THURSDAY, Nov. 5 (HealthDay News) -- The H1N1 swine flu virus is now the predominant flu strain worldwide, although it shows no signs of becoming more virulent and continues to produce mild-to-moderate symptoms in most people, the World Health Organization's flu chief said Thursday.

In some countries, the swine flu accounts for up to 70 percent of the flu viruses being sampled, Dr. Keiji Fukuda said during a press briefing, the Associated Press reported.

In the United States, virtually all flu activity right now is from the H1N1 virus, according to federal health officials.

But unlike seasonal flu, which typically strikes hardest at people over age 65, the H1N1 swine flu targets a disproportionate number of people under 65, Fukuda said.

"We remain quite concerned about the patterns that we're seeing," he said.

Fukuda said the H1N1 flu virus appears to be stable, with samples from around the globe very similar to those seen when the virus first emerged in Mexico and the United States in April, the AP said.

In the United States, a federal health official said Tuesday that the country had 31.8 million H1N1 flu vaccine doses available and was on track to have another 10 million ready by week's end.

So far, that hasn't been enough to prevent long lines at vaccination centers, but it is consistent with what officials had projected earlier this week.

"We're having a steady increase in the availability of vaccine, but not nearly as rapidly as we would have liked," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told reporters during a teleconference. "That is encouraging, but it is not nearly as much as we would like. We realize it is frustrating and inconvenient [to patients and to physicians]. As public health professionals, it is frustrating because, in part, many people who seek the vaccine will not get vaccinated later. They might not come back. We ask people to continue to be persistent."

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."

More information

There's more on H1N1 influenza at the U.S. Centers for Disease Control and Prevention.

SOURCE: Nov. 3, 2009, teleconference with Thomas R. Frieden, M.D., director, U.S. Centers for Disease Control and Prevention, Atlanta; Associated Press

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