Officials also said that more health-care workers than usual are availing themselves of the seasonal flu vaccine, which is in short supply in some areas due to heavy demand, according to published reports.
"We're seeing a higher uptake of the seasonal influenza vaccine by health-care workers than in previous years," said Frieden. "We'll have to wait and see how that goes and how extensive it is."
Demand for the regular, seasonal flu vaccine among the general population has also been unprecedented, Frieden added, with 90 million doses already distributed to providers and 114 million expected to be available by year's end.
However, virtually all of the flu being diagnosed right now is H1N1.
"We're seeing almost no seasonal flu," Frieden said. That doesn't mean the seasonal flu shot won't be needed, however. "What the rest of the season holds, only time will tell," he said.
Frieden also reiterated the importance of antiviral medications, such as Tamiflu or Relenza, regardless of whether the vaccine is available. That's especially true for people with certain chronic medical troubles, such as asthma and heart disease.
As always for everyone, the message of the season is, wash your hands frequently, cover your mouth when sneezing or coughing, and stay home if you are sick.
"The flu season lasts till May, and this flu season is unlike any other for at least 50 years," Frieden said. "We don't know what will happen, but we will continue to monitor and do everything we can to prevent or reduce the spread of flu."
There's more on H1N1 influenza at the U.S. Centers for Disease Control and Prevention.
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