission Our goals were to describe COVID 19 nosocomial cases during the first pandemic wave among patients in a French university hospital and compliance with hygiene measures We conducted a prospective observational study in Grenoble Alpes University Hospital from 01 03 2020 to 11 05 2020 We included all hospitalised patients with a documented SARS CoV 2 diagnosis Nosocomial case was defined by a delay of 5 days between hospitalisation and first symptoms Hygiene measures were evaluated between 11 05 2020 and 22 05 2020 Lockdown measures were effective in France on 17 03 2020 and ended on 11 05 2020 Systematic wearing of mask was mandatory for all healthcare workers HCW and visits were prohibited in our institution from 13 03 2021 and for the duration of the lockdown period Among 259 patients included 14 5 4 were considered as nosocomial COVID 19 Median time before symptom onset was 25 days interquartile range 12 42 Eleven patients 79 had risk factors for severe COVID 19 Five died 36 including 4 deaths attributable to COVID 19 Two clusters were identified The first cluster had 5 cases including 3 nosocomial acquisitions and no tested HCWs were positive The second cluster had 3 cases including 2 nosocomial cases and 4 HCWs were positive Surgical mask wearing and hand hygiene compliance were adequate for 95 and 61 of HCWs respectively The number of nosocomial COVID 19 cases in our hospital was low Compliance regarding mask wearing hand hygiene and lockdown measures drastically reduced transmission of the virus Monitoring of nosocomial COVID 19 cases during the first wave enabled us to determine to what extent the hygiene measures taken were effective and patients protected Trial registration Study ethics approval was obtained retrospectively on 30 September 2020 CECIC Rhône Alpes Auvergne Clermont Ferrand IRB 5891 