The SR was registered on the International Prospective Register of SR (PROSPERO) (CRD42022303714) (Appendix 1). This protocol is prepared in accordance with the Preferred Reporting Items for Systematic review and Meta-Analysis Protocols (PRISMA-P) statement  (Appendix 2); the PROSPERO record will be updated regularly.
The final SR will be conducted according to updated PRISMA statement  and will follow the guidelines of the actual Cochrane Handbook for SR, if applicable .
Eligibility criteria and information sources
Based on the examination of an environmental exposure, namely the COVID-19 pandemic, the research question was formulated within a Population-Exposure-Comparison-Outcome (PECO) scheme :
Population: General population ≤19 years in Europe
Exposure: COVID-19 pandemic
Comparison: Pre-pandemic baseline
Outcome: Depression or anxiety
The eligibility criteria were conducted in accordance to the PECO scheme and are presented in Table 1. Also, further categories were defined, e.g. effect measure, study design, language, time frame and publication status.
The search plan includes the following databases: MEDLINE (Pubmed), EMBASE, PsycINFO, Cochrane Central Register of Controlled Trials (CENTRAL), Web of Science and WHO COVID-19 database (also including pre-prints). Also, study registries (e.g. PROSPERO), relevant grey literature (e.g. government reports), related articles, reference lists of included articles and previous published reviews will be screened. Translating the research question into a search string was performed in accordance with the guideline for Peer Review of Electronic Search Strategies (PRESS)  and with the use of validated or recommended search filters where possible. The search strategy was peer reviewed before the searches were run by an expert in conducting SR in health sciences according to the evidence-based checklist PRESS Evidence-Based Checklist  to ensure a high-quality search strategies (search submission and peer review assessment are attached in Appendix 3 and 4). The draft search strategy for Pubmed is presented in Appendix 5.
The literature search was performed on March 18, 2022. Study selection will be conducted in three steps: (1) duplicates removal; (2) screening at title and abstract level; and (3) screening the full text. Several publications with the equal or similar study population will be considered once; duplicates with e.g. smaller sample sizes will be excluded. Any discrepancies between the two reviewers will be discussed and, if necessary, resolved by a third author. The reasons for study exclusion in step 3 will be reported in the Appendix of the final study. All screening procedures will be presented using the PRISMA flow diagram .
Data extraction will be conducted by two authors using specially developed (pilot tested) tabular data collection forms . Any discrepancies between the two reviewers will be discussed extensively and, if necessary, resolved by a third author.
Risk of bias assessment
The risk of bias (RoB) will be assessed using the RoB instrument for non-randomized studies of exposures . This instrument contains seven RoB items; the defined criteria for each category to assess the included studies will be described in the Appendix of the final study. Judgments for each RoB item and the overall study-judgement could be: “Low RoB”, “Moderate RoB”, “Serious RoB”, or “Critical RoB”.
The certainty of evidence for each outcome will be evaluated by using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach adapted to the use of non-randomized studies . GRADE assessments are determined through eight domains for study downgrading or upgrading. The use of the RoB instrument for non-randomized studies will allow to start at “high” initial certainty of evidence within GRADE . The GRADE approach finally specifies four levels of the certainty for a body of evidence for each outcome: high, moderate, low and very low.
The ratings for RoB and GRADE will be separately conducted by two reviewers, and disagreement will be resolved by discussion with involvement of a third author where necessary.
In accordance with the Cochrane Handbook  the characteristics of each study will first be summarized in a “Characteristics of included studies” table. Second data will be checked which studies are similar enough to be grouped within a comparison by comparing the characteristics across studies and it will be determined what data are available for synthesis. If appropriate, a statistical synthesis (meta-analysis) will be performed using the statistical software Stata. If a statistical pooling (meta-analysis) appears to be inappropriate, e.g. if data are highly heterogeneous or if study designs differ considerably, a tabular, graphical or narrative synthesis will be provided .
To determine whether the pooled results are robust, sensitive analysis will be conducted. This includes the repetition of the primary analysis or meta-analysis with different comparison categories  e.g. for different study types, different pandemic time points and RoB .
The SR will also address RoB due to missing results in a synthesis. Graphical and statistical methods will be used to provide information about the extent of missing results. Funnel plots will be generated and visually interpreted for signs of asymmetry, which could indicate that publication bias is present . When at least 10 studies of different sample size will be included in meta-analysis, statistical tests will also be used to test for funnel plot asymmetry .