WASHINGTON — U.S. long-term mortgage rates rose this week. They remain at historically low levels, now around a percentage point below a year ago.

Mortgage buyer Freddie Mac reported today that the average rate on the 30-year benchmark loan increased to 2.84% from 2.78% from last week. By contrast, the rate averaged 3.75% a year ago.

The average rate on the 15-year fixed-rate mortgage edged up to 2.34% from 2.32%.

Breaking their downward trend through most of this year, mortgage rates were bolstered by the news that a COVID-19 vaccine might be 90% effective, based on early and incomplete test results.

The vaccine announcement came two days after Joe Biden became president-elect upon defeating President Donald Trump by crossing the winning threshold of 270 Electoral College votes with a win in Pennsylvania.

The historically low borrowing rates have bolstered demand from prospective homebuyers. Demand for homes has remained strong despite a brief slowdown in the early days of the coronavirus pandemic, but at the same time the rise in home prices has stretched the limits of affordability for many would-be buyers.

The government reported Thursday that the number of Americans seeking unemployment benefits fell last week to 709,000, a still-high level but the lowest figure since March and a further sign that the job market might be slowly healing.