MBB vs SPMB

MBBAiShares MBS ETF

SPMBAState Street SPDR Portfolio Mortgage Backed Bond 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.

SPMB The no-frills way to own America's mortgage bonds. SPMB tracks the Bloomberg U.S. MBS Index for four basis points, a fraction of what the typical mortgage-bond ETF charges, and pays income monthly.

Stated mandate

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

SPMB The fund seeks to track, before fees and expenses, the price and yield performance of the Bloomberg U.S. MBS Index.

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.

• SPMB Four basis points a year, against a 0.35% median for mortgage-bond ETFs. Cost is where this fund does its loudest talking.
• Runs the same Bloomberg U.S. MBS Index as iShares' MBB at the same 0.04% fee, and undercuts MBBA, which tracks that index for far more.
• Roughly 2,700 mortgage bonds in a single line item, inside a multi-billion-dollar fund that trades actively, with distributions paid monthly.
• Trading since 2009, and one of the strongest implementations in its mortgage-bond peer group.

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.

• SPMB Homeowners refinance when rates fall, so mortgage bonds get paid back early and rally less than Treasuries. That quirk is built into the index, not the fund.
• Prices move with interest rates and the monthly payout floats with the portfolio. This is a bond fund, not a cash substitute.
• US mortgages only. No Treasuries, no corporates, no credit spice. A building block for a bond sleeve rather than the whole sleeve.

Full MBB profile⁠

Full SPMB profile⁠