About half of the girls acknowledged ever having sex; among them, the rate was 40 percent. While some teens define sex as only intercourse, other types of intimate behavior including oral sex can spread some infections.
For many, the numbers likely seem "overwhelming because you're talking about nearly half of the sexually experienced teens at any one time having evidence of an STD," said Dr. Margaret Blythe, an adolescent medicine specialist at Indiana University School of Medicine and head of the American Academy of Pediatrics' committee on adolescence.
But the study highlights what many doctors who treat teens see every day, Blythe said.
Dr. John Douglas, director of the CDC's division of STD prevention, said the results are the first to examine the combined national prevalence of common sexually transmitted diseases among adolescent girls. He said the data, from 2003-04, likely reflect current rates of infection.
"High STD rates among young women, particularly African-American young women, are clear signs that we must continue developing ways to reach those most at risk," Douglas said.
The CDC's Dr. Kevin Fenton said given that STDs can cause infertility and cervical cancer in women, "screening, vaccination and other prevention strategies for sexually active women are among our highest public health priorities."
The study by CDC researcher Dr. Sara Forhan is an analysis of nationally representative data on 838 girls who participated in a 2003-04 government health survey. Teens were tested for four infections: human papillomavirus, or HPV, which can cause cervical cancer and affected 18 percent of girls studied; chlamydia, which affected 4 percent; trichomoniasis, 2.5 percent; and herpes simplex virus, 2 percent.
Blythe said the results are similar to previous studies examining rates of those diseases individually.
The results were prepared for release Tuesday at a CDC conference in Chicago on preventing sexually transmitted diseases.
HPV can cause genital warts but often has no symptoms. A vaccine targeting several HPV strains recently became available, but Douglas said it likely has not yet had much impact on HPV prevalence rates in teen girls.
Chlamydia and trichomoniasis can be treated with antibiotics. The CDC recommends annual chlamydia screening for all sexually active women under age 25. It also recommends the three-dose HPV vaccine for girls aged 11-12 years, and catch-up shots for females aged 13 to 26.
The American Academy of Pediatrics has similar recommendations.
Douglas said screening tests are underused in part because many teens don't think they're at risk, but also, some doctors mistakenly think, '"Sexually transmitted diseases don't happen to the kinds of patients I see."'
Blythe said some doctors also are reluctant to discuss STDs with teen patients or offer screening because of confidentiality concerns, knowing parents would have to be told of the results.
The American Academy of Pediatrics supports confidential teen screening, she said.