(Bloomberg) -- More than 40% of all US mortgages were originated in 2020 or 2021, when the pandemic drove borrowing costs to historic lows and triggered a refinancing boom, according to data from Black Knight.

That’s good news for all the homeowners who locked in cheap loans — but maybe not so great for the Federal Reserve, as it seeks to cool the economy by raising interest rates. 

Almost one-quarter of all mortgages are 2021 vintage, according to Black Knight, a mortgage technology and data provider. That year, the average cost of a 30-year fixed-rate loan touched a low of 2.8%. Another 18% of home loans date from the previous year, when the pandemic hit.

The numbers illustrate one obstacle for the Fed, which is hiking rates at the steepest pace in decades to rein in inflation. 

One way that monetary tightening works is by damping consumer demand, as credit becomes more expensive. That’s having an impact on housing markets now, because new buyers have to pay 7% or more. But the large majority of American homeowners have fixed mortgages, mostly much cheaper than today’s going rate. Those who refinanced in the pandemic have locked in extra purchasing power for potentially decades ahead.

Things used to be different when more Americans had mortgages that carried variable interest rates. In a report this week, UBS economists estimated that the share of floating-rate debt in the US mortgage pile has shrunk to about 5%, from a peak of around 40% in 2006. That’s one reason for the “lower responsiveness of household credit to higher rates,” they wrote. 

Still, even if the shift to fixed-rate mortgages makes the Fed’s anti-inflation campaign a little harder, there’s an obvious upside. 

The last time the Fed hiked rates by a comparable amount was in the mid-2000s when adjustable-rate mortgages were widespread. The result: Housing markets crashed — and not long afterwards, so did the world economy.

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