During the COVID-19 pandemic, some children experienced psychological distress. Moreover, pandemic-related stressors were associated with changes in hair cortisol concentrations (HCC) in youth. Research has shown that parental distress influenced children's well-being during the pandemic, but it remains unclear whether parental distress is associated with children's HCC during the pandemic. Furthermore, as some preliminary evidence suggests that children's HCC may predict their emotional response to the pandemic, it is essential to assess whether children's HCC provides insight into their susceptibility to developing symptoms associated with stress-related psychopathologies. The present study aimed to (1) examine the association between parental pandemic-related distress and children's HCC
(2) investigate the moderating role of parental distress on the association between parent and child HCC
and (3) explore the association between children's HCC and their distress longitudinally. In June 2020, 71 parent-child (8-15 y/o) dyads provided a hair sample to assess pre-pandemic HCC (December 2019-March 2020) and pandemic HCC (March-June 2020) in Quebec, Canada. Post-traumatic stress, anxiety, depression, and stress symptoms were also assessed in dyads every three months from June 2020 to March 2021. Results showed that parental stress symptoms and HCC were positively associated with children's HCC during the pandemic. Moreover, children's pre-pandemic and pandemic HCC were independently negatively associated with children's anxiety symptoms during the pandemic. These results provide evidence of an association between parental physiological and psychological stress and their children's HCC during the pandemic and suggest that HCC may help identify youth at risk of developing anxiety symptoms during chronic stressful events.