With much anticipation and fanfare, the Federal Reserve is finally on track to reduce its MBS holdings. Guidance from the September FOMC meeting reveals that the Fed will allow its MBS holdings to “run off,” reducing its position via prepayments as opposed to selling it off. What does this Fed MBS Runoff mean for the market? In the long-term, it means a large increase in net supply of Agency MBS and with it an increase in overall implied and realized volatility.
MBS: The Largest Net Source of Options in the Fixed-Income Market
We start this analysis with some basic background on the U.S. MBS market. U.S. homeowners, by in large, finance home purchases using fixed-rate 30-year mortgages. These fixed-rate mortgages amortize over time, allowing the homeowner to pay principal and interest in even, monthly payments. A homeowner has the option to pay off this mortgage early for any reason, which they tend to do when either the homeowner moves, often referred to as turnover, or when prevailing mortgage rates drop significantly below the homeowner’s current mortgage rate, referred to as refinancing or “refis.” As a rough rule-of-thumb, turnover has varied between 6% and 10% per annum as economic conditions vary, whereas refis can drive prepayments to 40% per annum under current lending conditions. Rate refis account for most of a mortgage’s cash flow volatility.
If the homeowner is long the option to refinance, the MBS holder is short that same option. Fixed-rate MBS shorten due to prepayments as rates drop, and extend as rates rise, putting the MBS holder into a short convexity (gamma) and short vega position. Some MBS holders hedge this risk explicitly, buying short- and longer-dated options to cover their short gamma/short vega risk. Others hedge dynamically, including money managers and long-only funds that tend to target a duration bogey. One way or another, the short-volatility risk from MBS is transmitted into the larger fixed-income market. Hence, the rates market is net short vol risk.
While not all investors hedge their short-volatility position, the aggregate market tends to hedge a similar amount of the short-options position over time. Until, of course, the Fed, the largest buyer of MBS, entered the market. From the start of Quantitative Easing, the Fed purchased progressively more of the MBS market, until by the end of 2014 the Fed just under 30% of the agency MBS market. Over the course of five years, the effective size of the MBS market ex-Fed shrunk by more than a quarter. Since the Fed doesn’t hedge its position, either explicitly through options or implicitly through delta-hedging, the size of the market’s net-short volatility position dropped by a similar fraction.
The Fed’s Balance Sheet
As of early October 2017, the Federal Reserve owned $1.77 trillion agency MBS, or just under 30% of the outstanding agency MBS market. The Fed publishes its holdings weekly which can be found on the New York Fed’s web site here. In the chart below, we summarize the Fed’s 30yr MBS holdings, which make up roughly 90% of the Fed’s MBS holdings. 
Runoff from the Fed
Following its September meeting, the Fed announced they will reduce their balance sheet by not reinvesting run-off from their treasury and MBS portfolio. If the Fed sticks to its plan, MBS monthly runoff from MBS will reach $20B by 2018 Q1.
Assuming no growth in the aggregate mortgage market, runoff from these MBS will be replaced with the same size of new, at-the-money MBS passthroughs. Since the Fed is not reinvesting paydowns, these new passthroughs will re-enter the non-Fed-held MBS market, which does hedge volatility by either buying options or delta-hedging.
Given the expected runoff rate of the Fed’s portfolio, we can now estimate the vega exposure of new mortgages entering the wider (non-Fed-held) market. When fully implemented, we estimate that $20B in new MBS represents roughly $34 million in vega hitting the market each month. To put that in perspective, that is roughly equivalent to $23 billion notional 3yr->5yr ATM swaption straddles hitting the market each and every month.
While the Fed isn’t selling its MBS holdings, portfolio runoff will have a significant impact on rate volatility. Runoff implies significant net issuance ex-Fed. It’s reasonable to expect increased demand for options hedging, as well as increased delta-hedging, which should drive both implied and realized vol higher over time. This change will manifest itself slowly as monthly prepayments shrinks the Fed’s position. But the reintroduction of negative vega into the wider market represents a change in paradigm which may lead to a more volatile rates market over time.
 In the early 2000s, prepayments hit their all-time highs with the aggregate market prepaying in excess of 60% per annum.
 This is not entirely accurate. The short-vol position in a mortgage passthrough is also a function of its note rate (GWAC) with respect to the prevailing market rate, and the mortgage market has a distribution of note rates. But the statement is broadly true.
 The remaining Fed holdings are primarily 15yr MBS pass-throughs.