Post-traumatic stress symptoms (PTSS) were the most frequently reported mental health concern for youth during COVID-19, yet variations in youth's PTSS responses warrant empirical consideration. Features of the caregiving environment influence youth's responses to environmental stressors, and youth's parasympathetic nervous system regulation may qualify the magnitude and/or direction of these effects. This prospective investigation evaluated diathesis stress and differential susceptibility models of caregiving and parasympathetic influences on youth's PTSS responses to COVID-19.

Participants were 225 caregiver-youth dyads (youth 49.8% female at birth; 88.4% non-white) followed from childhood through adolescence and COVID-19. Youth's resting respiratory sinus arrhythmia (RSA; Mage = 6.11, s.d. = 0.21), caregiving features (i.e. attachment security [youth Mage = 12.24, s.d. = 0.35] and caregiver internalizing psychopathology [caregiver Mage = 39.29, s.d. = 6.78]) were assessed pre-pandemic. Youth's PTSS was assessed one year prior to the US COVID-19 pandemic (Mage = 14.24, s.d. = 0.50) and during the spring of 2020 at the height of the pandemic (Mage = 15.23, s.d. = 0.57).

Youth's PTSS increased during COVID-19. Youth with relatively high resting RSA evidenced the lowest PTSS when their caregiving environment featured high attachment security or low caregiver internalizing problems, but the highest PTSS when their caregiving environment featured low attachment security or high caregiver internalizing problems. In contrast, PTSS levels of youth with relatively low or average resting RSA did not differ significantly depending on attachment security or caregiver internalizing.

Results are consistent with a differential susceptibility hypothesis, wherein relatively high resting RSA conferred heightened sensitivity to caregiving environments in a for-better-and-for-worse manner during COVID-19.