The number of people who fainted after getting vaccines doubled after U.S. health officials recommended three new shots for adolescents in 2005 and 2006.

The number of people ages 5 and older who fainted after getting a vaccination increased to 463 in January 2005 through July 2007, from 203 during a similar period ending in 2004, according to a report today by the U.S. Centers for Disease Control and Prevention.

Fainting, or syncope, can be triggered by vaccinations and other medical procedures, the CDC said. The number of fainting episodes has risen since the CDC recommended Merck & Co.'s cancer vaccine, Sanofi Aventis's meningitis vaccine, and shots against whooping cough, tetanus and diphtheria sold by Sanofi and GlaxoSmithKline Plc. About 63 percent of the fainting episodes occurred after one of the three new shots.

"Potentially life-threatening injuries were described, and one fatality was documented" when a 15-year-old boy fainted and injured his head, CDC researchers said. "All providers administering vaccinations should be aware of the potential for syncope after vaccination and should take appropriate measures to prevent potential injuries."

The rate of fainting reported to the CDC also increased, to 0.54 cases per million doses distributed in 2006, from 0.3 per million in 2002. The actual rates may be higher because of unreported cases and unused vaccines, the CDC said.

The majority of people who fainted were girls ages 11 to 18, for reasons that aren't clear, the CDC said. Merck's Gardasil shot is approved for females ages 9 through 26 and is recommended as part of regular vaccinations for girls 11 and 12. The shot is designed to prevent infection from the sexually transmitted human papillomavirus, or HPV, a cause of cervical cancer.