Chicago ? The biggest study ever to look at the side effects of flu shots in children confirmed that the vaccine is safe for babies and toddlers.

Researchers studied 45,000 U.S. children and found almost no side effects requiring medical treatment during the six weeks after the youngsters were vaccinated.

The Centers for Disease Control and Prevention estimate that more than 90 children under 5 die of the flu each season.

Flu vaccine has a good safety record, the researchers wrote, though some formulations have been linked to Guillain-Barre syndrome, a rare paralyzing disorder.

With the shots now recommended for all children younger than 5, the findings are reassuring, said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University who was not involved in the study.

“Linus had a security blanket, and this is a huge security blanket,” Schaffner said. “This is a comfort to all providers, parents and policymakers that we can move ahead with great, great confidence.”

Researchers found a few more cases of mild nausea and diarrhea than expected within the first two weeks after the shot, but the number of cases – 13 – was extremely low considering the thousands of children studied.

After their flu shots, the children were less likely to get treated for upper respiratory tract and ear infections. That could have been because parents felt reassured that the symptoms were not signs of flu, said study co-author Dr. Simon Hambidge, an investigator at Kaiser Permanente Colorado’s clinical research unit.

The federally funded study appears in today’s Journal of the American Medical Assn.

“This is really reassuring for parents and for doctors who want to protect children from what’s a pretty nasty disease,” Hambidge said. “We know children in this age group get hospitalized for complications of influenza as much as elderly adults do.”

Nine of the study’s 19 co-authors reported financial ties to vaccine manufacturers, but the industry had no direct role in the study.