Inclusion Criteria:  admitted to the intensive care unit of the Amsterdam UMC, location 'AMC', between March 1, 2020 and May 31, 2020 (the first 3 months of the first wave of the national outbreak in the Netherlands) and between October 1, 2020 and December 31, 2020 (the first 3 months of the second wave in the Netherlands); RT-PCR confirmed coronavirus diseases 2019 (COVID-19); and having had received at least one chest radiograph (CXR) and one adjacent chest computed tomography (CT) scan during the period a patient was having acute respiratory failure.  Exclusion Criteria:  Age < 18 years.