The Co-FriSero study describes a COVID-19 outbreak at the Friedrichroda hospital in Thuringia, Germany, with 185 beds and 404 employees, at the onset of the pandemic between March 30th, 2020, and April 13th, 2020. This study aimed to analyze potential sources of SARS-CoV-2 transmission amongst hospital employees.After the outbreak, a comprehensive follow-up was conducted through a questionnaire and a seroprevalence study using two different immunoassays for IgG detection and a third for discordant results.PCR screenings confirmed SARS-CoV-2 infection in 25 of 229 employees, with an additional 7 detected through serology. Statistical analysis indicated that direct patient contact, exposure to high flow ventilation in non-isolated rooms, direct contact with colleagues, shared use of recreational rooms, and carpooling were associated with an increased infection risk. Conversely, contact with family and friends, public transportation, public events, and use of locker rooms were not associated with infection. Male gender showed a lower infection likelihood, independent of age and other risk factors.This study highlights the role of direct patient care and internal staff interactions in the spread of SARS-CoV-2 in the hospital setting. It suggests that non-traditional transmission routes like carpooling require consideration in pandemic preparedness.