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BACKGROUNDDue to the dissemination of vaccination against severe acute respiratory syndrome coronavirus 2 in the elderly, the virus-susceptible subjects have shifted to unvaccinated non-elderlies. The risk factors of COVID-19 deterioration in non-elderly patients without respiratory failure have not yet been determined. This study was aimed to create simple predicting method to identify such patients who have high risk for exacerbation.METHODSWe analyzed the data of 1,675 patients aged under 65 years who were admitted to hospitals with mild-to-moderate COVID-19. For validation, 324 similar patients were enrolled. Disease progression was defined as administration of medication, oxygen inhalation and mechanical ventilator starting one day or longer after admission.RESULTSThe patients who exacerbated tended to be older, male, had histories of smoking, and had high body temperatures, lower oxygen saturation, and comorbidities such as diabetes/obesity and hypertension. Stepwise logistic regression analyses revealed that comorbidities of diabetes/obesity, age ≥ 40 years, body temperature ≥ 38°C, and oxygen saturation < 96% (DOATS) were independent risk factors of worsening COVID-19. As a result two predictive scores were created: DOATS score, which includes all the above risk factors; and DOAT score, which includes all factors except for oxygen saturation. In the original cohort, the areas under the receiver operating characteristic curve of the DOATS and DOAT scores were 0.789 and 0.771, respectively. In the validation, the areas were 0.702 and 0.722, respectively.CONCLUSIONWe established two simple prediction scores that can quickly evaluate the risk of progression of COVID-19 in non-elderly, mild/moderate patients.SummaryThe risk stratification models using independent risks, namely comorbidity of diabetes or obesity, age ≥ 40 years, high body temperature ≥ 38□, and oxygen saturation < 96%, DOATS and DOAT scores, predicted worsening COVID-19 in patients with mild-to-moderate cases.