“…The latter included the following known prognostic factors: age, gender, health care worker, presence of comorbid condition, hospital, delay in admission, onset date, lactate dehydrogenase level, chest radiograph, oxygen saturation on room air, neutrophil count, platelets, and lymphocyte count, 20 and 3 other factors that may have been potentially important in treatment assignment, including date of onset, onset-to-admission delay, and hospital. Because some prognostic factors were only available for a subset of up to approximately 30% of the patients in Hong Kong and 60% of the patients in Toronto, 20 we used multiple imputation to facilitate inclusion of these factors in the propensity score model. 27 The resulting propensity scores were then used to weigh the case fatality ratios in each treatment group, allowing estimation of the mean causal effect, 26 that is, for each treatment group we estimated the overall case fatality ratios had all patients been assigned to that particular treatment.…”