In addition to the absolute case number, a rapid increase in the number of COVID-19 cases within a short time results in insufficiency of healthcare systems and further negatively affects patient outcomes. This study was conducted to investigate the association between the outcomes of COVID-19 patients and daily cumulative index (DCI), which was defined as the average daily number of new cases of COVID-19 and calculated by cumulative cases/number of days between the first reported case and March 6, 2020, by country. Spearman's rank correlation analyses were conducted to evaluate the relationship between mortality, incidence, and DCI. In this study, DCI was positively correlated with incidence (adjusted risk ratio [aRR] = 1.01, 95% confidence interval [CI] = 1.00-1.02, P < 0.01). Higher correlation was observed between mortality and DCI (mortality rate: r = 0.397, P = 0.018; mortality per 1 000 000 people: r = 0.0.428, P = 0.004) than between disease incidence and DCI. DCI remained statistically significantly associated with mortality per 1 000 000 people after adjustment of Health Care Index (aRR = 1.02, 95% CI = 1.01-1.03, P < 0.001) or Healthcare Access and Quality Index (aRR = 1.02, 95% CI = 1.01-1.04, P < 0.01. Reducing DCI through strict infection control measures can help slow the number of new COVID-19 cases and further improve outcomes in COVID-19 patients.