And those infections bring more than $1 billion in lifetime medical costs, the report shows.
"Society is not aware of how common STDs are, and how much not just personal stress and burden they cause, but financial burden," said Petra Jerman, a research scientist for the Oakland-based Institute and co-author of the study.
Published in the California Journal of Health Promotion, the study focused on eight major STDs: chlamydia, gonorrhea, syphilis, genital herpes, HPV, hepatitis B, trichomoniasis and HIV.
Researchers used mathematical models to calculate the incidence of the infections among 15- to 24-year-olds in each California county.
In 2005 alone, new STD cases totaled:
-62,417 in Alameda County;
-28,519 in Contra Costa County;
-15,440 in Solano County; and
-8,642 in San Mateo County.
Existing public health reports show only a fraction of those cases, partly because many STDs are not subject to mandatory reporting including human papillomavirus, or HPV, which accounts for more than half the estimated cases in the new report.
HPV is common partly because it's easy to transmit, Jerman said. It's also the target of a vaccine recently licensed for young adults by the federal Food and Drug Administration.
New HPV infections alone accounted for $460 million of the total medical costs. Though much less common, new HIV infections are even more costly, at $560 million.
Public health workers have long known that STD cases are under-reported, said Francie Wise, communicable disease program chief for Contra Costa Public Health.
"Some physicians aren't clear as to what's reportable. Some see these things as very private and they don't want to breach the patient's privacy," she said. "And patients say, 'Please don't report it.'"
Teenagers and young adults are biologically more susceptible to contracting STDs, Wise said, but may not realize the longterm effects the infections can have.
"Gonorrhea or chlamydia, especially if they go untreated, can lead to infertility, which is a lifetime issue for women," she said.
The new numbers underscore the importance of using protection if sexually active, and promptly seeking medical care for any symptoms, Wise said.
And since women can contract "silent infections" that go on for years without obvious effects, she said, "it doesn't hurt to have screening tests periodically (even) if you don't have symptoms."
The prevalence of STDs among teens and young adults shows the need for accurate, comprehensive sex education in schools, said Chris Lee, vice-president of public policy and advocacy for Planned Parenthood Shasta-Diablo.
"Kids are obviously still having sex," he said. "Abstinence-only is not the answer."
Researchers hope that seeing the long-term costs of STDs will help spur renewed public discussion about the need to invest in preventive measures, Jerman says.
She also hopes the county-specific data will catch the attention of high school and college-age youths.
"I hope they're surprised," she said. "I hope it makes them think, 'Oh my gosh, they're around us, we need to protect ourselves.'"
Read the full study, including charts and background material, at teenbirths.phi.org.