Sovereign bond-backed securities: a VAR-for-VaR and Marginal Expected Shortfall assessment

The risk reducing benefits of the sovereign bond-backed security (SBBS) proposal of Brunnermeier et al (2011, 2016, 2017) have been assessed in terms of the likely losses that different kinds of holders would suffer under simulated default scenarios. However, the effects of mark-to-market losses that may occur when there is rising uncertainty about defaults, or when self-fulfilling destablising dynamics are prevalent, have not yet been examined. We apply the "VAR-for-VaR" method of White, Kim and Manganelli (2015) and the Marginal Expected Shortfall approach of Brownlees and Engle (2012, 2017) to estimated yields of SBBS to assess how ex ante exposures are likely to playout under various securitisation structures. We compare these with exposures of single sovereigns and a diversified portfolio. We find that the senior SBBS has extremely low ex ante tail risk and that, like the lowest-risk single-named sovereigns, it acts as a hedge against extreme adverse movements in the yields on more junior tranches. The mezzanine SBBS has tail risk exposure similar to that of Italian and Spanish bonds. Yields on SBBS appear to be adequate compensation for their risks when compared with single sovereigns or a diversified portfolio.