At least half of the young children who are supposed to get annual flu shots haven't been doing so. Among children with chronic medical conditions such as asthma and heart disease, about two-thirds are missing out on the vaccine.

And, with medical experts expanding the age range of children who should be vaccinated this year, the percentage who fail to comply with the recommendations could go up. Health officials now say children between the ages of 6 months and 5 years should get flu shots; previously the range was 6 months to 2 years.

Many parents may not be aware of the change.

Health officials with the National Foundation for Infectious Diseases say Americans don't take flu seriously enough. This year, a vaccine shortage won't be a credible excuse for failure to get a shot. More than 100 million doses of vaccine will be available — the most ever.

"Vaccine that remains in the refrigerator cannot prevent influenza," said Dr. William Schaffner, a professor at Vanderbilt University.

The stakes are high when it comes to the flu, said officials who attended a press conference designed to raise awareness of the illness. The flu kills about 36,000 people each year and puts more than 200,000 people in the hospital. Most of the deaths and serious cases are among the elderly.

Still, for children between the ages of 2 and 5, there is a higher rate of clinic and emergency room visits due to flu-related illness than with other illnesses. Officials also said that vaccinating siblings is important for newborns. While children under the age of 6 months should not get vaccinated, it's important that their siblings and parents do, officials said.

"Influenza in the young should be prevented and can be prevented," said Dr. Julia McMillan of the Johns Hopkins University School of Medicine.

McMillan's estimate of about 50 percent getting a shot is based on a telephone survey conducted by state health departments. But another survey, which requires verification from a child's health care provider, indicates that the percentage of children getting a flu shot could be even lower — about 33 percent.

"The real message is, no matter what survey you look at, we're nowhere near protecting the number of children that we're supposed to," said Dr. Jeanne Santoli, a pediatrician at the CDC.