Lead author Dr. Katherine Poehling of Wake Forest Baptist and colleagues found children age 6 months and under had the highest hospitalization rates with flu.
"Parents should include a yearly flu shot to protect themselves and their children," Poehling said. "The best way to protect infants too young to receive the influenza vaccine is for pregnant women, the infant's family members and anyone with contact with the baby to get the shot, too."
The study involved 8,000 children seen in inpatient, emergency department and clinic settings with confirmed flu cases in children age 5 and younger in three counties in Ohio, New York and Tennessee from 2004 to 2009.
During the study period, the researchers found the overall flu vaccination coverage changed little -- less than 45 percent of children were vaccinated against the flu during a five-year study period, but the proportion of infants age 6 months and younger diagnosed with the flu increased to 48 percent compared with 28 percent in a previous study using data from 2000 to 2004, conducted by the same research team.
The study, published online ahead of next month's print edition of the journal Pediatrics, found seasonal flu remained an important cause of hospitalization, emergency department and outpatient visits among children and that vaccination and antiviral medications were underused.
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