Nosocomial case (NC) of COVID-19 infections is a challenge for hospitals. We report the results of a seven-month prospective cohort study investigating COVID-19 patients to assess unexpected cases (UC) (no COVID-19 precautionary measure application since admission) and NC. Investigation by an infection control team of 844 patients with COVID-19 infection hospitalized for more than 24 hours (cases). A total of 301 UC were identified (31% after contact tracing) with a total of 129 contact patients, and 27 secondary cases for 59 of them. In geriatric wards, 50% of cases were UC. NC represented 18% of cases (37% in geriatric wards), mainly identified after contact tracing of wandering cases. A rapid infection control response is essential to contain nosocomial transmission, along with detailed contact tracing and screening policy. Dealing with wandering elderly patients remain challenging for HCWs.