BackgroundThe volume-outcome relationship in patients with severe Coronavirus disease 2019 (COVID-19) is unclear and is important for establishing a system for the medical care of severe COVID-19. This study aimed to evaluate the association between institutional case volume and outcomes in patients with ventilated COVID-19.MethodsWe analyzed patients with severe COVID-19 on ventilatory control aged > 17 years who were enrolled in the J-RECOVER study, which is a retrospective multicenter observational study conducted between January 2020 and September 2020 in Japan. Based on the ventilated COVID-19 case volume, the higher one-third of institutions were defined as high-volume centers, the middle one-third as middle-volume centers, and the lower one-third as low-volume centers. The primary outcome measure was in-hospital mortality during hospitalization due to COVID-19. Multivariate logistic regression analysis for in-hospital mortality and ventilated COVID-19 case volume was performed after adjusting for multiple propensity scores and in-hospital variables. To estimate the multiple propensity score, we fitted a multinomial logistic regression model, which fell into one of the three groups based on patient demographics and prehospital factors.ResultsWe analyzed 561 patients who required ventilator management. In total, 159, 210, and 192 patients were admitted to low-volume (36 institutions, < 11 severe COVID-19 cases per institution during the study period), middle-volume (14 institutions, 11–25 severe cases per institution), and high-volume (5 institutions, > 25 severe cases per institution) centers, respectively. After adjustment for multiple propensity scores and in-hospital variables, admission to middle- and high-volume centers was not significantly associated with in-hospital death compared with admission to low-volume centers (adjusted odds ratio, 0.77 [95% confidence interval (CI): 0.46–1.29] and adjusted odds ratio, 0.76 [95% CI: 0.44–1.33], respectively).ConclusionsThere may be no significant relationship between institutional case volume and in-hospital mortality in patients with ventilated COVID-19.