Study period

The period between February 21, 2020, and April 1, 2020, which was amidst the COVID-19 outbreak, was defined as the ‘outbreak period,’ and the same time periods during 2018 and 2019 were defined as the ‘control period’ to compare patients during the two periods. The start date of the outbreak period was defined as the first day when the number of patients occurred more than 100 per day. The end date of the outbreak period is defined as the first date on which the daily number of patients is maintained at or below 100 until one week after the end date. The dates corresponding to the criteria were from February 21 to April 1, 2020 in Korea.