*1.1. Literature Review*

#### 1.1.1. Impacts of the COVID-19 Pandemic on Adolescents

Emerging research has shown that since the COVID-19 pandemic emerged, adolescents are at increased risk for various mental and physical health issues [7,10,11,13,15,16]. In a longitudinal study of adolescents in Shanghai, China conducted from January to March 2020, adolescents' physical activity decreased significantly, from an average of 540 min per week to 105 min per week [13]. In a 22-week longitudinal study of Australian adolescents, there was also a significant decrease in physical activity after the government of New South Wales implemented social distancing policies [17]. Similar findings were also reported in a sample of adolescents in southern Croatia, with physical activity levels particularly decreasing among boys [16].

Adolescents are also at increased risk for negative mental health outcomes since the beginning of the COVID-19 pandemic [10,11,15]. A review of the preliminary literature indicated an increased risk of posttraumatic stress disorder, depressive and anxiety symptoms among adolescents since the pandemic emerged [11]. Additionally, in a longitudinal study with Norwegian adolescents, the prevalence of mental distress increased significantly from February 2019 to June 2020 [15]. Moreover, in a survey of Canadian adolescents, stress related to the pandemic was associated with increased feelings of loneliness and depression [10].

#### 1.1.2. Determinants of Adolescents' Protective Behavior Changes Since COVID-19

Utilizing Bronfenbrenner's ecological perspective on human development, determinants at multiple levels of the ecological system (e.g., individual and family) impact adolescents' protective behavior changes since COVID-19 [18]. Such protective changes may include increased handwashing; wearing a face mask; keeping distance from others; and working or attending school from home [14]. One determinant of protective behavior changes among adolescents that has emerged in the literature at the individual level is gender [16,19–23]. Compared to their female counterparts, male adolescents in Norway, Poland and Jordan, and young adult men in Switzerland were less likely to report protective handwashing behaviors [19–21,23]. Further, female adolescents in Poland and young adult women in Switzerland reported higher compliance with social distancing behaviors than their male counterparts, while no statistically significant difference by gender was found in a sample of adolescents in the United States [20–22]. Across studies, females were also found to be more likely than males to use hand sanitizer, avoid touching their face, and wear a face mask [19–21].

Another determinant of protective behaviors is one's perception of the risk of COVID-19 [24–27]. In an online survey of Chinese adolescents, results showed that perception of COVID-19 risk positively affected their understanding of and participation in social distancing behaviors [27]. Additionally, results of a survey of adults in Qatar found that the more highly they rated the danger of COVID-19, the more likely they were to socially distance [24]. Similarly, in a sample of adults in Hong Kong, a structural equation model revealed that perceptions of COVID-19 risk significantly affected compliance with protective measures (e.g., handwashing, social distancing) [25]. Further, researchers administered a survey to adults in Portugal and found that anxiety regarding COVID-19 and fear of death from COVID-19 significantly predicted protective behaviors, mediated by one's perception of their own perceived risk [26].

Moreover, at the family level, the main determinant that has been examined in the context of protective behavioral changes since COVID-19 is low parental monitoring [21]. Among Swiss young adults, low parental monitoring was associated with lower COVID-19 protective behavior compliance [21]. However, family factors such as conflict and emotion expression were related with COVID-19 stressors [28]. Thus, it is of interest to determine how COVID-19 stressors affect such family factors, which thereby may influence protective behaviors against COVID-19 with the intention to reduce the spread of COVID-19 and associated stressors.
