MBB vs MBBA

MBBAiShares MBS ETF

MBBACiShares Mortgage-Backed Securities Active ETF

In short

MBB Launched in 2007, MBB is the plain-vanilla way to own U.S. agency mortgage bonds: one of the biggest and busiest funds in a small niche, tracking the Bloomberg U.S. MBS Index for four basis points.

MBBA Active management aimed at the plumbing of the mortgage market: iShares gives a manager room to pick agency MBS instead of mirroring the Bloomberg U.S. MBS Index, and charges less than the typical fund in this corner to do it.

Stated mandate

MBB The fund seeks to track an index composed of U.S. agency mortgage-backed securities.

MBBA The Fund seeks to maximize total return while generating income and applying prudent investment management.

Why people hold it

• MBB Four basis points to rent the entire agency mortgage market, against a category median near 0.35%. In low-coupon bond land, that gap compounds quietly.
• Scale is the real feature: one of the largest and most actively traded funds in the mortgage-bond category, which generally shows up as tighter spreads.
• Pays monthly, and the mandate stays narrow: an index of U.S. agency mortgage-backed securities, nothing exotic bolted on.
• Among the stronger builds in a thin peer group of mortgage-bond ETFs, on the combination of cost, trading friction and issuer staying power.

• MBBA Runs at 0.25% a year, under the 0.35% typical of MBS funds in its group. Active mandates usually charge up from the index, not down toward it.
• The mandate: maximize total return while generating income. The Bloomberg U.S. MBS Index is the yardstick, not a leash the manager has to hug bond for bond.
• Spread across roughly 500 US mortgage bonds in a plain 1940 Act open-end fund. No K-1, no derivative wrapper, no offshore structure to decode.
• Fund inception dates to 1998, meaning this mortgage playbook was running long before most of the ETFs it now sits beside existed.

Worth knowing

• MBB This is one slice of the bond market. Agency mortgage paper only, with no Treasuries or corporates alongside it.
• Mortgage bonds cut both ways with rates: borrowers refinance when yields fall and sit tight when they rise, so the portfolio's duration shifts underneath you.
• On price it's a photo finish: SPMB tracks the same index at the same 0.04%, and VMBS runs a basis point cheaper.

• MBBA Cheap for active, still pricier than the index route: MBB, VMBS and SPMB all sit at a few basis points. Here you are paying for a manager's calls.
• Because it is active, there is no tracking record to audit, and only a short window of risk history exists. Less to measure than a long-running tracker.
• Smaller and more lightly traded than the index heavyweights in the MBS category, which is where limit orders earn their keep.

Full MBB profile⁠

Full MBBA profile⁠