The study, reported online on Aug. 4 in Pediatrics, followed kids who had to be admitted to a pediatric hospital for the flu and other respiratory infections. Researchers stressed that they are much different from the vast majority of children who fall ill during cold-and-flu season.
In fact, they would likely represent only a small percentage of children seen in the ER for worrisome flu-like symptoms, said Dr. Rakesh Mistry, the lead researcher on the study and an emergency specialist at Children's Hospital Colorado in Aurora.
"These kids were really ill," Mistry said of the study group, which included 241 children brought to his center's ER between 2008 and 2010.
The children had to be admitted to the hospital and required IV fluids. All underwent testing to weed out the cause of the infection -- which is not routine for kids who land in the ER with flu symptoms, Mistry noted.
In the end, about 25 percent of the children tested positive for the flu virus. The rest had a mix of viruses that typically cause the common cold, but sometimes lead to more severe respiratory infections.
Overall, 35 percent of the children developed a serious complication, such as pneumonia, inflammation around the brain, or respiratory failure. Often, those kids had underlying medical conditions that can make respiratory infections dangerous -- including neurological disorders like cerebral palsy or muscular dystrophy.
"Those children may just not have the capacity to breathe when they get these infections," explained Dr. Vincenzo Maniaci, an attending ER doctor at Miami Children's Hospital who was not involved in the study.
More than one-quarter of those with severe complications had a history of asthma, according to the study. In addition, children with certain other conditions -- such as heart problems, chronic lung disease and sickle-cell disease -- are part of the "key group" at risk of complications from the flu and other viruses, Maniaci said.
Still, about 40 percent of the children in this study who suffered complications were otherwise "reasonably healthy," Mistry noted.
"That's important," he said, "because it means we have to be aware that even healthy children can develop these complications."
But he stressed that even though those children had no underlying high-risk conditions, they were so ill they had to be admitted to a children's hospital -- which would not be true of the vast majority of kids who come down with the flu or a bad cold.
"For most children, if they have a fever and cough, it's just a fever and a cough," Mistry said.
Maniaci agreed that parents of healthy kids should not be alarmed. Instead, he said, this is more evidence that parents and doctors should be aware of the possibility of complications in children with high-risk diseases.
Maniaci did suggest, though, that parents know the potential symptoms of pneumonia -- including fast, labored breathing and chest pain. Pneumonia is the most common complication of the flu and colds in healthy children, Maniaci noted.
When it comes to influenza, he said, a yearly flu shot might save children some misery. However, they'll still be susceptible to other common viruses.
The U.S. Centers for Disease Control and Prevention has more on children and the flu.