WASHINGTON (AP) — U.S. mortgage rates rose slightly this week but they remain far below year-ago levels, which has provided a boost to home sales.

Mortgage buyer Freddie Mac said Thursday that the average rate for a 30-year fixed-rate mortgage loan rose to 3.49% from 3.47% last week. That is down from 4.35% a year ago. The average rate on a 15-year fixed mortgage also increased slightly, to 2.99% from 2.97%.

Fears about the economic impact of China’s viral outbreak have caused investors to snap up U.S. Treasury securities, often seen as a safe haven in the event of an economic downturn. That has pushed the yield on the 10-year Treasury note sharply lower this year. Mortgage rates typically follow the yield on the 10-year note.

The rate for a 5-year adjustable mortgage was 3.25% versus 3.28% last week.