Babies, born to women who had flu vaccine while pregnant, were hospitalized at a lower rate than infants of unvaccinated mothers, U.S. researchers found.

Dr. Marietta Vazquez, assistant professor of pediatrics at Yale School of Medicine, said influenza is a major cause of serious respiratory disease in pregnant women and of hospitalization in infants.

Although the flu vaccine is recommended for all pregnant women and children, no vaccine is approved for infants less than six months of age. Preventive strategies for this age group include general infection control and vaccination of those coming in close contact with the infant.

During nine flu seasons from 2000-2009, Vazquez and colleagues identified and tracked more than 350 mothers and infants up to 12 months of age hospitalized at Yale-New Haven Hospital. They compared 157 infants hospitalized due to influenza to 230 influenza-negative infants matched by age and date of hospitalization.

"We found that vaccinating mothers during pregnancy was 80 percent effective in preventing hospitalization due to influenza in their infants during the first year of life and 89 percent effective in preventing hospitalization in infants under six months of age," Vazquez said in a statement.

The team presented the findings at the 47th annual meeting of the Infectious Diseases Society of America in Philadelphia.