Purpose During the COVID-19 pandemic in the Netherlands, governmental regulations resulted in a lockdown for adults as well as children/adolescents. Schools were closed and contact with other people was limited. In this cross-sectional, population-based study, we aimed to investigate the mental/social health of children/adolescents during COVID-19 lockdown. Methods Two representative samples of Dutch children/adolescents (8–18 years) before COVID-19 (2018, N = 2401) and during lockdown (April 2020, N = 844) were compared on the Patient-Reported Outcomes Measurement Information System (PROMIS) domains: global health, peer relationships, anxiety, depressive symptoms, anger, sleep-related impairment by linear mixed models and calculating relative risks (RR (95% CI)) for the proportion of severe scores. Variables associated with worse mental/social health during COVID-19 were explored through multivariable regression models. The impact of COVID-19 regulations on the daily life of children was qualitatively analyzed. Results Participants reported worse PROMIS T-scores on all domains during COVID-19 lockdown compared to before (absolute mean difference range 2.1–7.1 (95% CI 1.3–7.9). During lockdown, more children reported severe Anxiety (RR = 1.95 (1.55–2.46) and Sleep-Related Impairment (RR = 1.89 (1.29–2.78) and fewer children reported poor Global Health (RR = 0.36 (0.20–0.65)). Associated factors with worse mental/social health were single-parent family, ≥ three children in the family, negative change in work situation of parents due to COVID-19 regulations, and a relative/friend infected with COVID-19. A large majority (> 90%) reported a negative impact of the COVID-19 regulations on daily life. Conclusion This study showed that governmental regulations regarding lockdown pose a serious mental/social health threat on children/adolescents that should be brought to the forefront of political decision-making and mental healthcare policy, intervention, and prevention.

Importance: It is unknown how a lockdown during the COVID-19 pandemic impacts childrens and adolescents mental and social health. Objective: To compare mental and social health of children and adolescents during the COVID-19 lockdown versus before, identify associated factors, describe the change in atmosphere at home and qualitatively assess the impact of COVID-19 regulations on daily life. Design: Cross-sectional study comparing two Dutch representative samples of children and adolescents (8-18 years); before COVID-19 (Dec2017-July2018) and during the COVID-19 lockdown (April/May 2020). Setting: Population-based Participants: Children and adolescents aged 8-18 years (M=13.4, 47.4% male), representative of the Dutch population on key demographics. Exposure(s): COVID-19 pandemic lockdown Main Outcome(s) and Measure(s): Patient-Reported Outcomes Measurement Information System (PROMIS) domains: Global Health, Peer Relationships, Anxiety, Depressive Symptoms, Anger and Sleep-Related Impairment. Single item on atmosphere at home and open question regarding the impact of the regulations on the childs/adolescents daily life Results: Children and adolescents reported significantly worse PROMIS T-scores on all domains (absolute mean difference range, 2.1-7.1; absolute 95% CI range, 1.3-7.9) during the COVID-19 lockdown as compared to before COVID-19. More children reported severe Anxiety (during 16.7% vs. before 8.6%; relative risk 1.95; 95% CI 1.55-2.46) and Sleep-Related Impairment (during 11.5% vs. before 6.1%; relative risk 1.89; 95% CI 1.29-2.78). Fewer children reported poor Global Health (during 1.7 vs. before 4.6%; relative risk 0.36; 95% CI 0.20-0.65). More mental and social health complaints during the COVID-19 lockdown were found in children and adolescents growing up in a single-parent family, having >three children in the family, a negative change in work situation of parents due to COVID-19 regulations, and having a relative/friend infected with COVID-19. A small effect was found on atmosphere at home during the lockdown compared to before (mean difference, -3.1; 95% CI, -4.1 - -2.1). A large majority (>90%) reported a negative impact of the COVID-19 regulations on their daily life. Conclusions and Relevance: This study showed that governmental regulations regarding lockdown pose a serious mental and social health threat on children and adolescents that should be brought to the forefront of political decision making and mental health care policy, intervention and prevention.