Daily Life Patterns, Psychophysical Conditions, and Immunity of Adolescents in the COVID-19 Era: A Mixed Research with Qualitative Interviews by a Quasi-Experimental Retrospective Study

Background: This study investigated the daily lifestyle changes, prevalence of psychological depression, physical health status, and immunity of adolescents in Korea resulting from increased isolation and social restriction due to the COVID-19 pandemic. Materials and Methods: All subjects included 17-year-old male adolescents. A total of 117 subjects were assigned to one of four groups according to the degree of depression based on item #6 in the Center for Epidemiologic Studies Depression (CES-D) questionnaire as follows: no-depression group (NDG, n = 71; 61.0%), low-depression group (LDG, n = 23; 19.0%), moderate-depression group (MDG, n = 15; 13.0%), and high-depression group (HDG, n = 8; 7.0%). This study analyzed the data using quantitative and qualitative methods to understand how the COVID-19 pandemic affects adolescents’ daily lives, psychophysiological conditions, and immune function. Results: This study found that the COVID-19 pandemic significantly affects the daily lifestyle pattern, psychophysical condition, and immunocytes of adolescents. In terms of depression, 39.0% of adolescents felt depressed, and 7% of them felt depressed almost every day. Overall, HDG considered themselves unhealthy and felt prone to immune diseases, such as colds. HDG were prone to sleep late, eat more frequently, and work out less. Regarding physical fitness factors, the cardiorespiratory endurance, strength, and power of HDG were significantly lower than those of NDG, LDG, and MDG. Moreover, HDG had the worst body composition, including the lowest muscle mass. Finally, natural killer (NK) cells and T cells were significantly different among groups, with the levels in HDG being significantly lower than those of the other three groups. Conclusions: Since the COVID-19 pandemic negatively affects the daily lives, psychophysical conditions, and immunocytes of adolescents, there is an urgent need to create and provide solutions to adolescents with depression though the number of subjects is few.