Background: Coronavirus OC43 infection causes severe pneumonia in patients with comorbidities, but clinical signs alone are insufficient to identify the virus. Objectives: To analyze the acute manifestations of Coronavirus OC43 infections and the outcomes of patients admitted to an intensive care unit (ICU). Patients and methods: A retrospective, single-center study conducted during a Coronavirus OC43 outbreak. We used multiplex PCR to detect an OC43 outbreak on Reunion Island during the 2016 Southern Hemisphere winter: seven admissions to the ICU. Results: The mean age of patients was 71 [67;76] years; SAPS II was 42 [28;53]; the pneumonia severity index was 159 [139;182] compared to 73 [40.5;107] for patients in medical wards; and 43% required mechanical ventilation. Comorbidities included diabetes mellitus (87%), chronic respiratory failure (57%), and chronic renal failure (29%). One patient died from Haemophilus influenzae co-infection. Conclusion: As with MERS-CoV infections, underlying comorbidities influenced the clinical outcomes of OC43 infections.