2.1. Study Design and Participants
This longitudinal study evaluated KRG intake’s effect on COVID-19 antibodies after vaccination in Korean adults (Clinical Research Information Service, KCT0007342). The study was approved by the Institutional Review Board of Yongin Severance Hospital (IRB no. 9-2021-0101) and was conducted in compliance with the Declaration of Helsinki. Written informed consent was provided by all patients prior to participation. Sample size was calculated based on the following assumptions: the effect size (cohen’s d) 0.25, type I error of 0.05, power of 80%, and 1:2 ratio (KRG/control ratio). The number of subjects to be enrolled was determined to be 190 for the KRG group and 380 for the control group. The originally intended total sample size was 570, but the required number of enrollments was not achieved within the given study period. The KRG dose used in this study was 3 g of KRG tablet/day, containing ginsenoside, Rb1 (8.03 mg/g), Rb2 (2.80 mg/g), Rg3 (2.50 mg/g), Rg1 (1.18 mg/g), Rc (3.29 mg/g), Rf (1.47 mg/g), Re (1.29 mg/g), and Rd (1.0 mg/g). The KRG tablets were made by dehydrating extracts of KRG. A total of 534 subjects who received a COVID-19 vaccine were recruited in August 2021 and continued with follow up. Subjects were assigned to two groups: the KRG group was taking KRG continuously before the study KRG per day for four weeks from enrollment, and the control group did not take KRG. In total, 174 subjects were included in the KRG group and 331 in the control group. Because of incomplete follow up or refusal of a third dose of COVID-19 vaccine administration, 155 subjects were excluded. Thus, there were 149 subjects in the KRG group and 201 in the control group. All subjects completed five visits (