This year’s flu season may be deadlier than usual, and this year’s flu vaccine is a relatively poor match to a new virus that is now circulating, federal health officials warned on Thursday.

“Flu is unpredictable, but what we’ve seen thus far is concerning,” said Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention.

The C.D.C. has alerted doctors to the problem and has urged them to prescribe antiviral drugs like Tamiflu to vulnerable patients with flu symptoms without waiting for a positive flu test.

The season has only just begun, but 91 percent of the approximately 1,200 samples tested thus far are of the H3N2 subtype of influenza A, Dr. Frieden said. Almost all the rest were influenza B. There were almost no samples of the H1 subtype, a descendant of the 2009 swine flu strain.