3 RESULTS
In South Korea, a total of four COVID-19 waves occurred between January 20, 2020 to January 31, 2022. We examined the distributions of daily Ct values, new cases, and Rt and analyzed their associations for the entire period and by the periods of four COVID-19 waves.
During the entire period, Ct values declined in general, indicating an increased viral load over time. In terms of the new cases, the number of new cases surged after a substantial drop of Ct values. During the first wave, the number of new cases rose rapidly at a point in which Ct values decreased, and Ct values increase gradually and subsequently declined with a rapid drop prior to the onset of the second wave. During the second wave, the number of new cases surged after the Ct values hit bottom, and Ct values rose gradually and subsequently dropped, with a marked decline until the onset of the third wave. During the third wave, the number of new cases also surged after the Ct values hit bottom, and the Ct values rose substantially and then rose again. During the fourth wave, the Ct values remained high with a smaller degree of change compared to that in previous periods, and although the number of new cases increased when Ct values decreased, the degree of increase in the number of new cases was very large (Figure 1).
The median Ct values were compared by COVID-19 wave. The median Ct value was the highest (31.71) in the first wave and decreased over time, with the lowest value (21.27) in the fourth wave (Figure 2).
By categories of Ct values, the percentage of Ct values < 25 increased whenever the number of new cases surged during the first, second, and third waves. However, during the fourth wave, the percentage of Ct values < 25 remained high regardless of the number of new cases (Figure 3).