H3N2

Overall, this season's vaccine is about 36 percent effective in blunting or preventing flu, health officials say. That's better than earlier predictions, and good enough to prevent flu in thousands.

Health officials fear the U.S. may have a nasty flu season because the main flu virus circulating this year tends to hit people hard and the flu vaccine may be weaker than normal.

The main flu strain circulating now tends to send more people to the hospital than other strains. It also causes more deaths, especially among the elderly, children and people with health issues.