ATLANTA — The nation's health agency plans to closely watch flu complications  among children, who have swamped hospitals in some states and surprised doctors with the severity of their illnesses.

A new concern is the rise of a common drug-resistant staph infection that is complicating efforts to treat children with the flu, an official with the Centers for Disease Control and Prevention  said Monday.

Dr. Tim Uyeki, epidemiologist with the influenza branch of CDC, said that some children have died from the staph infections  — a phenomenon the CDC has not seen before.

Flu complications for children have always been dire: pneumonia, kidney and heart failure, possible brain damage.

"We've just never seen them in the proportions we've seen them this year," said Dr. Steve Schexnayder, chief of pediatric critical care at Arkansas Children's Hospital in Little Rock.

From Texas to California, children's hospitals have been swamped with sick children — many of them desperately ill.

The waiting room at Children's Hospital Central California in Madera was standing room only during the Thanksgiving holiday, and hospital officials said nearly every child tested positive for the flu.

Children's Medical Center in Dallas has seen more than 500 kids with the flu since October. One day last week, two dozen more appeared, most with enough lung disease to be put on ventilators, said Dr. Jane Siegel.

"Because it seems to be a strain that has not circulated in the U.S. before and is not well-covered by the existing vaccine, we're seeing far more cases," said Dr. James Todd, director of epidemiology of Denver Children's Hospital. "Just because you're seeing more cases, you're seeing more complications."

Doctors say some children are coming into hospitals with so much damage they are put on heart-lung bypass machines just to stay alive.

Others face additional problems: Nine-year-old Nick Collins at Arkansas Children's Hospital needed four chest tubes to drain fluid from holes in his lungs caused by bacterial pneumonia. Doctors are trying to prevent a blood clot from killing him.

He also had methicillin-resistant staphylococcus aureus (MRSA), a usually mild infection which led to his severe pneumonia. Staph bacteria are commonly found on the skin or in the nose and often go undetected.

Uyeki said the children with staph-related flu likely picked up the bacteria before they were hospitalized. In October CDC warned parents that many school athletes had been found to carry MRSA.

These infections don't normally cause pneumonia without the flu virus, said Dr. Frederick Hayden, a flu expert and professor of internal medicine at the University of Virginia.

But the flu virus can impair the body's ability to fight the bacteria and expel it. The bacteria, in turn, can produce enzymes that enhance the flu's ability to infect cells, he said.

Nick, a healthy boy until he got the flu in early November, is doing better, having been removed from a ventilator on Friday. But he'll likely have to stay in the hospital through the end of the year, his mother says.

"It's scary to find that something as common as the flu can cause something this major every year," said his mother, Kim Collins of Texarkana, Texas. "We sit around for days in awe of the fact the flu has caused all of this."

Flu and its complications are the sixth leading cause of death nationally among children age 4 and younger, according to the CDC.

Anecdotally, this flu season seems to be worse for children. But because the CDC doesn't keep track of flu deaths, it's unclear how much worse. This year the agency is planning to collect data on children who die from the flu, those with MRSA, and those who develop brain damage, said Dr. Keiji Fukuda, chief of epidemiology in the CDC's influenza branch.

In addition, some connected with the CDC say there may be a push to add school-age children to the list of those most strongly urged to get the flu shot — the best protection against the virus. The current recommendation for children covers those from 6 months to 2 years and those who have certain chronic conditions.

"My own prediction is what you'll continue to see is a broadening of the recommendation for influenza immunization," said Dr. Greg Poland, a Mayo Clinic professor and a member of the CDC advisory committee on immunization.

Pregnant women — urged to get the flu shot if they are in their second or third trimester — have also become a concern this year.

The CDC is looking closely at some cases in which pregnant women have displayed high pulse rates — which could be a symptom of a dangerous, and potentially fatal, inflammation of the heart, said Dr. William Schaffner, a flu expert with Vanderbilt University Medical Center. Texas in particular has reported several such cases.