*4.4. Limitations*

There are several limitations to the present study. One is the cross-sectional character of the research. The longitudinal study could reveal the cause—effect relationship between the proposed indices and mental health issues. Direct comparisons among countries are also limited due to the different pace and extent of public health restrictions imposed by governments and due to the situation with COVID-19 related deaths in the observed period in each of the observed countries. Another limitation is a self-selected study sample and data collection via self-reported questionnaires. Therefore, the data can be subject to retrospective response bias. Previous research showed that more depressive symptoms can be elicited for milder forms of depression through self-reported measurements compared to clinician-rating methods (interview) [87]. More educated and younger people usually score higher on self-rated scales than on clinician-rating scales [88]. However, it should be noted that even though a milder form of depression may be elicited among young adults, depressive symptoms have increased during the pandemic [89,90]. Finally, generalizing the results may be hindered by the lack of random sampling and representation of the student population being limited to specific regions in each country.

Considering strengths and limitations of this study, future research ought to examine mental health using a longitudinal design from the cross-cultural perspective.

#### **5. Conclusions**

Our study has shown risk factors for depression and anxiety and differences in mental health among university students in the nine countries during the first wave of the COVID-19 pandemic. We have revealed that even so common a risk factor as gender does not predict anxiety or depression in all the countries. Moreover, physical inactivity as a risk factor strongly depends on the country, and in most of the nine countries was a significant predictor neither for anxiety nor depression.

This research underlines the necessity of interpreting data within the cross-cultural context and argues that presenting mental health results during the COVID-19 pandemic only in one country can be challenging in terms of generalization. We demonstrated that, even though there are several risk factors associated with mental health issues in all of the nine countries (i.e., exposure to COVID-19, perceived impact of COVID-19 on students' well-being, including graduation, economic status, and relationships quality, general and comparative health), the multivariate models differed drastically among the countries. Therefore, despite the globalization of a homogeneous student population, our study showed varied mental health predictors in relation to cultural, political and economic situation in a particular country. Planning and implementation of psychological intervention programs for students should include differentiation by country concerning mental health risk factors.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/jcm10132882/s1, Table S1: Association between depression risk and other variables among university students from Colombia, Czechia, Germany, Israel, Poland, Russia, Slovenia, Turkey, and Ukraine during the first wave of the COVID-19 pandemic. Table S2: Association between anxiety risk and other variables among university students from Colombia, Czechia, Germany, Israel, Poland, Russia, Slovenia, Turkey, and Ukraine during the first wave of the COVID-19 pandemic. Table S3: Logistic regression for depression symptoms among university students from Colombia during the first wave of the COVID-19 pandemic. Table S4: Logistic regression for depression symptoms among university students from Czechia during the first wave of the COVID-19 pandemic. Table S5: Logistic regression for depression symptoms among university students from Germany during the first wave of the COVID-19 pandemic. Table S6: Logistic regression for depression symptoms among university students from Israel during the first wave of the COVID-19 pandemic. Table S7: Logistic regression for depression symptoms among university students from Poland during the first wave of the COVID-19 pandemic. Table S8: Logistic regression for depression symptoms among university students from Russia during the first wave of the COVID-19 pandemic. Table S9: Logistic regression for depression symptoms among university students from Slovenia during the first wave of the COVID-19 pandemic. Table S10: Logistic regression for depression symptoms among university students from Turkey during the first wave of the COVID-19 pandemic. Table S11: Logistic regression for depression symptoms among university students from Ukraine during the first wave of the COVID-19 pandemic. Table S12: Logistic regression for anxiety symptoms among university students from Colombia during the first wave of the COVID-19 pandemic. Table S13: Logistic regression for anxiety symptoms among university students from Czechia during the first wave of the COVID-19 pandemic. Table S14: Logistic regression for anxiety symptoms among university students from Germany during the first wave of the COVID-19 pandemic. Table S15: Logistic regression for anxiety symptoms among university students from Israel during the first wave of the COVID-19 pandemic. Table S16: Logistic regression for anxiety symptoms among university students from Poland during the first wave of the COVID-19 pandemic. Table S17: Logistic regression for anxiety symptoms among university students from Russia during the first wave of the COVID-19 pandemic. Table S18: Logistic regression for anxiety symptoms among university students from Slovenia during the first wave of the COVID-19 pandemic. Table S19: Logistic regression for anxiety symptoms among

university students from Turkey during the first wave of the COVID-19 pandemic. Table S20: Logistic regression for anxiety symptoms among university students from Ukraine during the first wave of the COVID-19 pandemic.

**Author Contributions:** Conceptualization, D.O., A.M.R., C.K.; data curation, D.O.; formal analysis, A.M.R., investigation, D.O., A.M.R., C.K., M.J., A.S., M.J.H., A.A., J.B., R.B., E.V.K., I.P., I.B., Z.K., I.A., O.Ç., Y.A.C.-A.; methodology, A.M.R., D.O.; project administration, D.O.; resources D.O., A.M.R., C.K., M.J., A.S., M.J.H., A.A., J.B., R.B., E.V.K., I.P., I.B., Z.K., I.A., O.Ç., Y.A.C.-A., M.W.-S.; supervision, D.O., A.M.R.; visualization, A.M.R.; writing—original draft preparation, D.O., A.M.R.; writing review and editing, D.O., A.M.R., C.K., M.J., A.S., M.J.H., A.A., J.B., R.B., E.V.K., I.P., I.B., Z.K., I.A., O.Ç., Y.A.C.-A., M.W.-S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the local IRB: University Research Committee at the University of Opole, Poland, decision no. 1/2020.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The materials and methods are accessible at the Center for Open Science (OSF), titled: Well-being of undergraduates during the COVID-19 pandemic: International study [91]. The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
