But these deaths are almost certainly an underestimation of the actual number, Dr. Thomas R. Frieden, director of the U.S. Centers for Disease Control and Prevention, said during an afternoon press conference Friday.

"We can provide information on the number of laboratory-confirmed hospitalizations and deaths -- we know that that's an underestimation of the total," he said.

Of the children who have died, two-thirds had chronic health problems such as asthma, cerebral palsy and muscular dystrophy, Frieden said.

"In a usual flu season, 90 percent of the deaths are among people over the age of 65. In H1N1, 90 percent of the deaths are in people under the age of 65," he said.

The swine flu continues to spread throughout the country; for most people the infections are mild to moderate. It is now widespread in 48 states, although there are signs it has begun to recede in some Southeastern states, he said.

Frieden said the latest statistics continue to show that H1N1 swine flu is a disease that afflicts younger people, and can be especially serious for those underlying medical conditions. "In the past two months, we have seen more hospitalizations in people under the age of 65 than in most entire flu seasons," he said.

"One of the things we have been surprised to see," Frieden added, "is that even among people who have an underlying condition, such as asthma or heart disease or lung disease, only half sought care for influenza-like illness. People with underlying conditions with fever and cough should see their [health-care] provider promptly."

While vaccine shortages persist, there are 26.6 million doses now in circulation, up from 16.1 million doses last week. The supply is "increasing steadily," said Frieden. "The gap between supply and demand is closing."

Production delays have resulted in far fewer doses of the vaccine being available than federal officials had hoped for by this time. The first estimates called for 40 million doses by the end of October and 190 million doses by year's end.

Since the genetics of the virus have not changed, the vaccine is a good match, Frieden said.

Frieden also said there is a stronger than usual demand for the seasonal flu vaccine, prompting some shortages. "Of the 89 million doses that have already been distributed, the overwhelming majority have already been given," he said. But more doses will be available in November and December.

Supplies of the antiviral drug Tamiflu should be plentiful, but there have been shortages of the liquid form given to children, Frieden said. To help overcome that shortage, the federal government is releasing the 234,000 remaining doses of liquid Tamiflu that have been stockpiled, he said.

More information

For more on the H1N1 swine flu, visit the U.S. Department of Health and Human Services.