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Officials from the Centers for Disease Control and Prevention, speaking on a conference call Friday morning, again urged Americans to keep getting flu shots. At the same time, they emphasized that the shots are not infallible: a preliminary study rated this year's vaccine as 62 percent effective, even though it is a good match for the most worrisome virus circulating. That is considered "moderately" effective — the vaccine typically ranges from 50 percent to 70 percent effective.

Even though deaths stepped into epidemic territory for the first time — barely — the C.D.C. officials expressed no alarm, and said it was possible that new flu infections were peaking in some parts of the country.

Flu outbreaks typically reach epidemic level for one or two bad weeks every flu season, and the mortality threshold is a wavy curve that dips down in summer and up in winter. How severe a season is depends on how high deaths climb and for how many weeks they persist.

The flu vaccine, said Dr. Thomas Frieden, director of the C.D.C., "is far from perfect, but it's by far the best tool we have to prevent influenza."

Michael T. Osterholm of the University of Minnesota's Center for Infectious Disease Research and Policy had a moment of self-satisfaction at hearing of the vaccine effectiveness study.

Although he still backs getting the shots, last year he released a study saying that the effectiveness of flu shots had been "oversold" and that new vaccines had to be developed.

But he said that no government and no private vaccine company was yet prepared to spend "the $1 billion needed to get a new vaccine across the Valley of Death" — by which he meant the huge expensive trials needed to get a vaccine approved.