and Ken West. The views expressed herein are those of the author(s) and do not necessarily reflect the views of the National Bureau of Economic Research.NBER working papers are circulated for discussion and comment purposes. They have not been peerreviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

We provide a simple and intuitive measure of interdependence of asset returns and/or volatilities. In particular, we formulate and examine precise and separate measures of return spillovers and volatility spillovers. Our framework facilitates study of both non-crisis and crisis episodes, including trends and bursts in spillovers, and both turn out to be empirically important. In particular, in an analysis of nineteen global equity markets from the early 1990s to the present, we find striking evidence of divergent behavior in the dynamics of return spillovers vs. volatility spillovers: Return spillovers display a gently increasing trend but no bursts, whereas volatility spillovers display no trend but clear bursts.

We propose several connectedness measures built from pieces of variance decompositions, and we argue that they provide natural and insightful measures of connectedness among financial asset returns and volatilities. We also show that variance decompositions define weighted, directed networks, so that our connectedness measures are intimately-related to key measures of connectedness used in the network literature. Building on these insights, we track both average and daily time-varying connectedness of major U.S. financial institutions' stock return volatilities in recent years, including during the financial crisis of 2007-2008.

Objective Coronavirus disease 2019 (COVID‐19) is a global health problem that can result in serious complications. The aim of this study was to investigate the prevalence and clinical importance of vitamin D deficiency in children with COVID‐19. Material and Methods This study includes 40 patients who were diagnosed to have COVID‐19 and hospitalized with the real‐time reverse transcription polymerase chain reaction method, 45 healthy matched control subjects with vitamin D levels. The age of admission, clinical and laboratory data, and 25‐hydroxycholecalciferol (25‐OHD) levels were recorded. Those with vitamin D levels which are below 20 ng/ml were determined as Group 1 and those with ≥20 ng/ml as Group 2. Results Patients with COVID‐19 had significantly lower vitamin D levels 13.14 μg/L (4.19–69.28) than did the controls 34.81 (3.8–77.42) μg/L (p < .001). Patients with COVID‐19 also had significantly lower serum phosphorus (4.09 ± 0.73 vs. 5.06 ± 0.93 vs. (U/L) (p < .001)) values compared with the controls. The symptom of fever was significantly higher in COVID‐ 19 patients who had deficient and insufficient vitamin D levels than in patients who had sufficient vitamin D levels (p = .038). There was a negative correlation found between fever symptom and vitamin D level (r = −0.358, p = .023). Conclusion This is the first to evaluate vitamin D levels and its relationship with clinical findings in pediatric patients with COVID‐19. Our results suggest that vitamin D values may be associated with the occurrence and management of the COVID‐19 disease by modulating the immunological mechanism to the virus in the pediatric population.

Standard-Nutzungsbedingungen:Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden.Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen.Sofern die Verfasser die Dokumente unter Open-Content-Lizenzen (insbesondere CC-Lizenzen) zur Verfügung gestellt haben sollten, gelten abweichend von diesen Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte. Abstract: We propose several connectedness measures built from pieces of variance decompositions, and we argue that they provide natural and insightful measures of connectedness among financial asset returns and volatilities. We also show that variance decompositions define weighted, directed networks, so that our connectedness measures are intimately-related to key measures of connectedness used in the network literature. Building on these insights, we track both average and daily time-varying connectedness of major U.S. financial institutions' stock return volatilities in recent years, including during the financial crisis of 2007-2008. Terms of use: Documents in EconStor may