Infants as young as six to 12 weeks could be safely protected by seasonal flu vaccines, U.S. researchers claim in a new study.

In this study available here on Wednesday, researchers at the University of Washington in Seattle randomly assigned 1,375 healthy infants to receive either two doses of the standard trivalent seasonal flu vaccine (protects against three strains of circulating flu virus) or a placebo vaccine.

The vaccines were given one month apart, in combination with standard recommended infant vaccines.

The researchers found no differences between the two groups of infants in terms of side effects or adverse events. About 11 percent of infants in both groups developed a fever within three days after vaccination. In the month after vaccination, serious adverse events related to the trivalent vaccine were rare, according to the study published in the February issue of The Pediatric Infectious Disease Journal.

Immunological tests showed that nearly half of the infants who received the flu vaccine developed protective antibodies against at least two of the three influenza strains covered by the vaccine. No evidence was noted that the flu vaccine interfered with infant responses to routine childhood vaccinations.

"Based on the results of this study, potential protection against influenza could be safely obtained in infants less than 6 months old of age who receive standard infant dose of inactivated influenza vaccine," said Dr. Janet A. Englund, who led the study.

These findings suggest that seasonal flu vaccine should be included in standard vaccinations for infants less than 6 months old, the researchers said. But they noted that more studies should be conducted to confirm the results.

The United States has not approved any flu vaccine for infants less than 6 months old, even though children in this age group are at high risk for flu and related complications.